The Wind River Basin, in western Wyoming, preserves mostly Eocene rocks, which are subdivided into the Indian Meadows, Wind River, Aycross, Tepee Trail, and Wiggins formations. In many places the Indian Meadows Formation and Wind River Formation overlay an angular unconformity of Mesozoic and Paleozoic rocks. Coarse sandstones and conglomerates dominate these two lower formations. Fossils from the Indian Meadows Formation indicate an early Eocene age, roughly 54-53 mya (Winterfeld & Conard, 1983). Below these formations, the middle to late Paleocene Waltman Shale and Shotgun members of Fort Union Formation represent marine and low energy terrestrial environments. Fossil shark teeth are common in both these formations indicating an early Cenozoic remnant of the Western Interior Seaway in Wyoming (Keefer, 1961).