Joshua Tree National Park
Joshua Tree National Park is defined by a Desert ecology.
Joshua Tree National Park is an American national park in southeastern California, east of Los Angeles and San Bernardino, near Palm Springs. It is named for the Joshua trees (Yucca brevifolia) native to the Mojave Desert. Originally declared a national monument in 1936, Joshua Tree was redesignated as a national park in 1994 when the U.S. Congress passed the California Desert Protection Act.[4] Encompassing a total of 790,636 acres (1,235.4 sq mi; 3,199.6 km2)[2]^ – slightly larger than the state of Rhode Island – the park includes 429,690 acres (671.4 sq mi; 1,738.9 km2) of designated wilderness. Straddling San Bernardino and Riverside Counties, the park includes parts of two deserts, each an ecosystem whose characteristics are determined primarily by elevation: the higher Mojave Desert and the lower Colorado Desert. The Little San Bernardino Mountains traverse the southwest edge of the park.[5]
Geology
Giant Marbles
The park's oldest rocks, Pinto gneiss among them, are 1.7 billion years old. They are exposed in places on the park's surface in the Cottonwood, Pinto, and Eagle Mountains. Much later, from 250 to 75 million years ago, tectonic plate movements forced volcanic material toward the surface at this location and formed granites, including monzogranite common to the Wonderland of Rocks, parts of the Pinto, Eagle, and Coxcomb Mountains, and elsewhere. Erosion eventually exposed the harder rocks, gneiss and granite, in the uplands and reduced the softer rocks to debris that filled the canyons and basins between the ranges. The debris, moved by gravity and water, formed alluvial fans at the mouths of canyons, and *bajadas*, where the alluvial fans overlapped.[30]
The rock formations of Joshua Tree National Park owe their shape partly to groundwater, which filtered through the roughly rectangular joints of the monzonite and eroded the corners and edges of blocks of stone, and to flash floods, which washed away covering ground and left piles of rounded boulders.[31] These prominent outcrops are known as inselbergs.[32]
Of the park's six blocks of mountains, five—the Little San Bernardino, Hexie, Pinto, Cottonwood, and Eagle—are among the Transverse Ranges, which trend generally east–west in locations between the Eagle Mountains on the east and the northern Channel Islands, in the Pacific Ocean west of Santa Barbara, on the west. Tectonic forces along the San Andreas Fault system compressed and lifted the crust material that formed these ranges. The San Andreas Fault itself passes southwest of the park, but related parallel faults, including the Dillon, Blue Cut, and Pinto, run through the park, and movements along them have caused earthquakes. The easternmost range in the park, the Coxcomb Mountains, runs generally north–south and is part of the Basin and Range Province.[30]