Figure 5.—Geologic time chart and the progression of life forms. Note Cretaceous Triceratops, Oligocene Titanotheres, and Miocene Moropus.

[[This map in a higher resolution]]

GEOLOGIC TIME
The Age of the Earth

The Earth is very old—4.5 billion years or more according to recent estimates. Most of the evidence for an ancient Earth is contained in the rocks that form the Earth’s crust. The rock layers themselves—like pages in a long and complicated history—record the surface-shaping events of the past, and buried within them are traces of life—the plants and animals that evolved from organic structures that existed perhaps 3 billion years ago.

Also contained in rocks once molten are radioactive elements whose isotopes provide Earth scientists with an atomic clock. Within these rocks, “parent” isotopes decay at a predictable rate to form “daughter” isotopes. By determining the relative amounts of parent and daughter isotopes, the age of these rocks can be calculated.

Thus, the results of studies of rock layers (stratigraphy), and of fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), attest to a very old Earth!

Figure 6.—Generalized paleogeographic map of the United States in Late Cretaceous time (65 to 80 million years ago), when most of the Great Plains was beneath the sea.

Figure 7.—Structural setting of the Great Plains. Williston basin and Anadarko basin are separated by a midcontinental arch.

Table 1.—Generalized chart of rocks of the Great Plains