A state historical marker on U.S. 20-26 about five miles west of Glenrock, Wyo. notes:
Big Muddy oil field is a typical Wyoming oil producing structure. The field, discovered in 1916, has produced over 30 million barrels of high quality oil. Strata here were arched upward at the time the Rocky Mountains originated about 60 million years ago, to form anticline, or dome. Because oil is lighter than water, it rose to the crest of the dome where it was trapped in pore spaces between sand grains. The Wall Creek sand lies at a depth of near 3,000 feet and the Dakota sand at about 4,000 feet. The first oil well in Wyoming was drilled in 1884. There are now about 100 oil fields in the state.