Fossil Lake, Lake Gosiute, and Lake Uinta formed a sub-tropical lake ecosystem commonly referred to today as the "Green River Lake System." The lakes were located in what now are the states of Wyoming, Utah, and Colorado. The original Fossil Lake was 40-50 miles long (north/south) and 20 miles wide (east/west). During its approximately two million-year life, its length and width varied considerably.

Today, the national monument protects small portions of the original Fossil Lake and the larger Green River Lake ecosystem. The park consists of 13 square miles (8,198-acres) of the 900-square-mile (595,200 acre) ancient Fossil Lake. One of the park's geological formations is formed from the ancient lake sediments. Scientists refer to these lake sediments, now rocks, as the Green River Formation. These rocks preserve a tremendous variety of fossils. In addition to the Green River Formation, the colorful Wasatch Formation, composed of river and stream sediments, is exposed in the national monument. The Wasatch Formation contains fossilized teeth and bone fragments of many Eocene mammal species, including early primates and horses. These fossils tell us about animals living near Fossil Lake, thereby adding the terrestrial component to Fossil Lake's story.