The Cutler Formation is an Early Permian rock unit spread across parts of the Four Corners region, formed from debris eroded off the rising Uncompahgre Uplift of the Ancestral Rocky Mountains. Near the uplift it contains very coarse alluvial fan deposits, including huge granite boulders, while farther west it changes into finer fluvial, windblown, and marine-influenced sediments. Because these rock types vary so much, it is treated as a formation near Ouray but as the Cutler Group across much of the Colorado Plateau, with several named subdivisions. First defined in 1905, it remains an important record of late sediment shed from the Ancestral Rocky Mountains and a key unit in regional basin geology.