The infinite variety: the beginning of life
Evidence of Evolution in the Rocks
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- Layers of Rocks give us clues to their age
| Evidence of Evolution in the Rocks
Occasionally some animals after dying may be covered in mud, where their bones can be preserved. Dead plant material may also accumulate and is turned to peat, in time peat is compressed and turned to coal. Great pressure from overlying sediments and mineral-rich solutions that circulate through them cause chemical changes in the calcium phosphate of the bones. Eventually they are turned to stone giving an accurate representation of the original bones. This process is called fossilization. 
A fossil Ammonite The most suitable sites for fossilization are in seas and lakes were sedimentary deposits like sandstone and limestone are slowly accumulated. Fossils are exposed when such deposits erode away. Fossils can often be dated with the discovery of radioactivity in the surrounding rocks. Some chemicals in rocks decay with time producing radioactivity, for example potassium turns to argon, uranium to lead and rubidium to strontium. The amount of change from one chemical to the other depends on the amount of elapsed time. Consequently the proportion of the second element to the first can be used to calculate the time when the rocks were first laid down around the fossil. Layers of Rocks give us clues to their age When rocks occur as undisturbed layers, we find that the lowest layers will be the oldest and topmost layers will be the youngest. Frequently rivers cut incisions into the earths's surface and expose such layers. The Grand Canyon in the U.S.A. is the deepest cleft on the earth's surface. 
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