FROM BUTTES TO BOWLS: REPEATED INVERSIONS IN THE LANDSCAPE OF THE COLORADO PIEDMONT

Mesas and buttes of the central Colorado Piedmont are composed of at least two distinct rock types, which differ in their cohesiveness and ability to withstand erosion. The lower parts are friable, Early to Middle Paleogene sandstones of the Dawson Formation. The caprock is composed of one or more resistant formations: Castle Rock Conglomerate, Wall Mountain Tuff, and Larkspur Conglomerate — all of late Paleogene age. All of these formations were originally deposited in topographic lows. The lower slopes of the buttes are armored with colluvium composed of fragments of the capping units and commonly form “talus flatirons” or relict faceted slopes.

Once the caprock of a butte or mesa has been removed by erosion, the poorly consolidated Dawson Formation quickly erodes out of the center. This leaves the armored lower slopes of the former butte as an erosionally-resistant, circular ridge standing as much as 100 meters above the surrounding topography. This process produces a topographic low where the peak of the butte once stood.

Some buttes have prominent alluvial fans that record the main phase of butte removal and excavation of the central part of the armored slopes. Soil profiles and height above modern streams suggest the oldest preserved gravel deposit is of middle Pleistocene age; the youngest alluvial fans were deposited during the Holocene.