The great Navajo sand sea has spread across much of the Wertern Interior. A continental arc has developed across the Southwest and some Navajo dunes spill westward into the topographically low arc. Farther north the arc is off shore separated from the continent by a back arc basin. The Stikine and Quesnell elements are separated by the Cache Creek interarc basin. A long, narrow, epicontinental seaway extends into the Western Interior from the north.
Dunes rim the southeast margin.
The elements of the McCloud arc collapse against the continent and thrusting in Nevada creates uplands. The large Wrangellia oceanic plateau approaches from the south. Most interpretations consider Wrangellia to be an exotic terrain. The uplift in Nevada reverses stream direction across the region and the Morrison fluvial system expands eastward across the Western Interior. Stresses related to the opening of the Gulf of Mexico oceanic basin caused oblique, transtensional basins to form across the southwest portion of North America.