| Home > | Evolution 101 | |
|
|
|
|
|
|||||
|
|
|||||
|
|||||
StratigraphyFossils 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.
|
)  |
|||||||||||||||||
|
|
 Important Events in the History of Life |
|||||||||||||||||
|
Search · Site Index · Navigation · Copyright · Credits · Contact Understanding Evolution For Teachers Home · Understanding Evolution Home Read how others have recognized the Understanding Evolution website Spanish translation of Understanding Evolution For Teachers from the Spanish Society of Evolutionary Biology. |
)
