In 1847, an estimated 50,000 emigrants toiled westward up the valley of the Sweetwater River after fording the North Platte near Casper, passing through Emigrant Gap, and traveling over the high rims beyond. Famous Oregon Trail landmarks lie along the Sweetwater River—Independence Rock, Devils Gate, Split Rock, Sweetwater Crossing, and others on the way to the Continental Divide at South Pass. A number of these landmarks consist of massive pink granite shot through with black diabase dikes. The rocks crop out in bald, stark knobs that stand above light-colored, flat-lying sedimentary rocks. The landscape is reminiscent of central Africa and parts of Australia, but it is actually the Granite Mountains of central Wyoming. This area separates the Wind River Basin from the Great Divide and Hanna basins, and in a general sense connects the southern end of the Wind River Range with the northern Laramie Mountains.
It seems odd to call these features “mountains,” with their relatively low relief and height above the surrounding basins when compared to other Wyoming mountains, but the geologic history of the Granite Mountains explains why. The area of the Granite Mountains is unique because it was once a much higher mountain range, similar to other mountains in the state that formed during the Laramide Orogeny. For example, the Crooks-Green-Ferris-Seminoe-Shirley mountain complex that bounds the Granite Mountains on the south has structural features like those found on the flanks of other Laramide mountain ranges in Wyoming; however, the Granite Mountains lack a high central core of Precambrian terrane. Also like other Wyoming mountain ranges, sedimentary basins on both sides of the Granite Mountains contain a thick sequence of lower Tertiary rocks, including thick conglomerate sequences that become coarser grained as they approach the Granite Mountains. The rock types found in the conglomerates, such as the giant boulders of Precambrian granitic rocks found in and on top of Green and Crooks Mountains south of the Sweetwater River, are exactly the same rock types found in the core of the Granite Mountains. Clearly, during early Tertiary (Paleocene and especially early Eocene) time, there must have been high mountains in the area to produce these structural features and the conglomerates.
So what happened to these high mountains? The details of the area’s geology provide the answers. The south and north margins of the Granite Mountains are defined by an obvious system of east-west trending normal faults which preserve late Tertiary sedimentary rocks in the downdropped blocks. These rocks were almost completely removed by erosion elsewhere in Wyoming. At one time, all Wyoming basins were completely filled with these late Tertiary rocks, and only the highest mountain peaks stood above this fill. The granite knobs along the Sweetwater that guided the emigrants represent the buried crest of the central core of a mountain range that collapsed and subsided in the late Tertiary (mostly in the Pliocene Epoch). It is estimated that at least several thousand feet of the Granite Mountains collapsed along these normal faults into a downthrown trough (graben). This collapse was probably caused by crustal extension that affected other Laramide uplifts as well, but evidently not to the same extent it did the Granite Mountains. The proximity of early Tertiary volcanic rocks of the Rattlesnake Hills on the north edge of the Granite Mountains may also be a factor, and the debate continues as to why this entire mountain range collapsed.