Grand Wash, a tributary of Colorado River in northwestern Arizona, lies in a wide valley whose position is determined by a zone of profound faulting. This valley marks throughout its length a sharp and definite boundary between two major physiographic and structural provinces. East of the valley the Grand Wash Cliffs rise precipitously, as a pair of giant steps, to an elevation averaging 4,000 feet above the valley floor. These cliffs form the abrupt western limit of a great tabular mass, the Shiwits division of the Colorado Plateau, in which the stratified rocks are nearly horizontal over a wide area. West of Grand Wash Valley and reaching into southern Nevada lies the Virgin Range, a complexly broken fault block, in which the same formations exposed in the Grand Wash Cliffs are tilted at high angles. Continuing to the west, other ranges structurally similar to the Virgin Range alternate with . . .