The Peach Springs Tuff, a welded ash flow of Miocene age, formerly blanketed a minimum area of 2,000 sq mi on both sides of the western edge of the Colorado Plateau in Mohave County, Arizona, and filled northeast-trending, pre–Colorado River canyons cut through the Paleozoic rocks.
Paleomagnetic direction, petrography, and field relations indicate that the tuff is a single cooling unit of trachytic composition and normal polarity. It was erupted across the region prior to the last major movement of the plateau marginal faults, the completion of a significant amount of Basin and Range block faulting, and development of the existing topographic relief. A pre-tuff period of uplift and erosion exposed the Precambrian basement rocks directly west of the plateau.
Gravel buried in canyons beneath the tuff on the edge of the Hualapai Plateau is evidence of northeast-flowing, incised drainage, in pre-tuff time, which probably continued northward along the Hurricane fault zone across the region where the younger canyon of the Colorado River is now. This ancient drainage system was disrupted by faulting, volcanism, and localized fluvial aggradation, followed by gradual incision of the modern Colorado River drainage.
South of the Truxton Valley, there is evidence of a pre–middle Miocene drainage divide along the edge of the present plateau.
All geologic evidence points to an early to middle Pliocene age for the modern Colorado River and associated tributary drainage on the Hualapai Plateau west of the Hurricane fault.