Blount and Lancaster proposed that by late Pleistocene time, the Colorado River was a highly competent stream flowing through the area which is occupied today by the massive western star dune zone. The seashore at this time was at least south of its present-day location. Primary bed loads of poorly sorted gravel were deposited from present-day Yuma, Arizona to an area south of the present-day Sierra del Rosario mountains. As rifting of the Gulf of California progressed to the northwest, and uplift along the coast began, the river channel shifted westward, leaving primary bedload deposits in the former channel and floodplain. Deltaic sediments beneath the Gran Desierto may be as much as deep.