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42 The theory of pediment formation in the study area proposed ...... c.u c.o w herein involves a cycle of events that has not been identified by other authors in other localities. This theory incorporates periods of massive rock slides and associated debris flows derived from the cliff escarpment contemporaneous with wet climates. separated by periods of headward expanding ephemeral stream basins in the piedmont contemporaneous with dry climates. Upland-derived streams and stream capture are not involved in this theory. Also, sudden changes in base level of the Colorado River are not required in order to give rise to the different elevations of pediments in the area. 5.3.3 Model of Pediment Origin Figure 5.8 represents the sequence of events that are proposed as the origin of the pediments. In Figure 5. 8a, . the area is depicted as a region of badlands. The piedmont drainage basins are formed by headward extending ephemeral streams during a period dominated by an arid climate. The badlands persist because of the progressive incision of the Colorado River into the easily erodible Mancos Shale and because of progressive scarp retreat (Hunt, 1956; Lohman, 1965). The cliff escarpment is steep and gradually retreat- ing, but no blocks of sandstone from the capping Mesa Verde Forma- tion are present except at the base of the escarpment. There are no blocks or boulders beyond the escarpment base because of the low mechanical strength of the cliff-forming sandstone. The sandstones are poorly cemented with calcite cement, highly porous, and of low permeability. As blocks fall down the escarpment during the slow scarp retreat, they either disintegrate by falling and rolling, or most