The Sherman Mountains are erosional remnants rising above the general surface of the Laramie Range in southeastern Wyoming. The Laramie Range (Laramie Uplift) is cored by Precambrian igneous rock, most of which is a coarse-grained plutonic complex called the Sherman Granite (early Mesoproterozoic, 1.4 billion years old). Portions of the Laramie Range also have Mesoproterozoic anorthosites.
The pic below shows "Turtle Rock" in the Sherman Mountains. The Sherman Granite here has three sets of joints. Differential erosion has resulted in distinctive, somewhat rounded-weathering granite blocks.