Today, most of the sedimentary rock units have been tilted from their originally horizontal positions by large-scale crustal movements described by the terms diastrophism and tectonic activity. A major period of crustal activity began in the Late Cretaceous and has continued intermittently until the present. This crustal unrest warped and fractured the crust and the overlying sediments, outlining the mountain ranges and basins and establishing the geologic framework that we see today.
This episode of mountain building elevated the rocks above sea level and, together with further elevation in late Tertiary time, provided the necessary stream gradients whereby the streams of the region could proceed to dissect the rocks into the existing landforms. The major event that ultimately governed the position of most larger streams in Wyoming was the accumulation of a vast sheet of younger sedimentary rocks in the basin areas, climaxed by a widespread sheet of volcanically-derived, fine-grained sediments. The major streams developed on this aggraded surface. Later, as these streams cut downward, their courses were locked into the harder underlying rocks and, as a result, such great canyons as the Wind River, the Bighorn, and the Platte evolved.
As a result of this geologic history, Wyoming is now divisible into three major physiographic categories: mountains, the Great Plains of eastern Wyoming, and basins. The landscape and underlying geology of features in each of these categories are very different.
In most of Wyoming's mountainous areas, the difference of elevation is in part due to uplift of large segments of the Earth's crust in the form of folds or wrinkles, or blocks bounded by fractures, or a combination of both. Most of Wyoming's larger mountain ranges also have an exposed core of very ancient Precambrian metamorphic and igneous rocks.
The frequently asked question, “How old are the mountains?” may be answered in two ways. We might consider that the age of the oldest rocks exposed to the elements and in the process of being eroded is the age of the mountains. In another sense we might consider the time elapsed since the rocks were elevated so that they began to be carved by erosion.