The Cutler Formation (Pennsylvanian-Permian) of the northwestern Colorado Plateau was deposited as an alluvial fan complex along the western margin of the ancestral Uncompahgre Uplift. Near Gateway, Colorado, and Fisher Towers, Utah, the Cutler comprises rapidly-deposited, poorly-sorted, conglomeratic sandstone and siltstone. During deposition, the climate was arid to semi-arid. The red color of the Cutler rocks resulted from the formation of hematite by intrastratal solution of iron-bearing minerals. The detritus at the Gateway section differs in mineral composition and texture from the Fisher Towers section. The Gateway section is characterized by an overall coarse grain size, a high potassium feldspar to total feldspar ratio, and a non-micaceous, non-opaque heavy-mineral assemblage rich in epidote, sphene, and garnet. The Fisher Towers section is finer grained, contains more quartz, and has a heavy-mineral assemblage dominated by garnet, sillimanite, and tourmaline, with rare epidote and sphene. These differences in mineralogy reflect regional difference in provenance. The source rocks for the Gateway section were largely Precambrian Vernal Mesa-type quartz monzonite and gneisses. At Fisher Towers, gneiss was the main source rock.