Most of the map area is covered by sand dunes and related features of Cactus Plain. The dune field of Cactus Plain is on a low plateau formed primarily on older eroded sediments, with minor bedrock hills protruding from the plain locally. The oldest late Cenozoic deposits in Cactus Plain are undeformed fan deposits composed primarily of crystalline metamorphic clasts, obviously derived from the east and northeast. These deposits are overlain by, and locally may interfinger with, fine-g rained clay, silt, sand and minor limestone deposits that we consider to be part of the Bouse Formation. Bouse carbonate deposits are also extensively exposed on the flanks of Black Peak and related hills near the northern margin of the map area. On the western, north eastern, and southern margins of Cactus Plain, there are extensive well-rounded, lithologically diverse gravel and sand deposits that we tentatively correlate with the early Pliocene Bullhead alluvium. T h is major river aggradation sequence is found all along the Colorado River below the mouth of the Grand Canyon (House et al., 2008; Howard et al., in prep.).