The rock layers in the bluff across the river are made of sediments - particles of sand, silt, and clay. These layers, called strata, were carried here by the ancient Snake River and were deposited as the river entered an ancient lake. This process usually occurred slowly, taking many years to build up just a few inches. During floods, however, strata could form rapidly, often trapping plants and animals within the sediments.
Fossils - traces or remnants of organisms from the geologic past - are buried in the layers. Bones become fossils when their organic cells are slowly filled or replaced with minerals. Fossils found here include mammals, birds, fish, reptiles, amphibians and plants that scientists have dated from three to four million years old.