When rocks are deposited one on top of the other, stratigraphy, as the study of rock layers and their relative ages is known, is like reading a book backwards. The oldest rocks, deposited first, are at the bottom. The newest rocks, deposited last, are at the top. But when geological forces - such as folding, faulting and erosion - occur they can mix up the layers and make it difficult for geologists to know which rocks were deposited first. To help sort out the confusion, geologists study the fossils found in each layer of rock. Fossils help geologists determine the rock's age by telling them what was happening on earth when the rocks were deposited.