Abstract

Within the western part of the Grand Canyon, over a distance of more than 84 miles, from Toro-weap Valley almost to the Grand Wash cliffs, flow remnants of olivine basalt occur along the Colorado river channel. The remnants in the downstream three-fourths of the stream segment individually pertain to single flows. The accordance of their upper surfaces with a gradient parallel to that of the present river and the failure to discover sources of lava in this segment suggest that the remnants belong to the same flow. In the upstream one-fourth of the stream segment several flows may be distinguished, the lowest of which is correlated with the downstream remnants on the basis of near-continuity and similarity in physiographic position. The source of this flow is in the vicinity of Lava Falls at the mouth of Toroweap Canyon where the farthest upstream remnants are found. Later flows from the esplanade of the Uinkaret Plateau locally and briefly dammed the Colorado River near the mouths of Toroweap and Queanto-weap valleys. After removing the dams, the Colorado River cut downward to approximately its channel prior to the first flow. At one place below Lava Falls the river channel is still being cut in basalt, but everywhere else it is being cut a few feet to a few tens of feet below the bottoms of the lowest flow remnants. This long flow was an important event in the later geological history of the Grand Canyon and affords a striking example of the fluidity of basaltic lava, involving a descent of 700 feet in 84 miles along a water grade.

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