The Gran Desierto de Altar is one of the major sub-ecoregions of the Sonoran Desert, located in the State of Sonora, in northwest Mexico.

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.

The southernmost extension of the San Andreas Fault cuts across the area and lies beneath several prominent granite inselbergs, most notably the Sierra del Rosario mountains, which are surrounded by the erg on all sides.

The Sierra Enterrada is a smaller inselberg almost completely buried by the sand near the boundary of the Gran Desierto and the Pinacate volcanic complex.

Carbon-14 dating for a midden from the Tinajas Altas Mountains shows assemblages of juniper and Joshua trees coexisting with contemporary Gran Desierto flora and fauna more than 43,000 years before present.

[5] Although midden studies do not provide information beyond the late Pleistocene, they do indicate that, in gross form, the climate of the Gran Desierto as recorded by plant communities has been desert-like since at least the peak of the Wisconsinan glaciation.

The well-documented pluvial epochs which occurred over much of the southwestern United States during the most-recent (Wisconsin) ice age may not have extended as far south as the Gran Desierto.

As a minimum, it may be assumed that onshore coastal winds from the south were less important to sand movement when the Wisconsin shoreline was located 45 km (28 mi) seaward of its current position.

[10] Vertebrate fossils found by Merriam within the deltaic deposits include Equus, Gomphotherium, and Bison and were assigned to Irvingtonian age (0.5 to 1.8 million years before present), dates consistent with the aforementioned capture of the lower Colorado River.

Blount and Lancaster proposed that by late Pleistocene time, the Colorado River was a highly competent stream flowing through the area which is occupied today by the massive western star dune zone.

The Gran Desierto is located adjacent to a rapidly subsiding tectonic basin, the Salton Trough, which is a northern extension of the Gulf of California, itself an embayment created by rifting initiated during the Pliocene along the East Pacific Rise and the San Andreas Fault system.

The central portion of the nearby Salton Trough is more than 70 metres (230 ft) below sea level; it is protected from marine embayment only by the natural dike of the Colorado River Delta.

Most seismicity within the Gran Desierto originates at depths of 5 to 6 kilometres (3.1 to 3.7 mi), corresponding to the transition between deltaic deposits and basement crystalline rocks.

The synchronous development of the Colorado River Delta and the associated Gran Desierto sand sink continues offshore into the Gulf of California.