Most fossils are formed when sediment is deposited by wind or water. These sedimentary fossils usually develop only from hard body parts. The soft body parts decompose before they can be replaced by hard minerals. The special conditions needed for the formation of fossils cause the fossil record to be less than complete. Because of this, the fossil record is open to interpretation. Different scientists can look at the same fossil and reach different conclusions about its place in the history of life.

The laws of stratigraphy are some of the best tools we have for understanding rock and sediment layers. These laws help with relative dating which cannot determine the actual age of these layers, only the relative order in which the layers formed. In order to understand these laws, we must assume that the geologic processes of today were the same in the past. This is called uniformitarianism.

The first law of stratigraphy is the law of superposition, which states that the oldest layer is at the bottom of the sequence, and the youngest at the top.

The second law is the law of original horizontality. This law states that sediment or rock layers were deposited in a horizontal orientation. If units are not horizontal, an event occurred subsequent to the deposition which caused the layers to fold or tilt.

The third law is the principle of lateral continuity. This states that deposits originally extended in all directions.

Note: topography controls the angle at which sediments are deposited locally. For example, sands deposited on a sand dune will result in angled layers. Original horizontality generally remains true when observing layers over broader regions.