Geology is a vast of subject to study for CSS Exam as it is a professional four year degree it covers a lot as much branches it have ...it has engineering geology and general geology papers...if you are not a geologist i will suggest you not to opt it as i am personally a geologist and preparing it i have done BS in geology still it takes a lot of time....if you are interested still so first you need to study a book PHYSICAL GEOLOGY By Charles .c .plummer. it is basic book for geology and you can cover a lot with in it..follow past papers so you should have an idea about it....