The Gran Desierto covers approximately , most of it in the Mexican state of Sonora. The northernmost edges reach across the international border into Organ Pipe Cactus National Monument and Cabeza Prieta National Wildlife Refuge in southwestern Arizona, United States. The region is dominated by sand sheets and dunes ranging in thickness from less than to greater than . The total volume of sand in the Gran Desierto is about . Most of that volume was delivered by the Colorado River during the Pleistocene, which flowed through the present-day Gran Desierto area approximately 120,000 years before present. This Pleistocene delta migrated westward concomitant with strike-slip faulting and rifting associated with the opening of the Salton Trough and the Gulf of California.