Stratigraphy

Fossils can be dated relative to one another by noting their positions in layers of rocks, known as strata. As shown in the animation (right), fossils found in lower strata were generally deposited earlier and are older.

Sometimes geologic processes interrupt this straightforward, vertical pattern (left). For example, a mass of rock may cut across other strata, erosion may interrupt the regular pattern of deposition, or the rock layers may even be bent and turned upside-down.

In the example at left, we can deduce that the oldest rocks are those that are cut through by other rocks. The next oldest rocks are those that are "doing the cutting" through the oldest rocks, and the youngest rocks lie on top of these layers and are not cut through at all. By making careful observations, we can detect these interruptions in the vertical pattern and use them to get more information about the relative ages of different layers.

By studying and comparing strata from all over the world, we can date rocks relative to one another. Using numerical dating techniques, such as those based on the radioactive decay of atoms, we can assign probable ages to these layers and the fossils they contain.

Venericardia planicosta, an Eocene bivalve

Certain fossils, referred to as index fossils, can be helpful as well. If an organism existed for a relatively short period of time and had a wide geographic distribution, then it can provide an index as to the age of the rocks in which it is preserved. For instance, Venericardia planicosta is known to have lived only during the Eocene, thus every time we find Venericardia planicosta, we can assume that the rocks containing this fossil must have been formed during the Eocene.