GEOMORPHOLOGY OF THE VALLEY OF THE LITTLE COLORADO RIVER, ARIZONA
The Little Colorado River, a tributary from the southeast, empties into the Colorado River at the head of the Grand Canyon. It drains an extensive desert area in northeastern Arizona. Upstream from a gorge 60 miles long in the Marble Platform it flows in a broad valley which is the subject of the present study. The stratigraphy, structure, and volcanic rocks are summarized in a geologic map as a basis for the geomorphic study. Three successive erosion surfaces are, from highest to lowest, the Black Point pediplane, the Wupatki pediplane, and the Little Colorado pediplane. Each is interpreted as having formed during a period of stability of the profile of the Little Colorado River. It is concluded that the Black Point and Wupatki pediplanes represent the culmination of long periods of erosion, each interrupted and rejuvenated by regional uplift. During the formation of the present Little Colorado pediplane, the stabilization of base level has been controlled by the occurrence of resistant beds along the course of the master stream. The history of the Grand Canyon of the Colorado has been postulated by previous writers to consist of two great cycles of erosion. The events of the valley of the Little Colorado are interpreted as having taken place during the second of these, “the canyon cycle”. The Black Point pediplane was well developed at the time of Stage 1 lavas which are of uppermost Pliocene or early Pleistocene age. The Wupatki pediplane was formed prior to Stage 2 lavas which are of early or middle Pleistocene age. The Little Colorado surface is still in the process of development and is found locally beneath Stage 3 and Stage 4 lavas.