[2] The following description is from USGS Bulletin 798 by C. R. Longwell in 1928:[3] In all the large intermontane valleys adjacent to the Muddy Mountains and neighboring ranges there are thick clays or silts and associated deposits that have been relatively little disturbed by crustal deformation.

They have invaded St. Thomas Gap in places, and in the broad Grand Wash Valley they are exposed wherever the surface waste has been sufficiently dissected.

The clays are conspicuous beneath the waste in the sides of the large valleys and small washes and in extensive badland tracts where the later capping has been stripped away.Owing to many interbedded layers of sandstone more or less firmly cemented, the clays tend to stand in steep fronts along wash courses.

The edges of the sandy layers are visible in all sections, standing out as narrow shelves or forming a horizontal banding almost concealed by clay carried from above and plastered on the slope by infrequent rains.

In places they are interrupted by persistent shelves formed on sandstone layers of unusual resistance, below which the typical patterns reappear.