THE TYPE AREA FOR THE BRIDGER FORMATION, UINTA COUNTY, WYOMING

The type area of the middle Eocene Bridger Formation is along a series of rims flanked by badlands east of Fort Bridger, Unita County, Wyoming. The area extends for 45 km from Church Butte along the Oregon Trail at the north, to Sage Creek Mountain on the south. The Bridger “Group” was named by F.V. Hayden in 1869 for exposures in the area that he could access by the Union Pacific Railroad. Hayden had first examined the Church Butte area in 1868. Hayden returned to the area in September of 1870 with his geological survey, travelling to Fort Bridger along the Oregon Trail. His survey members spent a day and a half in the Church Butte area collecting fossils and W.H. Jackson making a series of photographs of the area on September 10 and 11, 1870. O.C. Marsh with a party of Yale students travelled to Fort Bridger in August of 1870 and collected fossil in the badlands east and southeast of the fort, moving up Sage Creek to the Henrys Fork River. Joseph Leidy and E.D. Cope also visited the area and collected fossils in 1872. Leidy worked primarily in the exposures due east and south of the Fort, in an area then called the “Grizzly Buttes.” This name is not used on modern maps, but was a well known geographic feature to early vertebrate paleontologists.

In August 1902, Walter Granger and W.D. Matthew of the American Museum of Natural History started a five-year project studying the paleontology and geology of the Bridger Formation. Matthew (1909) split the Bridger into five subdivisions, which he labeled Bridger A-E that were bounded by widespread limestones that he called “white layers.” The type area includes the sequence from Matthew’s Bridger B through E units. Matthew’s Bridger A & B units were combined into the Blacks Fork Member and the Bridger C & D units were combined into the Twin Buttes Member by Wood in 1934. The Bridger E was named the Cedar Mountain Member by West and Hutchinson in 1981, but since this name is used for Cretaceous rocks in Utah, the member was renamed the Turtle Bluffs Member by Evanoff and others in 1998.

The Bridger Formation in the type area is 550 m thick and is mainly composed of a sequence of interbedded mudstone and sandstone beds. Thin but widespread volcanic tuffs and limestones also occur in the sequence that act as widespread marker beds. Twenty three marker beds occur in the Bridger Formation in the type area.