As soon as the Rocky Mountains crumpled skyward, 70 million years ago (MYA), erosional debris began to fill the valleys and basins between the ranges. This process continued throughout most of the Tertiary Period and, by the Miocene (20 MYA), the terrain across this mountain corridor was relatively flat. A second uplift of the region, from the late Miocene into the Pliocene, followed by the wet climate of the Pleistocene, spawned river systems which uncovered the ranges and scoured out the intervening valleys and basins.
One of these streams was the Wind River of central Wyoming which rose along the east flank of the Wind River range, flowed to the southeast and then turned northward, eventually entering the Yellowstone River in southeast Montana. En route, this river crossed two buried ridges, the Owl Creek Mountains of central Wyoming and the Pryor Mountains along the Wyoming-Montana line; with its course set in the overlying Tertiary sediments, the Wind River carved spectacular canyons through these walls of rock, now known, respectively, as Wind River Canyon and Bighorn Canyon. Since the strata of these east-west ridges were tilted during uplift and faulting, the river exposed rock layers stretching from the Precambrian to the Mesozoic; in Wind River Canyon, the oldest layer, Precambrian granite dating back almost 3 billion years, towers along the canyon's southern mouth while Triassic redbeds, deposited 225 MYA, adorn the canyon at its north entrance.
Today, the original Wind River crosses two major topographic basins (the Wind River and Bighorn Basins), separated by the Owl Creek Mountains; north of Wind River Canyon, the river is now known as the Bighorn River. The Wind River Basin is bordered by the Wind River Range on its west, the Owl Creek Mountains on its north, the southern end of the Bighorn Range on its east and a low divide along its southern edge, separating the Wind River watershed from that of the Sweetwater River. After leaving Boysen Reservoir and flowing northward through Wind River Canyon in the Owl Creek Mountains, the Wind River enters Bighorn Basin and becomes the Bighorn River; the Bighorn Basin is bordered by the Absaroka Range on its west (composed of Eocene volcanic rocks), the Owl Creek Mountains on its south and the Bighorn Range on its east. At the north end of the basin the Bighorn River cuts through the Pryor Mountains to form Bighorn Canyon (described above) and then enters the Yellowstone River, a major tributary of the Missouri.
