The Edge of the Desert

The monument lies on the southwestern border of the Mojave Desert. To the south lies the Colorado Desert, a division of the Sonoran Desert that includes Palm Springs, the Coachella Valley, and the Salton Sea, which in the Ice Age rivaled Lake Manly and which is now sustained mostly with irrigation water brought in from the Colorado River and used to grow most of the nation's winter vegetables. That once-mighty Ice Age lake returned from the dead between 1904 and 1907 when a break in an irrigation dike on the Colorado River caused the whole flow of the Colorado River to take a hard right-hand turn into the low-lying Coachella Valley. Desperate engineers finally sealed the breach and returned the river to its former path by building an elevated railway track across the breach and then dumping freight cars loaded with rock into the gap.

The Joshua tree area stands at an elevation of about 2,500 feet (833 m) above sea level, while the nearby Salton Sea is 235 feet (78 m) below sea level. The uplift in the 50–100 mile (80–160 km) span between these two landmarks defines the boundary between the Mojave and the Sonoran. It is a result of the tug, pull, stretch, and uplift along that tempestuous boundary between the Pacific and the North American crustal plates. The San Andreas Fault runs along the edge of that uplift dividing Mojave from Sonoran. Down in Palm Springs, the 11,000-foot-tall (3,667 m) San Jacinto Mountains on the south side of the valley were formed from a great bubble of granite several hundred miles to the south in what is now Mexico. As the mountain range rose, it moved north along the fault. On the north side of the Palm Springs area lie the much older San Gorgonio Mountains.

The continent continues to split apart along this fault line dividing the Sonoran from the Mojave. The San Andreas Fault, which starts in the sea bottom beneath the Gulf of California, is now a transverse fault, which means its two sides are moving past one another. But shifts in the mantle could easily change it back into a more conventional rift system, so that the long narrow Gulf of California could one day gape open to become a new Atlantic Ocean, splitting off much of California to become another wandering continent like Australia.

So the forces that define the edge of the North American crustal plate have also caused the Mojave Desert and Joshua Tree National Monument to rise in the past 15 to 11 million years. The mountains in this desert are formed of strikingly different rocks from Death Valley. They are mostly made of Mesozoic granite, molten rock forged, cooled, and crystallized miles beneath the surface between 240 and 65 million years ago. These rocks contain gleaming crystals of feldspar and a crisscrossing of fine white veins, made by molten quartz that squeezed into cracks in the deeply buried rock to eventually cool and harden into these bright veins. This deeply buried mass of molten rock was forced to the surface by deeper forces, perhaps going down 100 miles (160 km) to currents in the semi-molten mantle. The mass of rock rose either in great blocks or in dikes and ridges along fractures in the cooler, overlying rock.

Once the uplift that formed the basin and range province and the Colorado Plateau set in, these now-hard, igneous intrusions were lifted high above sea level. Now much harder than the surrounding sedimentary rock, erosion went to work like a sculptor. Erosion exposed these masses of crystallized rock to sunlight and littered the landscape with gargantuan piles of boulders.

The landscape in Joshua Tree National Monument consists of sculptural boulder piles interspersed with the equally otherworldly Joshua trees. These massive buried slabs of granite were often cracked and fractured in regular ways by the forces that uplifted them. Geologists believe that most of these low-angle extension faults formed some 30 million years ago when these rocks were still buried and the shifts at the edge of the continent were stretching the basin and range province to the breaking point. Erosion along these often regular, parallel fractures at the surface caused the boulder piles to weather into dramatic shapes and piles. Although they are called boulder piles, in fact these dramatic outcrops are generally formed of a single mass of cracked rock, so that the boulders remain connected to one another in the pile. Moreover, they are rounded by the flaking away of the large crystals in the granite matrix, rather than having been rolled in a river or succession of floods, which is what produces most rounded boulder shapes.

These striking boulder piles have formed in a gap between two mountain ranges, each uplifted by a sharp fault line. Like Death Valley, the desert area in the Monument is a graben, a block of land between two faults that has dropped as the mountains on either side rose. To the south, the Little San Bernardino Mountains are made of 1.8-billion-year-old gneiss, ancient seafloor deposits that were deeply buried, melted down, cooled, and thrust once again to the surface. Somewhere in that process the buried gneiss was invaded by molten granite, which is identical in composition to most types of lava and ash, but which never reached the surface before hardening.

All the rocks that were laid down between 500 million and 25 million years ago in the area have vanished. This means that the region was uplifted and all of these older rocks were eroded and carried off. However, the area did not become a desert until relatively recent times. It was probably a shrub-filled grassland until continued geological upheaval raised a low range of mountains that cut off the flow of moist air from the ocean, creating the conditions for a rain shadow desert. Although the area probably returned to grassland during the wet glacial periods, the dwindling of the Ice Ages some 10,000 years ago converted it into a high, cold, biologically distinctive desert.