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Upper Cenozoic terrestrial basin-fill sedimentary and basic volcanic rocks are common in the 20,700 km 2 Basalt area, which includes parts of the Gore, Sawatch, and southern Park Ranges, Elk Mountains, Grand Mesa, and White River Plateau. Principally on the basis of whole-rock K-Ar ages from basalt flows, the rocks can be placed in four groups. Group 1 rocks attain a thickness of 210 m and range in age from 24 to 20 m.y. (early Miocene). They consist primarily of flows of alkali-olivine basalt or of basalt flows interlayered with cross-bedded sandstone of the Browns Park Formation. Group 2 rocks, 14 to 9 m.y. old (late Miocene and perhaps early Pliocene), attain a maximum thickness of 180 m and are composed largely of basalt, basaltic andesite, and fine-grained tuffaceous fluvial, lacustrine, and eolian sedimentary rocks containing a rich vertebrate fauna indicative of a semiarid climate and a steppe vegetation. Toward the end stages of the accumulation of group 2 rocks, about 10 m.y. ago, the region was subjected to major tectonism, including uplift, reactivation of some Laramide (Late Cretaceous-early Tertiary) structural features, and creation of new warps, folds, and faults. The initiation of the Colorado River system apparently occurred at that time in response to increased precipitation stemming from an increase in elevation and relief. Downcutting began at this time and has continued intermittently to the present. By about 8 m.y. ago (late Miocene or early Pliocene, depending on boundary interpretation), the Roaring Fork River had downcut 600 m and had formed a broad flood plain, upon which thin alkali-olivine basalt flow units (group 3 rocks) were erupted. Downcutting appears to have been slow between about 8 and 1.5 m.y. ago; since then, an additional 300 m of valley deepening has occurred. Sporadic volcanism since 1.5 m.y. has accounted for several small cinder cones and flows of nepheline-normative alkali basalt (group 4 rocks); the last eruption occurred about 4,000 yr ago.