<P> Below 3,000 feet (910 m), the Colorado Desert encompasses the eastern part of the park and features habitats of Creosote bush scrub, Ocotillo, desert Saltbush and mixed scrub including Yucca and Cholla cactus (Cylindropuntia bigelovii). There are areas of such cactus density they appear as natural gardens . The lower Coachella Valley is on the southeastern side of the Park with sandy soil grasslands and desert dunes . </P> <P> The only palm native to California, the California Fan Palm (Washingtonia filifera), occurs naturally in five oases in the park, rare areas where water occurs naturally year round and all forms of wildlife abound . </P> <P> 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 as well as bajadas where the alluvial fans overlapped . </P> <P> 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 . These prominent outcrops are known as inselbergs . </P>

What kind of rock is in joshua tree
find me the text answering this question