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. It includes the only active erg (sand sea) dune region in North America. The desert extends across much of the northern border of the Gulf of California, spanning more than 100 kilometres (62 mi) east to west and over 50 kilometres (31 mi) north to south. It constitutes the largest continuous wilderness area within the Sonoran Desert. The eastern portion of the area contains the volcanic Pinacate Peaks region; together with the western portion, the area forms the El Pinacate y Gran Desierto de Altar Biosphere Reserve and a UNESCO World Heritage Site.
Geography The Gran Desierto covers approximately 5,700 square kilometres (2,200 sq mi), 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 1 kilometre (0.62 mi) to greater than 12 kilometres (7.5 mi). The total volume of sand in the Gran Desierto is about 60 cubic kilometres (14 mi3). 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. The eastern margin of the Gran Desierto abuts the Cenozoic volcanic complex of the Sierra Pinacate, a composite volcanic field covering more than 1,800 square kilometres (690 sq mi) with a summit elevation of 1,206 metres (3,957 ft). Aeolian sands have climbed onto many of the western slopes of the Sierra Pinacate, defining the eastern limit of the dune field. To the north, the sands thin out against the distal margins of alluvial fans from the Tinajas Altas and Tule Mountains along the Arizona–Sonora border. The southern border of the sand sea is the northern shore of 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.
Sand dune distribution The Gran Desierto is best known for its magnificent star dunes, many in excess of 100 metres (330 ft) high. More than two-thirds of the Gran Desierto is covered by sand sheets and sand streaks. The remaining area is split equally between a western population of star dunes and an eastern set of transverse or crescentic dunes. Some of the larger crescentic dunes in the northeastern sand sea exhibit reversing crests, a transitional morphological feature associated with star dunes.
Flora Fauna of the Gran Desierto includes a range of desert-adapted reptiles, small mammals, and migratory bird species typical of the Sonoran Desert ecosystem. Vegetation assemblages of the Gran Desierto are typical of the lower Sonoran Desert with a marked difference in vegetation type and density with location. Large areas of the southern and eastern sand sea, especially near the margins, have a moderately dense (up to 20%) cover of perennial low shrubs and herbs such as bursage (Ambrosia dumosa) and longleaf jointfir (Ephedra trifurca) with creosote bush (Larrea tridentata) in areas of thin sand cover. Palo verde/acacia/ocotillo communities occur on alluvial slopes on the northern side of the sand sea, particularly in arroyos and washes. The region's estimated total vegetation cover is 15% in the star dunes and about 10% in the low transverse or crescentic dunes areas. These percentages are substantially greater than in most active dune fields, where vegetation covers of 15% are more typical. Several teams have examined the middens built by pack rats as a proxy for ancient vegetation regimes. All have concluded that the Gran Desierto has been an ecological refuge for desert plants since at least the late Pleistocene. The Gran Desierto has served as a refuge for most dominant Mojave Desert plant species during cooler pluvial epochs as well. 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. 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.
Fauna
Climate The Gran Desierto has a warm to hot arid climate. Mean annual rainfall, most of which occurs between September and December, is 73 mm (2.9 in) at Puerto Peñasco, Sonora (located at the southeastern margin of the sand sea on the Gulf of California) and decreases northward toward Yuma, Arizona (on the northwestern edge) to 62 mm (2.4 in) per year. Mid-summer highs in excess of 45 °C (113 °F) are common in the central sand sea. Mid-winter lows of less than 10 °C (50 °F) are rare. Winds are controlled in part by the position and strength of the Sonoran Low in summer, creating southerly winds, and by the Great Basin High in winter, with north-to-northeasterly winds.
Paleoclimate 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. It appears that the climatic regime of the past 150,000 years at this site has been one of gradually increasing aridity with current hyper-arid conditions being firmly in place by at least 43,000 years ago. 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.
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