The mountains are faulted anticlines formed as part of the Sawatch Uplift during the Laramide orogeny around 65 million years ago (Mya).

The mountains were originally contiguous with the higher Sawatch Range to the west, but were separated during the formation of the rift valley at the headwaters of the Arkansas River about 35 Mya.

The eastern flank of the range along the rim of South Park is overlain by Paleozoic and Mesozoic sedimentary rocks.

The northern end of the range was glaciated during Pleistocene time, forming U-shaped valleys in the canyons.

Several of the high peaks of the range are prominently visible from the southern side of Fremont Pass along State Highway 91 between Climax and Leadville.