Distribution of Tertiary regional ash-flow tuffs indicates that the Colorado Plateaus and Basin and Range provinces in southwestern Utah became separate structural terrains some time after 29 m.y. ago; the structural differentiation probably was underway by 26 m.y. ago and had produced significant topographic contrasts between the two provinces by 24 m.y. ago. Initial differentiation may be older than the main phase of basin-range faulting in southwestern Utah, which appears to have begun about 21 to 20 m.y. ago.