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Old Tuesday, November 13, 2007
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Default Geography One - Landforms

CYCLE OF EROSION (Denudation)


EROSION

The rock grains or angular fragments that result from the disintegration and decomposition of rocks do not remain lying at the foot of the hills. As water, ice or wind move over the them, some of their material is drawn away as part of the flow. The angular fragments or rock grains, which are now part of the flow, when brought in contact with the earth’s surface, their friction with the surface of the earth produces etching (design); cut and groove the surface over which they pass. This is known as Erosion. In other words we can say that Erosion is the break up and removal of rock by moving natural agents.
The chief agents of erosion are running water, underground water, moving ice, wind and wave. These agents attack the earth’s crust by various means and produce minor relief features, the landforms of the third order.
The friction is the basis of erosion and there can be no friction without transportation. Hence transportation is necessary for erosion.

INTRODUCTION

The concept of the erosion cycle, widely popularized by William Morris Davis (1850-1934) in the 19th century, dominated geologic thinking for nearly 50 years. Many doubts are now being raised about the validity of the cycle concept.

GENERAL DESCRIPTION

All types of landforms undergo an evolution, a development through various processes. These processes start from up-arching or elevation of large area of a continent by internal earth movement. This provides a new landmass, which is then attacked by various erosional agents. Process of erosion becomes very much advance as time passes and then erosional process will be stopped. All these different processes of erosion are generally described as “Cycle of Erosion”.

STAGES OF CYCLE OF EROSION

Cycle of Erosion consists of following stages:
i. Young or Initial Stage;
ii. Mature or Sequential Stage; and,
iii. Old or Ultimate Stage

DIFFERENT CYCLES FOR HUMID & ARID REGIONS

The stages for cycle of erosion may be different for arid and humid climates. A single cycle of erosion will not cover all occurrences. There are differences between the landforms developed in humid and arid climates. The landmasses developed in humid climate, at initial stage, are relatively smooth surfaced, representing an even sea floor, broadly up-arched by epeirogenic crustal movement. The landmasses in arid climates are mountainous, which develop because of breaking and bending of rock during orogeny. Hence, the cycle of erosion for humid and arid climates are discussed separately. (Strahler 420)

I. Cycle of Erosion in Humid Climate

a. Initial Stage
A landmass formed by up-arching of a relatively smoothed sea floor would have gentle slopes inclined seawards from high central area; this is said to be in initial stage. The overland flow upon the new surface could drain off in downslope direction and would soon develop initial streams. In this stage, the stream starts down-cutting to trench youthful, v-shaped valleys in the initial landmass. Marshes and lakes, occupying shallow depressions in the initial surface, would soon be drained.

b. Youth Stage
After the initial stage, the landmass down-cutting, because of the stream, is very rapid. The relief of the area is now increasing rapidly, whereas between the streams there remains relative flat portions of the initial land surface. This stage is known as “Early Youth Stage”.
Now with the passage of time as valleys deepen, they also widen, because the rock waste has swept down the valley sides into the streams. The unconsumed areas between the valleys are reduced and the steep valley slopes increase in extent. Small tributary valleys branch out from the larger streams, further cutting the landmass. Now the remnants of the landmass between the valley system shrink in area. The greater proportion of the region consists of steep valley slopes; this stage is termed as “Late Youth Stage”.
Relief has been steadily increasing as streams have been down-cutting. The conditions show remarkable change. When larger streams become graded and begin to form their flood-plain, the increase in relief is halted. The remaining flat remnants of the initial surface are finally consumed, and valley slope intersect in narrow divides.

c. Mature Stage
When the late youth stage is attended by the landmass and the relief has reached its maximum, then this stage is called as “Mature Stage”. From this time onwards, the valleys floors are lowered with extreme slowness, whereas the inter-stream divides are rapidly lowered. Thus, the relief of the region decreases rapidly. Slopes become progressively lower in angle, slope erosion and mass wasting no longer so active as in previous stages.

d. Old Stage
After a time period much longer than was required for maturity to be reached, the both down cutting and side cutting have ceased, the streams no longer grading their course and aggradation begins. The landscape is reduced to a low rolling surface and this stage of landmass is termed as “Old Stage”.
By this time, most of the streams have low gradients and extensive flood plains. The ultimate goal which would be reached is the reduction of the land to a surface coinciding with sea level projected in land. This imaginary surface is called “base-level” and is attainable only in theory.
The land surface produced at old stage in cycle of erosion is called as “Peneplain”. A peneplain is not perfectly flat but has gentle slopes. Because the streams are sluggish (slow) and the land slopes are low, further erosion is very slow.
It is not easy to set a figure for the number of years required for a region to pass from initial stage to old stage, because it depends upon how high the landmasses are elevated and how resistant the rocks are to weathering and erosion. Perhaps it would be safe to say that in known cases of geologic records, several million years have been required to reduce a mountain mass to a peneplain.
Sometimes the region which is progressing through cycle of erosion contains patches or zones of rocks which are more resistant to weathering or erosion. As the cycle progresses from maturity to old stage, these harder rocks are left standing as prominent hills or isolated mountains, which rise above the surrounding peneplains. These isolated hard rocks are called monadnocks.(Strahler 438)

CRITICISM

An erosion cycle takes so long that no one has ever seen a complete cycle. Geologists try to piece together the parts of the cycle by examining different regions in the modern world and by looking at the geologic record which is spread over several million years.
The idealized erosion cycle is based on the presumption that nothing interrupts the regime of erosion. Actually many interruptions can take place. Once formed, a peneplain is usually elevated again by crustal movement. Sometimes this uplift of land occurs concurrently as cycle progresses. The uplift has taken place either in youthful or during maturity. The landscape and the particular stage is modified to a very great extent.
The cycle of erosion may be modified by the factors of climate. The modification by temperature and rainfall is most dominant in humid tropics and temperate climates. In humid tropics, due to heavy rainfall and chemical decomposition of rocks, the landscape abounds (overflows) in conical hills with concave slopes.
The stages of youth and maturity can be illustrated by many modern examples, but the stage of old age is where the doubts about the Davis Cycle have been raised. The old age stage is supposed to be a surface of low relief across which streams meander (maze). Many alleged modern plains have proved to be alluvial plains that appear the same regardless of their ages. From the geologic record, we know that ancient peneplains did exist. But no peneplains have been proved in the modern world. Therefore, the details of the shift from the stage of maturity to that of old stage are conjectural (hypothetical) and controversial.

ENVIRONMENTAL ASPECTS OF THE CYCLE

The environmental importance of initial stage in the cycle of erosion is very great. Regions in the initial stage of cycle of erosion are relatively flat plains, with poor drainage and marshy lands. Sandy beach deposits left by waves as the land surface emerge from beneath the sea usually produce infertile soils. The porous sands permit plant nutrients to be leached out of the soil. Not all regions in an initial stage have emerged from sea; some were built by aggrading streams and, although remarkably flat, are well-drained and do not have extensive marshes. The high plains posses a high productivity in wheat, not only because the soil and climate are favorable but also because the flatness of the land permits enormous fields to be cultivated and harvested by machines.
Still other areas of initial land surface are found by lava flows, poured out to inundate the previous topography and produce a high undulating lava patterns.
A region in youth of the cycle supports its population on the relatively flat areas between deep v-shaped valleys. Because these valleys are in a young stage, they have no flood plains; hence roads, railways, cities and farms are situated on uplands. A mature region, on the other hand, has no flat uplands remaining hence is not favorable to habitation, agriculture or transportation. Many of the world’s mountainous regions are in the stage of maturity of erosion cycle. Hence extremely great relief and steep slopes are the result of recent, high uplift of these portions of the earth’s crust. In some mature regions, the larger streams have already reached full maturity and have sizeable flood plains and the surrounding region is extremely rugged. Under such conditions, human activity is concentrated in the valley floors.
Regions with a humid climate in late mature or old stage of the cycle are usually favorable to agriculture. Slopes are moderate or low and are well-drained. Soils tend to be thick. Roads and railways cross the rolling surface without great difficulty or follow extensively developed flood plains.

II. CYCLE OF EROSION IN ARID CLIMATES
FEATURES OF ARID CLIMATIC REGION


In this cycle of erosion, we imagine a mountainous region formed by folding or fracturing of the earth’s crust and lying in an interior part of the continent. Relief is at the maximum in the initial stage and is reduced throughout successive stages. Many large depressions exist between mountain ranges. These are not filled up with water to form lakes as in humid climate but remain dry because of excessive evaporation in hot, dry climate. The flat central parts of such depressions provide the beds for temporary lakes known as Playas. Playa lakes are shallow and fluctuate considerably in level, often disappearing entirely for long periods. Because they have no outlets, playas lakes contain saline water, often more strongly saline than ocean water. (Strahler 445)

a. Initial Stage
The erosion takes place with heavy rains, which a particular locality of dry desert may experience once in several years, along with other weathering agents. When the rainfalls, the stream channels carry water and perform the same work as the constantly flowing streams of moist region.

b. Young Stage
Excess water runs off from the valley slopes into the streams washing down rock particles into the channel. Since the vegetation into the dry deserts is meager, hence the few small shrubs and herbs that survive offer little or no protection to bedrock. The swift downslope flow of water sweeps excessive quantities of coarse rock debris into the stream and in few minutes a dry channel is transformed into a raging flood, heavily charged with rock fragments.

c. Mature Stage

Throughout this erosion cycle, the intermountain depressions are filled with rock waste and alluvial fans are built out from adjoining mountain masses. When the basins are filled with alluvium and mountain masses are cut into an intricate (complex) set of canyons, the region is said to be in “Mature Stage”. As the maturity progresses, the mountains are worn lower, at the same time shrinking in size as the alluvium of the fans encroaches progressively inward upon the mountain base. (Strahler 445)

d. Old Stage
As the cycle progresses and further erosion takes place, then the mountains are further disintegrated and decomposed, and are represented by small island-like remnants. This is the “Old Stage” of the cycle. These remnants are compared to monadnocks on a peneplain but these are further eroded away and a vast plain remains. This surface is a type of peneplain, but it has not been developed with reference to sea level as a base because no streams drain out to the sea, therefore it may lie hundreds of feet above sea level. It contains shallow depressions occupied by playas rather than by flood plains of meandering rivers.
The old stage surface of the arid climatic landmass cycle, which consists in part of pediments is called as “Pedeplain”, which is equivalent to the term peneplain in humid climate landmass cycle. A pedeplain consists of alluvial fan and playa surfaces as well as pediments; it is thus partly erosional and partly depositional. (Strahler 447)

ENVIRONMENTAL ASPECTS OF THE CYCLE

Since the arid region includes rugged (uneven) mountains, broad sloping pediments, and perfectly Playa Lake floors, all these three landscapes have different environmental aspects.
The rugged mountains consist of valuable mineral deposits, which are an outstanding source of economic development.
The alluvial fan slopes or pediments are virtually worthless except in the few places where wells bring in a flow of water for the needs of isolated communities. The playas provide mineral wealth of different soil, including salts of calcium, sodium and potassium.


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MASS WASTING



INTRODUCTION

Mass wasting refers to the down slope movement of rock, regolith, and soil under the direct influence of gravity. Once weathering weakens and breaks rocks apart, mass wasting transfers the debris downslope, where a stream, acting as a conveyor belt, carries it away. Although there may be many intermediate stops along the way, the sediment is eventually transported to its ultimate destination, the sea.

ASSOCIATED FEATURES

The combined effects of mass wasting and running produce stream valleys, which are the most common and conspicuous landforms at the earth’s surface. If streams alone were responsible for creating the valleys in which they flow, valleys would be very narrow features. However, the fact that most river valleys are much wider than they are deep, is a strong indication of the significance of mass wasting processes in supplying material to streams.

FACTORS CAUSING MASS WASTING

Although gravity is the controlling force of mass wasting, other factors play an important part in bringing about the downslope movement of material.

1. WATER
Water is one of the important factors causing mass wasting. When the pores in sediments become filled with water, the cohesion between the particles is destroyed, allowing them to slide past one another with relative ease. Saturation reduces the internal resistance of materials, which are then easily set in motion by the force of gravity. Water also adds considerable weight to a mass of material. The added weight in itself may be enough to cause the material to slide or flow downslope.

2. SLOPES
Over steeping of slopes is another cause for many mass movements. Loose, undisturbed particles assume a stable slope called the Angle of Repose – the steepest angle at which material remains stable. Depending upon the size and shape of the particles, the angle varies from 25 to 40 degrees. The larger, more angular particles maintain the steepest slopes. If the angle is increased, the rock will be adjusted by moving downslope. There are many situations in nature where this takes place. A stream under cutting a valley wall and waves pounding against the base of a cliff are but two familiar examples. Further more, through their activities, people can create over steeped and unstable slopes that become prime sites for mass wasting.

CLASSIFICATION OF MASS WASTING PROCESSES

Generally the different types are divided and described on the basis of the type of material involved, the kind of motion that is displayed, and by the velocity of the movement.

Classification based on Material Involved
The classification of mass wasting processes based on the material involved in the movement depends upon whether the descending mass began as unconsolidated material or as bedrock. If soil and regolith dominate, terms such as debris, mud or earth are used in the description. On the other hand, when a mass of bedrock breaks loose and moves downslope, the term rock may be part of the description.

Classification based on the Type of Movement
The way in which material moves may also be important. Generally the kind of motion is described as either a fall, a slide, or a flow.

1. Fall
When movement involves the free-fall of detached individual pieces of any size, it is termed a fall. Fall is a common form of movement on slopes that are so steep that loose material cannot remain on the surface. The rock may fall directly to the base of the slope or move in a series of leaps and bounce over other rocks along the way. Many falls result when freeze and thaw cycles or the action of plant roots loosen rock to the point that gravity takes over.

2. Slides
Slides occur whenever material remains fairly coherent and moves along a well-defined surface. Sometimes the surface is a joint, a fault, or a bedding plane that is approximately parallel to the slope

3. Flow
Flow occurs when material moves downslope as a viscous fluid. Most flows are saturated with water and typically move as lobes or tongues.
Classification based on Velocity of Movement
Rates of movement can be spectacularly sudden or exceptionally gradual. During events called Rock Avalanches, rock and debris can move downslope at speeds well in access of 200 km/hr. Most mass movements, however, do not move with the speed of a rock avalanche. Infact, a great deal of mass wasting is imperceptibly (gradually) slow. One process termed Creep results in particle movements that are usually measured in millimeters or centimeters per year. Although various types of mass wasting are often classified as either rapid or slow, such a distinction is highly subjective because there is a wide range of rates between the two extremes. Even the velocity of a single process at a particular site can vary considerably from one time to another.

PROCESSES OF MASS WASTING

1. Slump

Slump refers to the downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface. Usually the slumped material does not travel spectacularly fast nor very far. This is a common form of mass wasting, specially in thick accumulation of cohesive material such as clay. The surface of rupture beneath the slump block is characteristically spoon shaped and concave upward or outward. As the movement occurs, a crescent shaped scarp (cliff) is created at the head and the block’s upper surface is tilted backwards.
Slump commonly occurs because slope has been over-steeped. The material on the upper portion of the slope is held in place by the material at the bottom of the slope. As this anchoring material at the base is removed, the material above is made unstable and reacts to the pull of gravity. Slumping may also occur when a slope is overloaded causing internal stress on the material below. This type of slump often occurs where weak, clay-rich material underlies layers of stronger, more resistant rock such as Sand Stone. The seepage of water through the upper layers reduces the strength of clay and slope failure results. [Strahler 406, 407]

2. Landslide / Rockslide
Rockslides occur when blocks of bedrock break loose and slide downslope. Such events are among the fastest and most destructive mass movements. Usually rockslides take place in a geologic setting where the rock strata are inclined, or joints or fractures exist parallel to the slope. If the rock is undercut at the base of the slope, it looses support and the rock eventually gives way. Sometimes the rock slide is triggered when rain or melting snow lubricates the underlying surface to the point where friction is no longer sufficient to hold the rock unit in place. As a result, rock slides tend to be most prevalent during the spring when heavy rains and melting are greatest. Earthquakes are another mechanism that triggers rock slide and other movements.
Aside from occasional great catastrophes, rock slides do not have strong environmental influence because of their sporadic (occasional/isolated) occurrence in thinly populated mountainous regions. Small slides may, however, repeatedly block or break an important mountain highway or railway line. [Strahler 406, GOH 40]

3. Mudflow
Mudflow is a relatively rapid type of mass wasting that involves a flowage of debris containing a large amount of water. Mud flows are most characteristic of semi-arid mountainous regions and because of their high water content and the predominance of fine particles, they tend to follow canyons and gullies. Although rains in semi-arid regions are infrequent, they are typically heavy. When a cloud-burst or a rapidly melting mountain snow cause a sudden flood, large quantities of soil and regolith are washed into nearby stream channels because there is usually little or no vegetation to anchor the surface material. This creates a flowing tongue of well-mixed mud, soil, rock and water. Its consistency may range from that of wet concrete to a soupy mixture not much thicker than muddy water. The rate of flow, therefore, depends not only on the slope but on the water content as well. When dense, mud flows are capable of carrying or pushing large boulders, trees and even houses with relative ease. Mudflows are a serious hazard in dry mountainous areas such as portions of Southern California. [Strahler 405]
Mudflows are also common on the slopes of some volcanoes in which case they are termed Lahars. Lahars result when highly unstable layers of ash and debris become saturated with water and flow down steep volcanic slopes generally following existing stream channels. Some are initiated when heavy rainfalls erode volcanic deposits. Others are triggered when large volume of ice and snow are suddenly melted by heat flowing to the surface from within the volcano or by the near molten debris emitted during a violent eruption.

4. Earthflow
Unlike mudflows, which are usually confined to channels in semi-arid regions. Earth flows most often form on hill slides in humid areas as the result of excessive rainfall. When water saturates clay-rich regolith on a hill slope, the material may break away and flow a short distance downslope, leaving a scar on the hillside. Depending upon the steepness of the slope and the materials’ consistency, the speed of an earthflow may very from a few meters per hour to several meters per minute. However, since the earthflows are quite viscous, they generally move more slowly than the more fluid mudflows. In addition to occurring as isolated hillside phenomena, earthflows commonly take place in association with large slumps. In this situation, they may be seen as tongue-like flows at the base of the slump block.
Shallow earthflows, affecting only the soil and residual overburden, are common on sod-covered slopes that have been saturated by heavy rains. An earthflow may affect a square yards, or it may cover an area of several acres. If the bedrock is rich in clay, earthflow sometimes include millions of tons of bedrock, moving by plastic flowage like a great mass of thick mud. [Strahler 405]

5. Soil Creep
Creep is a type of mass wasting that involves the gradual downhill movement of soil and regolith. One of the primary causes of the creep is the alternate expansion and contraction of surface material caused by freezing and thawing or wetting and drying. Freezing or wetting lifts the soil at right angle to the slopes, and thawing or drying allows the particles to fall back to a slightly lower level. Each cycle, therefore, moves the material a short distance downhill. Creep may also be initiated if the ground becomes saturated with water following a heavy rain or snow melt, a water logged soil may loose its internal cohesion, allowing gravity to pull the material downslope. Although the movement is imperceptibly slow, its effects are recognizable. Creep causes fences and telephone poles to tilt, and tree trunks will often be bent as a consequence of this movement. [Strahler 404, GOH 40]

6. Solifluction (Soil Flow)
In the frigid zones found at high latitudes solifluction (Latin word meaning soil & to flow) is of major importance. It is a variety of earthflow. In such regions, like temperate and tundra, only the ice in the upper few meters of regolith melts during spring and summer, while the ground below remains permanently frozen (permafrost). Since the water in the upper zone has no where to go, this layer remains saturated and slowly flows down even the gentlest slope. In this manner the overburden is removed and the unweathered rock below is exposed. When the newly exposed rock is eventually weathered, it too will be removed through solifluction. Flowing almost imperceptibly, this saturated soil forms terraces and lobes that give the mountain slope a stepped appearance. In Ireland, such flows are knows as Bog Bursts. [GOH 40]

7. Rockfall and Debris Avalanche
Most rapid of all mass wasting processes is rockfall, the free falling or rolling of single masses of rock from a steep cliff. Individual fragments may be as small as boulders, or as large as city blocks. Large blocks disintegrate upon falling, strewing (overspreading) the slope below with rubble and leaving a conspicuous scar on the cliff face.
A related phenomenon of high alpine mountain chains, where glacial erosion has produced extremely steep valley gradients and where large quantities of glacial rock rubble (moraine) and relict glaciers are perched precariously at high positions, is the Alpine Debris Avalanche. This glacial ice can produce a tongue of debris travelling down valley at a speed little less than that of a freely falling body.

MAN INDUCED MASS WASTING

Man’s activities induce mass wasting in forms ranging from mudflow and earthflow to rockslide and slump. These activities include:

1. piling up of waste soil and rock into unstable accumulations that fail spontaneously; and

2. removal of support by undermining natural masses of soil, overburden, and rock.
Spoil banks produced by strip mining of coal are unstable and a constant threat to the lower slope and valley bottom below. When saturated by heavy rains and melting snows, the spoil generates earth flows and mud flows that descend upon houses, roads and forests. Examples of both large and small earthflows induced or aggravated by man’s activities are found in the Palos Verdes Hills of Los Angeles County, California.

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WEATHERING



INTRODUCTION

Weathering is the combined action of all processes, whereby rock is decomposed and disintegrated because of exposure at or near the earth’s surface. Weathering normally changes the hard, massive rock into finally fragmented, soft residual overburden, the parent matter of the soil. It depends both on the nature of the climatic elements involved and on the character of the rock; its chemical composition, its hardness, its texture and its permeability. The chief agents of weathering are insolation, frost, rainwater, gases of the atmosphere, and organisms.

WEATHER PROCESSES

Weathering processes may be subdivided into two large groups; Physical or Mechanical Weathering and Chemical Weathering. Mechanical Weathering tends to breakdown the rock into progressively smaller fragments while Chemical Weathering forms residual materials; the joint result is the production of a loose layer, which can readily be removed by the agents of transportation. The influence of man, plants and animals, although not strictly part of weathering, may be considered as a biological agency which directly contributes to the layers of rock waste (Biological Weathering).

Physical / Mechanical Weathering
Mechanical weathering involves the destruction of rocks through the imposition of certain stresses. This is carried out in deserts by rapid changes of temperature or in mountains by the action of frost. Mechanical weathering is also carried on, to a limited extent, by plants and trees. In regions of dry climates, wind is also a powerful agent of weathering. The physical or mechanical processes of weathering produce fine particles from massive rock, but do not change its chemical composition.

1. Frost Action

One of the most important physical weathering processes in cold climates is Frost Action, the repeated growth and melting of ice crystals in the pore spaces or fractures of rocks.
At night the temperature of water drops on cooling to 4oC (contraction). Then from 4oC to 0oC, water expands and freezes at 0oC. When the water freezes, it exerts a pressure of almost 2,000 lbs/in2.
As water in joints freezes, it forms needle-like ice crystals extending across the openings. As these ice needles grow (water increases in volume about 9% as it freezes, i.e., 1/10th of its volume), they exert tremendous force against the confining walls and can easily pry apart (move, push/pull) the joint blocks. Even massive rocks can be shattered by the growth of ice crystals created from water that has previously soaked into the rock. On mountain peeks, this process creates sharp pinnacles and angular outlines. Such peeks are described as Frost-Shattered Peaks.
Freezing water strongly affects soil and rock in all middle and high latitude regions having a cold winter season, but its effects are most striking in high mountains, above the timber line. Here the separation and shattering of joint blocks may produce an extensive ground surface littered with angular blocks. Such a surface is termed a Block Field or Felsenmeer (rock sea), or Boulder Field. Where cliffs of bare rocks exist at high altitudes, fragments fall from the cliff face, building up piles of loose blocks into conical forms, termed Talus Cones or a Scree Slope. [Strahler 398]
Example of frost action is Millstone Grit Areas of the Pennines (England).

2. Growth of Salt Crystals
Closely related to the growth of ice crystals is the weathering process of rock disintegration by growth of salt crystals (process called salt wedging). This process operates extensively in dry climates or arid regions and is responsible for many of the niches (holes, corners), shallow caves, rock arches, and pits in sandstone formations.
During long drought periods, ground water is drawn to the surface of the rock by capillary force. As evaporation of the water takes place in the porous outer zone of the sandstone, tiny crystals of salt are left behind. The growth force of these crystals is capable of producing granular disintegration of the sandstone, which crumbles into sand and is swept away by wind and rain. Specially susceptible are zones of rock lying close to the base of a cliff, for here the ground water tends to sweep outward.

3. Swelling and Shrinking of Soil

An important but little appreciated process of physical weathering is the continual swelling and shrinking of soils as the particles of fine silt and clay absorb or give up soil water in alternate periods of rain and drought. Shrinkage forms soil cracks in dry periods, making the infiltration of rainfall much more rapid in early stages of an ensuing (resultant) rain. In clay rich sedimentary rocks, such as shale, the swelling is largely responsible for a spontaneous breakup known as Slaking, in which the shale crumbles into small chips or pencil-like fragments when exposed to the air.
In tropical regions like Malaysia, short downpours saturate the rocks while the hot sun quickly dries them again. In coastal areas, the wet rocks may be quickly dried by sun and wind between the tides. Water expands the outer layers of the rock while drying causes shrinkage.

4. Diurnal Range of Temperature
Most crystalline solids, such as minerals of the rocks, tend to expand when heated and to contract when cooled. When rock surfaces are exposed daily to the intense heating of the sun alternating with nightly cooling, the resulting expansion and contractions exerts powerful forces upon the rock. Given sufficient time (tens of thousands of such daily alternations), even the strongest rock may develop fractures. Breakage can take the form of exfoliation (onion peeling) or granular disintegration.

5. Unloading
One process of physical disintegration of rocks is known as Unloading or Pressure Release. If overlying layers of rocks are removed by denudation (exposure), the release of this weight-caused pressure may allow the newly exposed rock to expand and form new curvilinear (well-developed round) joints, causing curved rock-shells to pull away from the mass, a process known as Sheeting. Granite appears to be particularly prone (liable) to this. The enormous domes of the Yosemite Valley in California are probably the result of this outwards expanding force of pressure release on little jointed granite.

6. Wedging of Plant roots
In physical weathering processes, the wedging of plant roots deserve consideration as a possible mechanism, whereby joint blocks may be separated. We have all seen, a tree whose lower trunks and roots are firmly wedged between two great joint blocks of massive rock. The growth of tiny rootlets in joint fractures are of great importance in loosening countless small rock scales and grains, particularly when a rock has already been softened by decay or fractured by frost action.

7. Wind
The atmosphere has a mechanical action through wind. Fantastic weathering results when wind blows fine grit or sand against the exposed rock. The winds act as a sand blast and cause uneven weathering according to the hardness and softness of the rock, e.g., Brimham Rocks, Yorkshire, England.
This action is most pronounced in sandy regions. In some towns on the West Coast of the United States, the windows facing the shore have become so worn that they appear to be made of frosted glass. Trains that cross desert have to repainted more frequently due to the action of dust-laden wind. In many sea side areas, cliffs are fantastically weathered as a result of the wind blown sand.
In dry regions, wind aids in disintegration by cracking and peeling away the layers of rocks in shell-like formations. This is known as exfoliation.

8. Rainfall
Rain also acts mechanically by washing away the loose particles of insoluble material. The carving out of Gullies or miniature Valleys can be seen very well on the pit heaps of mining areas. Each successive rain storm, beating on the loosely piled material cuts the gullies deeper and deeper. The material is then carried to the foot of the slope partly by the force of rain.
The sides of an active volcano show such a carving with narrow gullies radiating out of the mountain like the spikes of a wheel.
The activity of rain as an erosive/weathering agent can be seen in the formation of Earth Pillars. These are formed in regions of soft soil or clays containing scattered blocks of rock. The rain washes away the soft material, except where a small boulder occurs. The boulders protect the underlying soil from the action of rain. As a result, pillars of earth are formed, each one in the early stages, having a CAP of rock. These pillars are not long-lived, for in time the cap falls and the rain destroys the uncapped softer material. Most famous earth pillars are in Bolzano, Italy.

Chemical Weathering

Chemical weathering is chiefly carried on by rain water and the atmospheric gases. The atmospheric moisture in conjunction with various gases, brings about a chemical change in the rocks. The rocks begin to decompose and they decay. High temperature and humidity, specially favor this sort of activity and, therefore, it is most common in the hot wet regions of the world. Oxygen, CO2 and water affect the composition of the rocks.
Some minerals such as quartz resist this alteration quite successfully, but other dissolve easily, such as the calcium carbonate or lime stone. In any rock made up of a combination of minerals, the chemical breakdown of one set of mineral grains leads to the disintegration of the whole mass. In granite, for example, the quartz resist chemical decay much more effectively than the feldspar, which is chemically more reactive and weathers to become clay. So even a rock as hard as granite cannot withstand the weathering process forever.
In Malaysia, the surface of the exposed granite is found to be pitted and rough. This is because the granite is made of three main minerals, i.e., quartz, feldspar, and mica. The feldspar is more quickly weathered and worn away. The quartz crystals are eventually loosened and form a course sandy residue.
The weathered material (regolith; including soil, broken rock, volcanic ash and glacial material overlying the bedrock also called mantle rock) may be taken away by the erosive agents or it may stay in position forming basis of soil. That soil contains regolith (minerals) and organic materials.
The chemical weathering may be said to take the following forms:

1. Oxidation
The presence of dissolved oxygen in water in contact with mineral surfaces in the soil leads to oxidation, which is the combination of oxygen ions with metallic ions, such as calcium, sodium, potassium, magnesium and iron; abundant in the silicate minerals. When oxygen reacts with iron, the latter gets a coating of iron oxide or rust, which weathers it away and the piece of iron decades. Rocks, which contain a mineral content of iron, are oxidized during the rainy season and whole rock mass decomposes.
The products of oxidation are compounds of iron and aluminum, which account for the reddish color seen in so many rocks and soils. In tropical areas, oxidation is the dominant chemical weathering process; e.g. Grand Canyon.
The rock surfaces of old tombstones or buildings are decaying e.g., Oxford and Cambridge Colleges. The carvings on Cleopatra’s Needle have become far less distinct during the 80 years than the monument has been on Thames Embankment. It had stood in the dry climate of Egypt for 4000 years.

2. Carbonation
Various circumstances may convert water into a mild acid solution, thereby increasing its effectiveness as a weathering agent. With a small amount of CO2, for example; water forms Carbonic Acid, which in turn reacts with carbonate minerals such as Lime Stone and Dolomite (a hard relative of limestone; a carbonate of calcium and magnesium). This form of chemical weathering, carbonation, is specially vigorous in humid areas where percolating waters contain CO2, derived from its passage through the atmosphere, it act as a dilute acid upon calcareous rocks such as lime stone and chalk dissolving and removing them in the form of calcium bicarbonate.

3. Hydrolysis
Water itself combines with certain mineral compounds in a reaction known as Hydrolysis. This process in not merely a soaking or wetting of minerals, but a true chemical change producing a different compound and a different mineral. It may also lead to volume expansion which in turn contribute in the breakdown of rocks. The reaction is not readily reversible under atmospheric conditions, so that the products of hydrolysis are stable and long lasting. The hydrolysis of granite with accompanying granular disintegration and exfoliation of thin scales, produces many interesting boulder and pinnacle (tower) forms by rounding of angular joint blocks. These forms are particularly conspicuous in arid regions because of the absence of any thick cover of soil and vegetation. There is ample moisture in most deserts for hydrolysis to act, given sufficient time. In warm, humid climates, hydrolysis of susceptible (exposed, vulnerable) rocks goes on below the soil and may result in the deep decay of igneous and metamorphic rocks to depths as much as 100-300 ft.

4. Hydration
Certain minerals have the property of taking up water and thus expanding, so stimulating the disintegration of the rock containing them; this is hydration.

5. Solution
The rain water is able to dissolve certain minerals and leach the soil. Through this process, many minerals are washed out of the soil and rocks so that their chemical composition changes. Some minerals, such as quartz are virtually unaffected; others such as Olivine, Augite, Hornblende, Biotite, Orthoclase, and Muscovite are very susceptible; and a few such as Rock Salt can be completely removed in solution.
Solution is the most potent weathering process in lime stone regions because the rain water dissolves the calcium carbonate (CaCO3). The removal of CaCO3 (marble and chalk), leaves joints and cracks which are slowly widened and the whole system of caves and passages is worn out. The chemical action of rain on the feldspar in granite leads to the formation of Kaolin or China Clay.
River Thames carries down to the sea in solution 250,000 tons of chalk and limestone everyday.

Biological or Organic Weathering

Plants assist in surface weathering by both chemical and mechanical means. Algae, mosses, lichens (fungus) and other vegetation retain water on the surface of the rock, and various organic acids help to decay the rock beneath, so that a tuft of moss may lie in a small and growing hollow in the rock. The presence of vegetation increases the acid content of the soil-water, which will be effective in chemical disintegration of calcareous rocks. Water containing bacteria can assist the decomposition of some rocks particularly Lime Stones. The mechanical disintegration effect of vegetation is mainly due to the penetrating and expanding power of roots, which exert considerable force as they grow and help to widen cracks and crevices (fissures), thus allowing water and air to enter.
Various forms of animal life such as warms, rabbits and moles, may have a contributory effect. Warms bring large quantities of fine material to the surface in the form of casts (inclination), while burrowing animals in some measure help to loosen the surface material.
Technically, humans are also agents of biological weathering. Human activity contributes to various forms of weathering in a number of ways; by polluting the air with substances that greatly accelerate some chemical weathering, specially in and around large urban centers; by quarrying and mining, we accelerate mechanical, chemical and biological weathering through exposure of deep strata to these processes.

WEATHERING IN VARIOUS CLIMATIC REGIONS

The effects of the weather upon rocks vary accordingly to the potency of the different climatic elements.

1. Equatorial Latitudes
In equatorial latitudes, where both humidity and temperature are consistently high, chemical weathering is continuously active, and it is generally much more rapid and effective than the transport and removal of the weathered material.

2. Desert Areas
In desert areas there is little weathering by ordinary leaching, but considerable mechanical weathering. While chemical weathering takes place by the drawing of strong solutions to the surface by capillary action.

3. Mid Latitudes
In mid latitudes, frost is by far the most powerful agent, while solution, particularly in lime stone areas, exert great effects.

4. Polar Conditions
Under polar conditions, great areas of permanent snow prevent any ordinary weathering, but where nunataks project from ice-sheets, frost action is rampant. Chemical and organic agencies here seem to be negligible for their effects. CO2 is more soluble at low temperatures than at high, and as the melted water has therefore a higher carbonic acid content, chemical weathering may be quite active under a glacier or at the edge of an ice-sheet.

CONCLUSION

The weathering processes, both physical and chemical, work universally but produce few distinctive large land forms or spectacular activities that would draw the attention of the average person. Nevertheless, these processes are of enormous importance in slope development for that they prepare the bedrock for soil formation and for erosional removal by the agents of land sculpture. Without the weathering processes, vegetation could not thrive as we know it today, nor could the great continental land masses be easily reduced by the agents of denudation (exposure).
{Also see GOH’s Chapter 4}

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GLACIER


INTRODUCTION

What is a glacier?

Glacier is a thick mass of ice that originates on land from the compaction and re-crystallization of snow and shows evidence of past or present movement. Since snow is the raw material that eventually produces glacial ice, glaciers must form in areas where more snow falls in winter than melts during the summer. A region that has such a net accumulation is termed a Snowfield and its outer limits are defined by the Snowline. The elevation of the snowline varies greatly. In the frigid polar realm, it may be sea level, while in tropical areas near the equator, the snowline exists only high in the mountains, often at elevations exceeding 4,500 meters. If the accumulation in the snow field is great enough, the pressure of overlying layers transforms the snow below into glacial ice.

Existence

The term ablation is used by glaciologists to include both evaporation and melting of snow and ice. thus, each year a layer of snow is added to what has already accumulated. As snow compacts, by surface melting and refreezing, it turns into a granular ice, then is compressed by overlying layers into hard crystalline ice. When the ice becomes so thick that the lower layers become plastic, outward or downhill flow commences, and an active glacier has come into being.

Movement

Glacier moves under the continuous pressure from the accumulated snow above. The rate of movement is greatest in the middle where there is little obstruction. The sides and the bottom are held back by friction with the valley side spurs and the valley floor. This is demonstrated by planting stakes across the glacier path, as they take a curved shape. (GOH 59)
Movements of different glaciers are:
In the Alps 1 m a day
In Greenland 15 m a day
In Antarctica a few millimeters a day
The glacier has differential movement due to different layers and composition. Veins and fissures are formed due to tension and compression. Melting proceeds due to the action of sunrays. Tiny streams are formed. Yawning gaps called Crevasses are formed due to which ice turns into a series of ridges, pinnacles and Seracs (stepwise like a cascade).
Fine dust causes melting and formation of dust wells. The water freezes at night forming ice needles, the collection of which is called Glacial Star. Sometimes rounded boulders come to rest on the glacier surface causing the formation of the Ice Pyramid.
The ice-sheets reaching the sea often extend into the polar waters and float as Ice Shelves. They terminate into precipitous Cliffs, when they break into individual blocks, these are called Icebergs. At the foot of the mountain ranges, several glaciers may convert to form an extensive ice mass called a Piedmont Glacier. The best known such glacier is the Malaspina Glacier of Alaska which is spread over 4,200 km2.
Load of the Glacier
1. Rock fragments from valley walls, blown by winds or brought by avalanches.
2. Material extracted from the floor.
3. Big boulders and fine particles.
Transportation
1. Near the bottom or dragged along
2. At the top
3. Embedded in layers.
Work of the Glacier
1. Difficulty in studying
2. 3 schools of thought regarding the work done by the glacier.
a. Protectionist School
By Hein (1885); glaciers are the protectors of the crust.
b. Erosionist School
By Hess (1904); glaciers are powerful agents of erosion and excavation.
c. Modern Thought
Stable ice does the work of protection while moving ice does erosion.

THE ICE AGE & TYPES OF GLACIER

Today, only two ice caps are present, in Greenland and in Antarctica. The former covers an area of 1,872,000 km2, while the later ins more than 13 million sq. km. They are made up of compact sheets of ice, hardened and crystallized to a depth of over a kilometer. A part form Antarctica and Greenland, glaciation is still evident on the highlands of many parts of the world, which lie above the snow line.
The glacier or ice masses of the world can be said to belong to two distinct classes:

1. Valley or Mountain Glaciers

These are also known as Alpine Glaciers and flow like tongues of ice through the mountain valleys. These glaciers are formed by the descending Neve or Firn (the water in an effort to percolate down through the glacier gets frozen again leading to the formation of granular ice which is known as Neve or Firn. It is the main source of nourishment for the glacier), and later on are also fed by direct snowfall, avalanches and drift of snow blown by the wind from higher levels. Its shape and form undergoes transformation according to the configuration of the valley. The mountain glacier are ordinarily not very extensive, their length ranging from 2 to 3 miles. But certain mountain glaciers are so vast that they go on flowing for a distance of 50 to 60 miles and appear like huge rivers of ice. The thickness of ice also varies from 10 ft. to tens of thousands feet.

2. Continental Glaciers or Ice Sheets

In the Greenland and Antarctica, all the precipitation is in the form of snow. The snow that falls from year to year goes on being accumulated because very little of it is wasted either by melting or evaporation (ablation). The result is that these regions are covered by an extensive ice-mass, a gigantic ice-dome that hides beneath it all the surface irregularities irrespective of its height or depth. This is known as ice-sheet or continental glacier. It is estimated that the thickness of the ice-sheets of Greenland varies from 2,000 to 7,000 feet while the average thickness of an ice-sheet of Antarctica is 4,000 feet. The ends of the ice sheets break into blocks due to action of sea waves and keep floating as Icebergs. The coastal regions are free from the covering of ice and mountain peaks stand out as Nunataks. In these glaciers streams flow beneath the ice mass. The upper ice layers move faster than the lower ones, this phenomenon is called Englacial Drift.
This type of glacial does not contain any debris so it does no erosional work.

Difference b/w Valley & Continental Glaciers
These two distinct classes of glaciers differ not only in their location but also in form, manner of modifying the relief, mode of nourishment, and nature of topography that results from their actions.

1. Form
Mountain or valley glaciers are contained within a rock basin or valley; while the continental glaciers are like a dome or sheet or shield, which covers beneath it all the irregularities of relief and configuration.

2. Manner of modifying the Relief
The mountain glaciers produce hollows and caves increasing the ascent of the relief. But the continental glaciers level down or iron out the irregularities of the surface over which they flow.

3. Mode of Nourishment
The mountain glaciers are nourished by Nivation, which consists of frost action resulting in snow niches (corners) that gradually dig themselves in and become the starting point of mountain glacier. The continental glacier, receive their nourishment from low level clouds, descending currents and the ice-needles descending from the clouds.

4. Resulting Topography
The regions visited by mountain glaciers show a concave relief or topography in which the curve is sharp towards upper end. The marks left by continental glaciers are, on the other hand, convex in shape and their curvature is slight. The mountain glaciers leave behind a topography predominantly marked with erosional features while the continental glaciers leave behind a predominance of depositional features.

GLACIAL EROSION

Introduction

Glaciers are capable of carrying-on great amounts of erosional work. The ice has scraped, scoured (polished), and torn rock debris from the floors and walls of the valleys, and carried them downslope.
Ways in which erosional work takes place
Glaciers primarily erode land in two different ways:

1. Plucking

As a glacier flows over fractured bedrock surface, it loosens and lifts blocks of rocks, incorporates them into flow and carries them off. This process, known as plucking, occurs when melted water penetrates cracks and joints along the rock floor of the glacier and refreezes. As the water expands, it exerts a tremendous leverage that pries (pushes) the rock loose. In this manner, sediments become part of the glacier’s load.

2. Abrasion

It is the second major erosion process. As the ice, with its load of rock fragments moves along, it acts as a giant rasp (wear) file and grinds the surface below as well as rocks within the ice. The pulverized rock produced by the glacial grist mill is appropriately called Rock Flour. When the embedded material consists of large fragments, long scars and grooves called Glacial Striations (texture) may gouge out. These linear scratches on the bedrock surface provide clues to the direction of glacial movement. On the other hand, all abrasive action produces striations. When the sediments consist primarily of fine silt particles, the rock surfaces over which glacier moves may become highly polished.

Difference b/w Continental & Alpine Glacial Erosion

The erosional effect of Alpine and Continental glaciers are quite different from each other. A visitor to an Alpine-Glaciated region is likely to see sharp and very angular topography. The reason is that as Alpine glaciers move down valley, they tend to accentuate the irregularities of mountain landscape by creating steeper canyon walls and making bold peaks even more jagged. By contrast, continental ice-sheets generally over-ride the terrain and hence tend to subdue rather than accentuate the irregularities they encounter. Although erosional accomplishment of continental glaciers can be tremendous, landforms carved by these ice-masses usually do not inspire the same degree of wonderment or awe as do the erosional features created by Alpine glaciers. In regions where the erosional effects of continental ice-sheets are significant, glacial scoured surfaces and subdued terrain are the rule. By contrast, in mountainous areas, erosion by Alpine glaciers yield many truly spectacular features. Much of the rugged mountain scenery so celebrated for its majestic beauty is the product of glacial erosion.

Erosional Landforms (Landforms of Highland Glaciation or Alpine)

1. Corrie, Cirque or Cwm

Cirques are bowl-shaped, steep sided depressions in the bedrock with gently slopping floors. It is formed by the downslope movement of the glacier from the snow covered valley head and the intensive shattering of the upland slopes. The depression so produced accumulates the Firn or Neve.
The process of plucking also steeps the back-wall and the movement of ice deepens the depression into a steep, Horse-Shoe-Shaped-Basin, called a Cirque (in French), Corrie (in Scotland) and cwm (in Wales).
There is rocky ridge at the exit of the corrie and when the ice eventually melts, water collects behind the barrier to from a Corrie Lake or Tarn. (GOH 61, Strahler 524)

2. Aretes and Pyramidal Peaks

When two corries cut back on opposite sides of a mountain, knife edged ridges are formed called aretes ( a French word). When the aretes are horizontally arranged they give rose to the Comb-Ridges.
When three or more cirques cut back together, their ultimate recession will form an Angular Horn or Pyramidal Peak. (GOH 61)

3. Bergschrund

At the heads of the glacier, where it begins to leave the snowfield of a corrie, a deep vertical crack opens up called a bergschrund (in German) or rimaye (in French).
This happens in summer when, although the ice continues to move out of the corrie, there is no new snow to replace it. In some cases, not one but several cracks occur. The bergschrund represents a major obstacle to the climbers. Further down where the glacier negotiates a bend or sudden slope, more Crevasses or cracks are formed. (GOH 61)
The above characteristic forms belong to upper slopes above the level of the glacier’s surface. Erosion here is accomplished by Sapping and Undermining.
In valleys and the lower slopes erosion is carried out by Plucking and Abrasion.

4. U-shaped Glacial Trough

The glacier as it moves down the narrow v-shaped valley, tends to iron out the irregularities of the side walls straighten them. It is fed by many corries-like tributaries that join a river. It wears away the sides and floor of the valley. It scratches and grinds the bedrock removing any rock debris and surface soil. The interlocking spurs are thus blunted to form Truncated Spurs and the floor of the valley is deepened.
The glaciated valley takes the characteristic U-shape with a wide, flat floor and very steep sides. After the disappearance of the ice, the deep sections of the glacial troughs may be filled with water forming ribbon lakes. (GOH 62)

5. Hanging Valleys

The main valley is eroded much more rapidly and to a greater extent than the smaller tributary valleys, as it contains a much larger glacier.
After the ice has melted, a tributary valley, therefore hangs above the main valley so that its stream plunges down as a water fall. Such hanging valleys may form a natural head of water for generating Hydroelectricity.
6. Rock Basins and Rock Steps (Bastions)
A glacier erodes and excavates the bedrock in an irregular manner. The unequal excavation give rise to many rock basins later filled by lakes in the valley trough.
Ribbon-like falls result from the hanging valleys. The steep walls over which these walls are formed are not quite smooth. The tributary glaciers have cut them into Steps due to the additional weight of ice at the point of convergence of the two valleys. A series of such steps may be formed due to different degrees of resistance to glacial erosion of the bedrock. This series is called Rock Bastion (battlement).

7. Moraines

The moraines are made up of the pieces of rock, boulders that are shattered by frost action, imbedded in the glaciers and brought down the valley.
The moraines are important from the point of view of the depositional work done by the glaciers. The glacial pavement bears the streaks and scratches and is smoothened into a rounded surface.

8. Fjords (body of water)

when the floor of a trough open to the sea lies below sea level, the sea water will enter as the ice front recedes, thus producing a narrow estuary known as a Fjord. Among the most spectacular landforms associated with glacial erosion are fjords. Fjords may originate either by submergence of the coast or by glacial erosion to a depth below sea level. These are deep, steep sided inlets of the sea that exist in many high latitude areas of the world where mountains are adjacent to the ocean. Norway, British Columbia, Greenland, New Zealand, Chile and Alaska all have coastlines characterized by fjords. They represent glacial troughs that were partially submerged as the ice left the valley and sea level rose following the Ice Age. The depth of fjords are often dramatic, in some instances exceeding 1,000 to 15,000 meters. The great depths of these flooded troughs is only partly explained by the post Ice Age rise in sea level. Fjords are observed to be opening up today along the Alaskan coast, where some glaciers are melting back rapidly and the fjord waters are extended along the troughs. (Strahler 529)

9. Horns

These are sharp, pyramid-like peaks. These are found by the enlargement of cirques produced by plucking and frost action. A group of cirques along the single high mountain create the spires of rock called horns. As the cirques enlarge and converge, an isolated horn is produced. The most famous example is the Matterhorn in the Swiss Alps.

10. Col

When on the opposite sides of a ridge, two cirques go on widening the boundary wall, the cutback action is able to reduce the intervening ridge. If the two cirques have been at the same level or height, they would come to become tangent to each other at some point. When this happens, the intervening rock wall gives way and a saddle-like formation comes into existence. This is called a col and is often used as a pass through the mountain ranges. The Canadian Pacific Railway passes through such a col.

11. Tarn

Small lake occupying a rock basin in a cirque or a glacial trough is called “Tarn”.

12. Trough Lakes

The major troughs usually contain large elongated trough lakes sometimes also called Finger Lakes. Finger Lakes are like tarns but bigger in scale.

(Landforms of Glaciated Lowlands)

1. Roche Moutonnees
Unlike the running water, glacier is not diverted from its path by slide barriers or obstacles. On the contrary, in motion tries to level down whatever comes in its way. The irregularities of the surface are ironed out so that they become rounded on the onward side (stoss side) while their leeward sides remain broken and irregular. Such rounded hummocks appear from a distance like sheep back in a flock and they are called Roche Moutonnees. These are asymmetrical ridges. They are supposed to be the residuals of the relief before the glaciers passed over them. Roche Moutonnees are most prominently developed where the ice action has been most powerful and continuous. Typical profiles of this type are seen in the passes of the Alps, in Southern Sweden and Finland, in the valleys of the Adirondacks and also on the borders of the Ice-sheet in Greenland. (Strahler 534)

2. Crag and Tail
The crag is a mass of a hard rock with a precipitous slope on the upstream side which protects the soft-ward leeward side slope from completely worn down by the oncoming ice. It therefore has a gentle tail, strewn with the eroded rock debris. For example; Castle rock of Edinburgh, Scotland.

Glacial Deposition

Glaciers are capable of acquiring and transporting a huge load of debris as they slowly yet steadily advance across the land. Ultimately these materials must be deposited when the ice melts. In regions where glacial sediments are deposited, the sediments can play a truly significant role in forming the physical landscape. For example in many areas once covered by continuous ice-sheets of the recent ice age, the bedrock rarely exposed because glacier deposits of tens or even hundreds of meters thick completely mantle (blanket) the terrain. The generation of these deposits is to reduce the local relief and thus level the topography. Indeed much of the country’s scenery results directly from glacial deposits.

Types of Glacial Deposits

The term glacial drift has long been applied to include all varieties of rock debris deposited in close association with glaciers. Drift is of two major types:

1. Till
This is an unsorted glacial deposit or a heterogeneous mixture comprising of a variety of eroded materials; boulders, angular stones, sticky clay and fine rock flour. It is spread out in sheets, not mounds and forms gently undulating till or drift plains. It is deposited directly form the ice without water transport. The landform is rather monotonous and featureless. The degree of fertility of such glacial plains depends very much on the composition of the depositional materials. Such plains are found in East Anglia, and the north mid-west of USA (arable lands).
Moraines of Valley Glaciers are composed largely of till, whereas the Valley Train (deposit of alluvium extending down valley from a melting glacier is the valley train) is composed of stratified drift or out wash.

2. Out wash or Stratified Drift
Materials deposited by glacial melt-water are called out wash. Out wash is sorted according to the size and weight of the fragments. Since ice is not capable of sorting activity, these sediments are not deposited directly by the glacier as the till is, but rather they reflect the sorting action of the glacial melt-water that was responsible for dropping them. Accumulations of out wash often consist largely of sand and gravel, i.e., bad load material, because the finer rock flour remains suspended and is commonly carried far from the glacier by the melt-water streams. An indication that out wash consists primarily of sand and gravel can be seen in many areas where these deposits are mined as aggregate for road works and other construction projects.

Depositional Landforms of Alpine Glaciers

1. Moraines

Perhaps the most wide spread feature created by glacial deposition are moraines, which are simply layers or ridges of till. Several types of moraines are identifiable; some are common only to mountain valleys, and others are associated with areas affected by either continental or Alpine glaciers. Lateral and Medial moraines fall in the first category, while End moraines and Ground moraines in the second.

a. Lateral Moraines
The sides of an Alpine glacier accumulate large quantities of debris from the valley walls. When the glacier wastes away, these, these materials are left as ridges, called Lateral Moraines, along the sides of a valley. (Strahler 525)

b. Medial Moraines
These are formed when two alpine glaciers or ice streams coalesce to form ice streams. This medial moraine rides upon the ice in mid stream. The till that was once all along the edges of each glacier joins to single dark stripe of debris within the enlarged glacier. The creation of the stripes within the ice stream is one obvious proof that glacial ice moves, because Medial moraine could not form if they did not flow down the valley. (Strahler 525)

c. End Moraine or Terminal Moraine
As the name implies, End Moraines form at the terminus of a glacier. At the terminus of a glacier debris accumulates in a heap. This heap is usually in the form of curved embankment lying across the valley floor and bending up-valley along each wall of the trough to merge with the lateral moraines, creating a ridge of till hundreds of meters high. As the end of the glacier wastes back, scattered debris is left behind. Successive halts in ice-retreat produce successive moraines, termed recessional moraines. (Strahler 525, 527)

d. Ground Moraine
As the glacier recedes layer of till is laid down, forming a gentle undulating surface of Ground Moraine. This cover is often inconspicuous because it forms no prominent or recognizable topographic feature. Nevertheless, the ground moraine may be thick and may obscure or entirely bury the hills and valleys that existed before glaciation. Where thick and smoothly spread, the ground moraine forms an extensive, level till plain, but this condition is likely only in regions already fairly flat to start with.

e. Interlobate Moraine
Moraine formed between two adjacent lobes of an ice sheet.
Depositional Landforms of Continental Glaciers

1. Out wash Plains
At the same time the end moraine is forming, water from the glacier cascades over the till, sweeping it out infront of the growing ridge as debris. Melt water generally emerges ice in rapidly moving streams that are chalked with suspended material and is substantial bed local as well. As the water moves the glacier, it moves onto the relatively flat surface beyond and rapidly loses velocity. As a consequence, much of its bed load is dropped and the melt-water begins weaving a complex pattern of braided channels. In this way a broad, ramp-like surface called an out wash plain is built adjacent to the downstream edge of most end moraines. The deposits are in reality great alluvial fans upon which are spread layer upon layer of sands and gravel. The term glaciofluvial is often applied to stream laid stratified drift.

2. Kettles
Often out wash plains are pockmarked with basins or depressions known as kettles. Kettles also occur in deposits of till. Kettles form when a block of stagnant ice becomes wholly or partially buried in drift and ultimately melts, leaving a pit in glacial sediment. Likewise typical depth of most kettles is kettles is less than 10 m, although the vertical dimensions of some approach 50 m. In many cases, water eventually fills the depression and forms a pond or lake.

3. Drumlins
These are swarms (crowd) of oval, elongated whale back hammocks composed wholly of boulder clay, with their elongation in the direction of the ice flow, i.e., on the downstream side. They are low hills varying from a few meters to 120 m in heights and may be a kilometer or two long. They appear a little steeper at the onset side and taper off at the leeward end. They are arranged diagonally and so are commonly described as having a basket of eggs topography. Large numbers of them are found in County Down in Northern Ireland and the glaciated plain around the Great Lakes in North America. It consists of glacial till. Drumlins invariably lie in a zone behind the terminal or recessional moraines. Drumlins are formed under moving ice, by a kind of plastering action in which layer upon layer of bouldery clay is spread upon the drumlin. This would have been possible only if the ice were so heavily choked with debris that the excess had to be left behind.(GOH 64).

4. Esker
These are long, narrow, sinuous ridges composed of sand and gravel which mark the former sides of sub-glacial melt-water streams (ice tunnel). They vary from a few meters to 60 m in height and may be several kilometers long.
As the sand and gravel is highly porous, water is rapidly drained off from their crests, so they do not support trees except in Finland.
Examples are found in Maine, USA (length = 160 km) and in Scandinavia e.g., Punkaharju Esker of Finland. The Eskers may take various shapes or get arranged in groups according to the pattern of glacier withdrawal. (Strahler 535,537)

5. Kame
These are a number of hillocks and ridges of sand and gravel. They are not terminal moraines. They may be in the form of alternating ridges and depressions (lakes).
They are deposited by sub-glacial streams and are arranged asymmetrically in the direction of the ice-front. Due to the presence of kettle lakes, the topography is called “Knob and Kettle“ Topography.

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LANDFORMS MADE BY UNDERGROUND WATER


INTRODUCTION

The whole process of circulation of water between the land, sea and atmosphere is known as the Hydrological Cycle.

The rainwater is distributed on the earth in various ways:
1. Some is evaporated immediately.
2. Some is absorbed by the plants and later transpired.
3. Some flows into streams and rivers, eventually reaching the seas and oceans. This is known as runoff.
4. A considerable portion of water from precipitation percolates downwards into the soil and rocks forming what is known as Ground Water.
The ground water is present in the joints and pore spaces and plays an important part in weathering and mass movement besides being an important natural storage.
Of all the world’s water, only six-tenths of 1% is found underground. Nevertheless, the amount of water stored in the rocks and sediments beneath the earth’s surface is a vast and significant natural resource. The US geological survey estimates that the quantity of water in the upper 800 meters of the continental crust is about 3000 times greater than the volume of water in all rivers at any one time, and nearly 20 times greater than the combined volume in all lakes and rivers. In many parts of the world, wells and springs provide the water needs not only for great number of people, but for crops, livestock and industry as well. In addition, subsurface water is important as an equalizer of stream flow and as an agent of erosion. It is the work of subsurface water that creates Caverns (subterranean passage) and Sink Holes. (Strahler 214, GOH 42)

GEOLOGICAL WORK OF GROUNDWATER

The groundwater does flow or travel but its flow is so slow that it has no energy to erode the surface over which it flows. Therefore, the work of groundwater is of three types: (Gupta 313)

a. Solution (Dissolution of matter)
The rainwater, as it moves into the earth gets mixed up with such chemicals as CO2, Nitrogen and Sulphur. This water becomes powerful enough to act chemically on rocks through which it travels and in the course of time dissolves a large part of them (weathering) so that they become hollow and ultimately sink downwards.
This underground water is most active over chalk and limestone and therefore its effects are more marked in limestone regions.

b. Deposition
When a large proportion of minerals has been dissolved by the water, the capacity of water to dissolve any further comes to an end and it begins to aggrade the minerals that it has dissolved. The fact is that as the temperature becomes higher, the water is transformed into vapors leaving behind its deposits of dissolved minerals. Besides, if the temperature becomes low, solution power of water declines and aggradation begins. Sometimes the oxides – specially the CO2 – which make the water powerful enough to do the solution work begin to escape and the groundwater, rendered powerless in that way, has to deposit its load held in solution.

c. Petrification
The groundwater often replaces the original mineral substances by another. This process is known as Petrification and most commonly it is seen that carbonate of lime is replaced by silica or iron sulphide. In this way, the form, shape and content of any mineral is entirely changed. The petrification done by underground water is a very useful activity. It helps in gathering useful minerals scattered at various places. It dissolves those minerals and holds them in solution. Then it deposits them at one place and thus facilitates their exploitation later on.

EROSIONAL LANDFORMS ASSOCIATED WITH GROUNDWATER

The primary erosional work carried out by groundwater is that of dissolving rock. Since soluble rocks, specially LIMESTONE, underlies millions of square kilometers of the earth’s surface. It is here that groundwater carries on its rather unique and important role as an erosional agent. Although nearly insoluble in pure water, limestone is quite easily dissolved by water containing small quantities of Carbonic Acid. Most natural water contains this weak acid because rainwater readily dissolves CO2 from the air and from decaying plants. Therefore, when groundwater comes in contact with limestone, the carbonic acid reacts with calcite in the rocks to form calcium bicarbonate, a soluble material that is then carried away in solution.

Karst Topography

Many areas of the world have landscapes that to a large extent have been shaped by the dissolving power of groundwater. Such areas are said to exhibit Karst Topography. The term is derived from a plateau region located along the northeastern shore of the Adriatic Sea in the border areas between Yugoslavia and Italy where such topography is strikingly developed. In the United States, Karst landscapes occur in many areas, including portions of Kentucky, Tennessee, Alabama, Southern Indiana, and Northern Florida. Generally, arid and semi-arid areas do not develop Karst topography. When solution features exist in such regions, they are likely to be remnants of a time when more humid conditions prevailed.
The Karst Topography shows a series of stages in its development. (Strahler 482, GOH 76)
In the first stage of the Karst Topography only fretting and fluting of the surface takes place so that lapies, swallow/sink holes and dolines are formed. The surface drainage disappears through these to a place beneath the surface and a sort of subterranean reservoir is formed.
In the second stage, underground chambers, ponors and caverns are formed and enlarge swallow holes, dolines and sink holes collapse or are so breached that more and more water is added to the underground reservoir. The water table is reduced in height. Intermittent drainage of this stage makes the surface topography dry and barren. In late maturity, dolines and swallow holes coalesce to form uvalas, whose roofs are very thin. These become 50 to 60 miles wide.
The third and the final stage is marked by the collapse of the roofs of these subterranean caverns, formation of Poljes with residual hums standing here and there and revival of surface drainage. The water has done its job and it can percolate no further. With the collapse of the roofs of the caverns, it is ultimately exposed to view once more and along with it is exposed the underlying impervious strata.

Characteristic Features of Karst Topography

Lapies

As rain water mixed with CO2 proceeds over a region of limestone rock, it begins to dissolve certain portions of the rock. The system of joints is widened by solution and the surface is fretted (grooved) and fluted (hollowed). This is known as Lapies in French, Karrens in German and Bogaz in the Serbian language. (Gupta 313)

Sink Holes

Gradually, lapies are further widened to form sink holes and swallow holes, through which surface streams disappear, and begin to flow beneath the ground.
Sink holes commonly form in one of two ways. Some develop gradually over many years without any physical disturbances to the rock. In these situations, the limestone immediately below the soil is dissolved by downward-seeping rainwater, that is freshly charged with CO2. These depressions are usually gentle slopes. By contrast, sink holes can also form suddenly and without warning when the roof of a cavern collapses under its own weight. Typically the depressions created in this manner are steep-sided and deep. When they form in populous areas they may represent a serous geologic hazard.
In the limestone areas of Florida, Kentucky, and Southern Indiana, there are literally tens of thousands of these depressions varying in depth from just a meter or two to a maximum of more than 50 meters.

Dolines

If the dissolution is very intense, the sink holes may become very wide and such wide and deep funnel shaped sink holes are called Dolines. The typical doline is found in the Karst Region along the Adriatic Sea and its characteristic feature is a funnel-shaped top, the diameter being 30 to hundreds of feet. Sometimes it is deeper than broader. In barren regions their sides are steep and bottoms deep. On the other hand in regions with a vegetation cover, dolines are shallow and have rounded outlines.
Limestone Caverns
Among the most spectacular results of groundwater’s erosional handiwork is the creation of limestone caverns. Most are relatively small, yet some have spectacular dimensions. In the United States, Carlsbad Caverns in southeastern New Mexico and Mammoth Cave in Kentucky are famous examples.
Although caverns may develop in folded, faulted, and steeply dipping limestone layers, most caverns occur in areas of flat-lying strata.
In the first stage, the action of calcium carbonate is concentrated just below the water table. Products of solution are carried along in the ground-water flow paths to emerge in streams and leave the region in the stream flow. In a later stage, the stream has deepened its valley and the water table has been correspondingly lowered to a new position. The caverns system previously excavated is now in the zone of aeration. Evaporation of percolating water on exposed rock surfaces in the caverns now begins the deposition of carbonate matter, known as travertine. Encrustation of travertine take many beautiful forms – stalactites, stalagmites, columns, drip curtains, and terraces. (Strahler 481)

Natural Bridge

Beneath the surface of the earth the underground drainage is able to dissolve out very long and wide caverns. Sometimes roofs of such caverns are incaved, and rock bridges may be formed. These are known as natural bridges and are circular in outlines.

Uvalas

Due to the collapse of cavern roofs and coalescence (coming together) of dolines and swallow holes, often very vast depressions are formed which are known as Uvalas or Poljes. Uvalas follow the system of fissures in their outline. On their floors develop Blind Valleys with steep sides and terminating in a steep wall where the surface stream disappears.

Poljes

Poljes are basins closed from all sides by steep walls. In outline they resemble an ellipse but their floors are almost flat with independent drainage. These depressions result when a Karst land suffers from Block Faulting movement at a sufficiently late stage in the youth of its cycle. They may be called Grabens of Rift Valleys of the Karst land.

Natural Tunnels

When a considerable portion of the roof of the tunnel is retained over the valleys (originally cavern), it is called a Natural Tunnel.

Hums

In the midst of Uvalas and Poljes, residual masses of limestone rise here and there. These are known as Hums.

DEPOSITIONAL LANDFORMS ASSOCIATED WITH GROUNDWATER

Depositional work by the groundwater goes on in the caverns.

1. Drip Stones
The features that arouse the greatest curiosity for most cavern visitors are the stone formations that often exhibit quite bizarre patterns and give some caverns a wonderland appearances. These features are created by seemingly endless dripping of water over great spans of time. The calcite that is left behind produces the lime stone we call Travertine. These cave deposits, however, are also commonly called drip stones, and obvious reference to their mode of origin.
Although the formation of caverns takes place in the zone of saturation, the deposition of drip stone is not possible until the caverns are above the water table in the zone of aeration. This commonly occurs as nearby streams cut their valleys deep, lowering the water table as the elevation of the river drops. As soon as the chamber is filled with air, the conditions are right, the decoration phase of the cavern building to begin.

2. Stalactite
Of the various drip stone features found in caverns, perhaps the more familiar are stalactites. These icicle-like (ice cube like) pendants (hanging ornaments) hang from the ceiling of the cavern and from where water seeps through crater (valley) above. The water containing limestone drops from the roof of the cavern. As the water evaporates it leaves behind solidified calcium carbonate (CaCO3). The stalactites are sharp, slender, downward growing pinnacles (peaks) that hang from the cave roof. They look like a set of concentric rings of CaCO3 placed over one another with the broadest one at the roof and the narrowest being at the downward extremity. The stalactite is appropriately called a Soda Straw. (GOH 76)

3. Stalagmites
The drops of water that fall on the floor of the cavern after trickling down the roof still contain some lime which is left behind on the floor after the water has evaporated. Thus on the same lines as a stalactite, a pillar begins to rise upwards from the floor. This is known as Stalagmite. It is conical in shape and comparatively thicker. Because the drops, as they fall down the roof, are scattered away, the stalagmite is blunt and irregular in form but at its top it has generally a crater like depression.

Environmental Problems Associated with Groundwater
As with many of our valuable natural resources, ground water is being exploited at an ever increasing rate. Overuse threatens the ground water supply in some areas. In other places, ground water withdrawal has caused the ground and everything resting upon it to sink. Still other localities are concerned with the possible contamination of their ground water supply.

1. Sinking of Grounds

The ground may sink when water is pumped from wells faster than the natural recharge processes can replace it. This effect is particularly pronounced in areas underlain by thick layers of unconsolidated sediment. As water is withdrawn, water pressure dorps and the weight of the overburden is transferred to the sediment. The greater pressure packs the sediment grains tightly together and the ground subsidies.
Many areas may be used to illustrate land subsidence resulting from the excessive pumping of ground water from relatively loose sediment. A classic example in the United States occurred in the San Joaquin Valley of California [Atlas 14-B3]. Here, in a region of extensive irrigation, the water table beneath the valley has gradually been drawn down by as much as 30 meters. As a consequence, the land has subsided by 3 m in some places.

2. Pollution of Groundwater

The pollution of ground water is a serious matter, particularly in areas where aquifers supply a large part of the water supply.

a. Sewage

A very common source of ground water pollution is sewage, which results from an ever-increasing number of septic tanks, as well as inadequate or broken sewer systems, and barnyard wastes.
If water contaminated from bacteria enters the ground water system, it may become purified through natural processes. The harmful bacteria may be mechanically filtered out by the sediment through which the water percolates, destroyed by chemical oxidation and / or assimilated (absorbed) by other organisms. In order for purification to occur, however, the aquifer must be of the correct composition. For example, extremely permeable aquifers such as highly fractured crystalline rock, coarse gravel, or cavernous limestone have such large openings that contaminated ground water may travel long distance without being cleansed. In this case, the water flows too rapidly and is not in contact with the surrounding material long enough for purification to occur. On the other hand, when the aquifer is composed of sand or permeable sand stone, the water can sometimes be purified within distances as short as a few tens of meters. The openings between sand grains are large enough to permit water movement, yet the movement of the water is slow enough to allow ample time for purification.

b. Sanitary landfills and garbage dumps

Sanitary landfills and garbage dumps are another source of pollutants that may endanger the ground water supply of an area. As rain water oozes through the refuse, it may dissolve a variety of organic and inorganic materials, some of which may be harmful. If water containing material is leached from the landfill reaches the water table, it will mix with the ground water and contaminate the supply. Since ground water movement is slow, the polluted water may go undetected for a considerable time. When the problem is finally discovered, the volume of contaminated water may already be very large. Thus, even if the source of pollution is eliminated immediately (which is most unlikely), the problem could linger for many years until the contaminated water has migrated from the area of use.

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LANDFORMS MADE BY RIVER



HOW RIVERS ARE FORMED?

Raindrops may fall thousands of meters before they hit the ground. The impact of falling raindrops can move sediments. Large amounts of soil can be moved this way. Precipitation that has neither soaked into the ground nor evaporated may form runoff. Runoff is water that flows across the land. It runs downhill on even the gentlest slopes. It is called the consequent stream. As the water runs downhill, it cuts into the soil forming many tiny grooves called Rills (small streams). If erosion continues, the rills widen and deepen, becoming Gullies. Gullies act as channels for runoff. Gullies are actually tiny stream valleys. When gullies reach a low lying area, runoffs from several gullies merge together. A larger stream is formed as a result. The greater part of the water, which falls as rain, runs off the surface of the earth to form streams. These unite to make rivers, which in turn flow into the sea or lakes. (See GOH 47)

GEOLOGICAL WORK OF RIVERS

The geologic work of streams consists of three interrelated activities: Erosion, Transportation and Deposition. (GOH 48)
Erosion by a river is the progressive removal of mineral material from the floor and sides of the river, whether this be carved in bedrock or in residual or transported overburden.
Transportation of rock particles by rivers can take place in one of four ways. When carried in suspension, small particles remain in the water current without settling to the bottom. This is made possible by turbulence in the flowing water. Some what larger particles may be transported by saltation, a hopping or jumping motion in which the particle occasionally strikes the channel bottom. If larger still, a rock fragment is turned over and over along the channel floor, a process termed rolling. The forth mode of transportation is in chemical solution. Here the particles are individual units of matter of molecular size and are of course invisible. Generally speaking, fine clays and silts are carried in suspension, sands and gravels by saltation, and cobbles and boulders by rolling.
Deposition is the progressive accumulation of transported particles upon the stream bed and flood plain, or on the floor of the standing body of water into which the river empties.
Obviously, erosion cannot occur without some transportation taking place, and the transported particles must eventually come to rest. Therefore, Erosion, Transportation and Deposition are simply three phases of a single activity. Perhaps as much as 95% of all landforms are sculptured by rivers.

LANDFORMS ASSOCIATED WITH RIVER EROSION

WAYS IN WHICH RIVER ERODES

Rivers erode in various ways, depending upon the nature of the river materials, and the tools with which the current is armed.

a. Hydraulic Action
The force of the flowing water, alone, exerting impact and a dragging action upon the bed can erode poorly consolidated alluvial materials such as gravel, sand, silt and clay, a process termed Hydraulic Action. (GOH 50)

b. Abrasion (Corrasion)
Where rock particles carried by the swift current strike against the bedrock or river walls, chips of rock are detached. The rolling of cobbles (pebbles) and boulders over the river bed will further crush and grind smaller grains to produce an assortment (variety) of grain sizes. These processes of mechanical wear and combined under the general term Abrasion, which is the principal means of erosion in bedrock too strong to be affected by simple hydraulic action.

c. Corrosion
The chemical processes of rock weathering – acid reactions and solutions – are effective in removal of rock from the rivers and may be designated as corrosion. Effects of corrosion are mostly marked in limestone, which is a hard rack not easily carved by abrasion, but yielding readily to the action of carbonic acid in solution in the river water.

d. Attrition
This is the wear and tear of the transported materials themselves when they roll and collide with one another. The coarser boulders are broken into smaller stones; the angular edges are smoothed and rounded to form pebbles. The finer materials are carried further downstream to be deposited.

LANDFORMS

a. Pothole
One interesting form produced by river abrasion is the pothole, a cylindrical hole carved into the hard bedrock of a swiftly moving river. Potholes range in diameters from a few inches to several feet; the larger ones may be many feet deep.

b. Waterfall
A waterfall is formed when a river flowed first over hard bedrock and then over soft bedrock. Abrasion wore away the soft bedrock more quickly than the hard bedrock. In time, the level of the river flowing over the soft bedrock becomes lower than the level of river flowing over the hard bedrock. Thus a waterfall is formed.

c. Canyon
A deepening Gorge or Canyon is perhaps the most striking landforms associated with erosion. The gorge is steep-walled and has a v-shaped cross section. The river occupies all the bottom of the gorge. Form the steep walls much weathered rock material is shed into the river. Landslides occur frequently, large fallen masses sometimes temporarily damage the stream. A young gorge may afford the only passage through a mountain range. The Royal Gorge of the Arkansas River, in the Rocky Mountain Front Range of South Colorado, is a striking example.

d. Rapids
A river in its youth stage, flows down a steep slope along a fairly straight path. As a result, the water in the river flows swiftly, often forming rapids. (GOH 51)

e. V-shaped Valley
The fast moving water erodes the land quickly. It can carry much sediments and cut deeply into the bedrock. Such erosion produces steep-sided v-shaped valleys. With the passage of time, as erosion continues, the valley’s v-shape become wider and less steep. The valley floor becomes flat.

LANDFORMS ASSOCIATED WITH RIVER DEPOSITS

Streams deposit sediments when they slow down or decrease in volume. The slower the water is moving, the less carrying power it has. The smaller the volume of the river the less water there is to carry sediment. Many landforms develop a result of stream deposition. These landforms are found in almost all parts of the world.

a. Ox-bow Lakes
As a river develops, meanders become more and more curved. In time, the river cuts through the land between the ends of a meander. Flowing through this shortcut, or cutoff, the river picks up speed. Slower water in the meanders deposits sediments near the cutoff. The sediments seal off the meander. The cut off meander dry up or become an Ox-bow Lake. (Strahler 424, GOH 53)

b. Natural Levees
Rivers that occupy valleys with broad, flat valley floors on occasion build a landform called Natural Levee that parallels its channel. Natural levees are built by successive floods over many years. When a river over flows its banks, its velocity immediately diminishes, leaving coarse sediments deposited in strips bordering the channel. As the water spreads out over the valley, a lesser amount of fine sediments is deposited over the valley floor. This uneven distribution of material produces the very gentle slope of the natural levee. The natural levees of the lower Mississippi rise 6 meters above the valley floor.
The area behind the levee is characteristically poorly drained for the obvious reason that water cannot flow up the levee and into the river. Marshes called Back Swamps result. A tributary stream that attempts to enter a river with natural levees often has to flow parallel to the main stream until it can breach the levee. Such streams are called Yazoo Tributaries after the Yazoo River, which parallels the Mississippi for over 300 kilometers. (Strahler 425)

c. Flood Plains
Rivers in their lower course carry large quantities of sediments. During annual or sporadic(occasional) floods, these materials are spread over the low lying adjacent areas. A layer of sediment is deposited during each flood, gradually building up a fertile floodplain. Flood plains make good farmland, the flood deposits enrich the soil.
The depositing streams generally have either meandering or braided (tangled) courses, depending on the kind of sediment in transit. Both types construct flood plains, and there are similarities and differences in the surface forms associated with each. The flood plains of meandering rivers are more suitable for agriculture use than those of braided streams because the alluvium has a finer texture and hence retains more moisture and plant nutrients.
d. Alluvial Terraces
These are flat benches of alluvium situated on flood plains. They mark former levels of deposition below which the rivers have since cut their present graded plain. Such terraces may result from lowering of base level or from a sudden decrease in load.

e. Deltas
A delta is a landform made from deposits at the mouth of a river. A river slows down and stops at its mouth. The mouth of a river is the point where it flows into an ocean or other large body of water. By the time it reaches the mouth, a river has already deposited much of its load of sand, silt and clay. Such sediments build the Delta.
Delta plains are generally favorable areas for agricultural land use because of their fine textured alluvial soils, but flood danger, poor drainage, and salinity in swells, and depressions can be local problems. The delta plains in Eastern and Southeastern Asia have some of the densest rural populations in the world, based primarily on the cultivation of rice. Other intensively cultivated delta plains include those of the Mississippi, Nile, Po, Rhine, Tigris – Euphrates and Colorado Rivers. The broad delta of Amazon is unique in that it is largely unused and supports only a sparse pastoral (rural) population.
Some coastal plains are produced through the joining of adjacent deltas. The plains along the Eastern side of Sumatra, the Northern part of Java, the Coast of the Gulf of Guinea, in Western Africa, and parts of the Guiana Coast of Northern South America [Atlas 18-D2] are of this type. Many alluvial plains that border the ocean in such Tropical areas as these are almost always bordered by dense tangles of mangroves, and swamp palms that hinder the reclamation of the seaward margins of deltas. (Strahler 429)

f. Alluvial Fan

An alluvial fan is a landform made of deposits from a river flowing onto a level land. A fast mountain river slows down when it reaches flatter land. At this point, much of the stream’s load settles to bottom of the river. The settled load can collect into a landform called Alluvial Fan. Usually, the coarse material is dropped near the base of the slope, while finer material is carried farther out on the plain. An alluvial fan is much like a delta that built on land instead of in a large body of water.

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WINDS


INTRODUCTION

As in humid regions, running water and in colder latitudes moving ice are the chief agents of denudation, similarly in arid regions - in the hot and temperate deserts of the world – wind is the chief agent of denudation. The chief factor that limit the action of the wind or which define the limits of the region where wind action is specially powerful are scarcity of rainfall and absence of vegetation cover. Besides these, excessive insolation and evaporation are the other controlling factors. In the absence of roots that bind the loose earth particles and the humidity of the soil that keeps it from being blown off, the wind attains special force.

WORK OF THE WIND

The wind carries with it thousands of dust particles. These dust particles collide with each other and become smaller. With the help of these sand particles the wind erodes the earth’s surface but it can do so when its velocity is very high. As soon as the speed of wind declines, it feels difficulty in carrying these particles. Consequently they are deposited in the form of hillocks (heaps).

WIND EROSION

Moving air, like moving water is capable of picking up loose debris and moving it to another location. Although wind erosion is not restricted to arid and semi-arid regions, it does its most effective work in these areas. In humid regions, moisture binds particles together and vegetation anchors the soil so that wind erosion is negligible. For wind to be effective, dryness and scanty vegetation are important prerequisites. When such circumstances exist, wind may pick up, transport and deposit great quantities of fine sediment.

Difference b/w Wind Erosion & Stream Erosion
Wind erosion differs from stream erosion in two significant ways. First, wind has a low density compared to water; thus it is not capable of picking up and transporting coarse materials. Second, because wind is not confined to channels, it can spread over large areas, as well as high into the atmosphere.

Ways in which erosion takes place

1. Deflation
This involves the lifting and blowing away of loose materials from the ground. Such unconsolidated sand and pebbles may be carried in the air or rolled along the ground depending on the grain size. The finer dust and sand may be removed kilometers away from their place of origin, and be deposited even outside the desert margins. Deflation results in the lowering of the land surface to form large depressions called deflation hollows. The Qattara Depression of the Sahara Desert lies almost 135 m below sea level. (GOH 69)

2. Abrasion
The sandblasting of rock surfaces by winds, when they hurl sand particles against them, is called abrasion. The impact of such blasting results in rock surfaces being scratched, polished and worn away. Abrasion is most effective at or near the base of rocks, where the amount of material, the wind is able to carry, is greatest. This explains why telegraph poles in the deserts are protected by a covering of metal, a meter or two above the ground. A great variety of desert features are produced by abrasion.

3. Attrition
When wind-borne particles role against one another in collision, they wear each other away so that their sizes are greatly reduced and grains are rounded into millet seed sand. This process is called attrition.

Erosional Landforms


1. Blowouts or Deflation Hollows
The most noticeable result of the deflation in some places are shallow depressions called blowouts or Deflation Hollows. In some places, layers and layers of loose dry sediments are removed by deflation. The result is a blowout. In the Great Plains Region, from Texas north to Montana, thousands of blowouts can be seen. They range in size from small dimples (depressions) less than 1 m deep and 3 m wide to depressions that are over 45 meters deep and several kilometers across. In wet years, they may be covered by grasses, some may even fill with runoff, forming shallow lakes. In dry years, however, the grasses die and the lakes dry up. Then the wind continues to deflate the bare soils. (Strahler 567, GOH 69)
Similarly, minor faulting can also initiate depression and the eddying action of oncoming winds will wear off the weaker rocks until the water table is reached. Water then seeps out forming oases or swamps, in the deflection hollows or depressions. The Faiyum Depression in Egypt lies 40 m below sea level. Large areas in western USA, stripped-off their natural vegetation for farming were completely deflated when strong winds moved materials as dust storms, laying waste crops and creating what is now known as the Great Dust Bowl.

2. Desert Pavement
In portions of many deserts the surface is characterized by a layer of coarse pebbles and gravel. Such a layer, called Desert Pavement, is created as the wind removes fine material, leaving the coarse particles behind. Once desert pavement becomes established, a process which may take hundred of years, the surface is effectively protected from further deflation. In North Africa such a pebble-covered surface is called reg. The desert pavement of quartzite fragments was formed by action of both wind and water on the surface of an alluvial fan in the desert of southeastern California.

3. Mushroom Topography
The sandblasting effects of winds against any projecting rock masses wears back the softer layers so that an irregular edge is formed on the alternate bands of hard and soft rocks. Grooves and hollows are cut in the rock surfaces, carving them into fantastic and grotesque-looking pillars called Rock Pedestals. Such rock pillars will be further eroded, due to abrasion, near their bases where the friction is greatest. This process of undercutting produces rock of mushroom shape called mushroom rock or gour as in the Sahara. (GOH 70)

4. Zeugen
These are tabular masses, which have a layer of soft rocks lying beneath a surface layer of more resistant rocks. The sculpting effects of wind abrasion wear them into a weird-looking “ridge and furrow” landscape. Mechanical weathering initiates their formation by opening up joints of the surface rocks. Wind abrasion further eats into the underlying softer layer so that deep furrows are developed. The hard rocks then stand above the furrows as ridges or zeugen, and many even overhang. Such tabular blocks of zeugen may stand 3 to 30 meters above the sunken furrows. Continuous abrasion by wind gradually lowers the zeugen and widens the furrows. (GOH 70)
Sometimes the pedestals of softer rocks or pillars are completely removed by undercutting so that the whole mass is overturned or turned turtle. This feature is also known as Zeugen. (Gupta 347)

5. Yardangs
Quite similar to the “ridge and furrow” landscape of zeugen are the steep-sided yardangs. Instead of lying in horizontal strata upon one another, the hard and soft rocks of yardangs are vertical bands and are aligned in the direction of prevailing winds. Wind abrasion excavates the bands of softer rock into long, narrow corridors, separating the steep-sided over hanging ridges of hard rock, called yardangs. They are commonly found in the Atacama Desert, Chile, but the more spectacular ones with yardangs rising 8 to 15 m are best developed in the interior deserts of Central Asia where the name originated.

6. Inselberg
This is a German word meaning “island-mountain”. They are isolated residual hills rising abruptly from the level ground. They are characterized by their very steep slopes and rather rounded tops. They are often composed of granite or gneiss, and are probably relics of an original plateau which has been almost entirely eroded away. Inselbergs are typical of many desert and semi-arid landscapes in old age, e.g., those of northern Nigeria, western Australia, and the Kalahari Desert in Botswana. Sometimes these hills are pyramid like with a cap of hard rock and sometimes they are dome shaped. These represent an island like emergence from a rock plain. (GOH 71)

7. Dreikanter or Ventifacts
These are pebbles faceted by sand blasting. They are shaped and thoroughly polished by wind abrasion to shapes resembling Brazil nuts. Rock fragments, mechanically weathered from mountains and upstanding rocks, are moved by wind and smoothed on the windward side. If wind direction changes, another facet is developed. Such rocks have characteristic flat facets with sharp edges. Among the ventifacts, those with three wind-faceted surfaces are called dreikanter. These wind faceted pebbles form the desert pavement, a smooth, mosaic region, closely covered by the numerous rock fragments and pebbles.

8. Hamada
In desert areas the exposed bedrock surfaces become the sides of the attack by sandblast and as the matter is moved to and fro, the less resistant parts gradually reduce to sand and conveyed beyond the limits of the region. Ultimately what remains behind is the rock pavement made of resistant rock stuff over which are spread dreikanter-shaped pebbles. Such a rock flow represents the base level of erosion in deserts – the last stage of works in the cycle of dessert erosion. This flat, bare rock floor is called Hamada. The best known rocky deserts are those of the Sahara Desert, e.g., the Hamada el Homra, in Libya, which covers an area of almost 52,000 km2.

9. Earth Pillars
It is seldom that wind action goes all alone. The water does intervene to modify the results of wind action. The desert areas of the world have occasional cloud bursts which produce sheet floods near the isolated mountainous borders and swell the wadis. These temporary masses of moving water are quite ephemeral (brief, temporary) in their existence and they carry a load which is much more than their capacity to carry. The result is that these under-fit streams are suffocated with the material they carry and produce very characteristic profiles of deposition. But due to its terrific nature, the rainwater helps in erosion and produce earth pillars each having at its cap some pebbles. These are also known as Hoodos or Demoiselles (like female child). These earth pillars are round or many faceted, smooth or rough accordingly as the wind has attacked them from one side or from all the sides steadily. (Gupta 349)

10. Castellated Chimneys
The rain water tends to sharpen the yardangs so that they begin to look like pointed needles or a series of chimneys known as Castellated Chimneys (built in the style of castle with battlements).

Wind Deposition
Although wind is relatively unimportant as a producer of erosional landforms, wind deposits are significant features in some regions. Accumulation of wind blown sediments are particularly conspicuous landscape elements in the world’s dry lands and along many sandy coasts.

Types of Wind Deposits
Wind deposits are of two types:

1. Loess
The fine dust carried to the borders of the desert or beyond and deposited on neighboring lands is called loess. It is yellow, friable (flaky) material and usually very fertile. The name loess comes from a village in Alsace (France) where such deposits occurred. Loess is infact, fine loam, rich in lime, very coherent and extremely porous. It is very pervious to water and thus water streams cut deep valleys. In this way Badland Topography may develop.
The thickest and most extensive loess deposits occur in western and northern China, where accumulations of 30 m are not uncommon and thickness of more than 100 m have been measured. It is this fine, buff colored sediment, which comes from Gobi Desert (Magnolia), and gives the Yellow River (Huang-He) and the adjacent Yellow Sea their names. The source of China’s 800,000 km2 of loess are the extensive desert basins of Central Asia.
In the United States, deposits of loess are significant in many areas, including South Dakota, Nebraska, Iowa, Missouri, and Illinois as well as portions of the Columbia plateau in the Pacific Northwest.
In Germany, France, Belgium, it is called Limon. In USA, loess is derived from ice-sheet called Adobe.

2. Sand Dunes
Like running water, wind releases its load of sediments when its velocity falls and the energy available for transport diminishes. Thus sand begins to accumulate where ever an obstruction across the path of the wind slows the movement of the air. Unlike deposits of loess, which from blanket like layers over large areas, winds commonly deposit sand in mounds or ridges called Dunes. Dunes may be live or fixed. (Strahler 569)

Process of Formation


As moving air encounters an object, such as a dump of vegetation or a rock, the wind sweeps around and over it leaving a shadow of more slowly moving air behind the obstacle as well as a smaller zone of quieter air just infront of the obstacle. Some of the salting sand grains moving with the wind come to rest in these wind shadows. As the accumulation of sand continues, increasing efficient wind barrier forms to trap even more sand. If there is a sufficient supply of sand and the wind blows steadily long enough, the mound of sand grows into a dune.
Although often complex, dunes are not just random heaps of sediments, rather they are accumulation that usually assumes surprisingly consistent patterns.

a. Barchan or Crescent Dunes
Solitary sand dunes shaped like crescents and with their tips pointing downwards are called Barchan Dunes. They are also spelled barcan, barkhan or barchane. These dunes form where supplies of sand are limited and the surface is relatively flat, hard and lacking vegetation. They migrate slowly with the wind at a rate of upto 15 meters/year. Their size is usually modest with the largest barchans reaching a height of about 30 m while the maximum spread between their horns approaches 300 m. When the wind direction is nearly constant, the crescent form of these dunes is nearly symmetrical. However, when the wind direction is not perfectly fixed, one tip becomes longer than the other. (GOH 72, Strahler 569)

b. Transverse Dunes
In regions where vegetation is sparse or absent and sand is very plentiful, the dunes form a series of long ridges that are separated by troughs and oriented at right angles to the prevailing wind. Because of this orientation, they are termed Transverse Dunes. Typically, many coastal dunes are of this type. In addition, they are common in arid regions where the extensive surface of wavy sand is sometimes called Sand Sea.

c. Longitudinal Dunes
These are long ridges of sand that generally form parallel to the prevailing wind and where sand supplies are limited. Apparently the prevalent wind direction must vary somewhat, but not by more than about 90o. Although the smaller types are only 3 or 4 meters high and several tens of meters long, in some large deserts, longitudinal dunes can reach great size. For example, in portions of North Africa, Arabia and Central Australia, these dunes may approach a height of 100 m and extend far distances of more than 100 m. (GOH 73, Strahler 573)

d. Parabolic Dunes
Unlike the dunes that have been described thus far, parabolic dunes form where vegetation partly covers the sand. The shape of these dunes resembles the shape of barchans except that their tip points into the wind rather than downwind. Parabolic dunes often form along coasts where there are strong onshore winds and abundant sand. If the sand’s vegetative cover is disturbed at some spot, deflation creates a blowout. Sand is than transported out of the depression and deposited as a curved rim which grows higher as deflation enlarges the blowout. (Strahler 571)

Last edited by Aarwaa; Tuesday, November 13, 2007 at 08:05 PM.
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