Granites, grusses, and the Sherman erosion surface, southern Laramie Range, Colorado-Wyoming

Abstract

The Sherman erosion surface is developed principally on the Precambrian Trail Creek granite facies of the Sherman Granite. A distinctive petrographic characteristic of the Trail Creek granite is extensive, high-temperature oxidation of Precambrian age. Minerals so affected were principally the opaques and biotite. Later weathering processes exploited the oxidized biotites to produce vermiculite, montmorillonite, kaolinite, and gibbsite, all contributing to additional biotite expansion which has shattered the near-surface rock. Disintegration to gruss is so rapid and complete that outcrops are scarce, and a smooth, gently rolling topography results. Other crystalline rocks, lacking the early alteration, weather more slowly and selectively, providing the parkland-tor topography.