Boysen Dam, completed in 1951, marks the southern margin of the east-west trending Owl Creek Mountains and Wind River Canyon. Drained by the north-flowing Wind River, the canyon is 14 miles long, 1.3 to 2 miles wide, and 2,400 feet deep where it crosses the axis of the mountain range north of here. Its steep walls expose Precambrian and Paleozoic rocks ranging in age from several billion to 235 million years. The Wind River originally flowed across flat terrain above the buried Owl Creek Mountains, which formed 50 million years ago as an upward fold in the earth's crust and were later covered by younger sediment. About 4 million years ago, a broad regional uplift caused the river's channel to cut into the rising crust, creating a canyon that crosses the mountains at a right angle, an unusual pattern since rivers usually flow around or parallel to mountains. Erosion then removed the sediment that had covered the Owl Creek Mountains, shaping the landscape that remains today, and this process continues. Parts of the canyon highway follow an ancient trail used by Native Americans, early fur trappers, traders, and U.S. military expeditions. Native Americans, especially the Shoshone, traveled this route to reach the hot springs in present-day Thermopolis. In 1825, William Ashley, called the "general" of the fur trade, floated a valuable cargo of pelts through the canyon on his way to St. Louis. In the late 1800s, the canyon was also a well-known escape route for Native American warriors, and it remains sacred to contemporary Native American tribes in the region.