While the field of plate tectonics has solved many geologic mysteries, others remain. One of these is Unaweep Canyon in the northern portion of the Uncompahgre Plateau, in western Colorado; unlike other canyons, it has two mouths and a divide along its course, which is now traversed by Colorado Route 141.
Geologists have long debated the natural history of Unaweep Canyon. Some have suggested that, since the canyon is U-shaped, it was carved by mountain glaciers during the Pleistocene; however, closer examination of its floor has revealed that it is a V-shaped, stream-cut canyon, since filled with erosional debris to produce the flattened bed. Others, realizing that the small streams that now flow east and west from the divide could have not produced this magnificent canyon, with its massive walls of hard, Precambrian rock, theorized that the Colorado River originally sculpted Unaweep before changing course to skirt the northern edge of the Plateau.
The most recent evidence suggests that the Gunnison River, which also carved the Black Canyon, once cut through the Uncompahgre Plateau, joining the Dolores River on its west side before flowing northwestward into the Colorado. Based on examination of gravels and other sediments, it appears that the Gunnison gradually sliced through the Uncompahgre ridge as it rose with the rest of the Colorado Plateau during the Miocene-Pliocene Uplift, 25-10 million years ago. A landslide is thought to have disrupted the Gunnison's flow within the Canyon (presumably about 6 million years ago), causing it to back up and eventually flow northward to join the Colorado in the Grand Valley. The abandoned Canyon is now drained by East and West Creeks, too weak to remove the erosional debris of the powerful Gunnison. Time and more field work will tell if this is the true solution to the Unaweep Canyon mystery.