The K-Ar age of the basal basalt flow at the bottom of the “Lower Canyon group” of lavas near Toroweap fault is 1.16 ± 0.18 standard deviation (sd) m.y. This represents a minimum age of Grand Canyon, for at the time the lava formed, the canyon was essentially as deep as it is today. Since that time the Colorado River has cut through the 550-ft lava dam at the mouth of Toroweap Valley, an additional 50 ft of Paleozoic strata below, and through one or more younger lava dams in the area; downstream it has cut through 100 feet of younger intracanyon lavas.

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