Segment from A Science Odyssey: "Origins."Geologists have calculated the age of Earth at 4.6 billion years.But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.
Before then, the Bible had provided the only estimate for the age of the world: about 6,000 years, with Genesis as the history book.Sediments were deposited in ancient seas in horizontal, or flat, layers.If sedimentary rock layers are tilted, they must have moved after they were deposited.By measuring the ratio of lead to uranium in a rock sample, its age can be determined.Using this technique, called radiometric dating, scientists are able to "see" back in time.Erosion may have worn away some of the rock, but layers on either side of eroded areas will still “match up.” Look at the Grand Canyon in Figure below. You can clearly see the same rock layers on opposite sides of the canyon.