The purpose of this paper is to describe in a general way the occurrence of certain detrital accumulations in the southwestern part of the United States, and to suggest a line of investigation which apparently makes clear the recent geologic history o f that region. The statements contained in the paper are supported by evidence which must be here omitted for want of space. On the other hand, the fact is recognized that in a region so extensive and so little known as that in which the detrital accumulations occur, much more work must be done before final conclusions can be reached. The observations on which this paper is based were made mainly during rapid reconnaissance trips in western Arizona, although detailed work was done in a number of places in the Southwest, notably in Salt River valley in central Arizona, Owens valley in eastern California, and in the . . .