Though to the lay person it might not be obvious, the jumbles of rock piled about Joshua Tree got their start deep underground via volcanic machinations. It was the upward pumping of monzogranite -- a particular form of molten rock -- that eventually gave birth to the landscape that now greets us.
Of course, that's the short story. It would take a geology class or two to fill in all the details, for they've come to be over a period of roughly 100 million years.
But, as the short story goes, these underground intrusions (see the NPS graphic to the left) steadily forced their way up towards the surface, coursing through an overlaying formation of gneiss today called the "Pinto." As the molten granite cooled, it also cracked to form loose joints in the resulting formations.
Then, as erosion came into play, the surrounding soils slowly were washed away, the joints expanded, and the monzogranite's angular shoulders became rounded, almost as if someone used a sanding block to take their edges off. While much of the central core remained intact, many of the outer edges broke off to create the rubble seen today.
As the Park Service is quick to note, Joshua Tree's "geology shows the effects of plate tectonics, volcanism, mountain-building, and stark erosion. With elevations ranging from 900 feet to over 5,000 feet above sea level, the park is home to a diversity of biological communities found nowhere else in such proximity. Sand dunes, dry lakes, flat valleys, extraordinarily rugged mountains, granitic monoliths, and oases are all found within its nearly 800,000 acres."
Geologic fault zones are key to Joshua Tree, for they bring life-saving water to the surface in the form of springs. These springs, in turn, create oases that nourish vegetation and wildlife. The Pinto Mountain Fault wraps the northwestern lobe of the park, the San Andreas Fault undercuts the southern boundary, and the Blue Cut Fault traverses the center of the park from west to east.
Of the 158 desert fan palm oases in North America, five are located within the national park. Among the more notable oases at Joshua Tree is the 49 Palms Oasis. Another is the Oasis of Mara at the visitor center in Twentynine Palms that marks the Pinto Mountain fault. Bighorn sheep, Gambel's quail, and coyotes are among the wildlife drawn to the oases.
While this exposed geology is fascinating in its own right, it's also a magnetic draw for climbers, who attack some of these features with relish. Day visitors who prefer to keep their feet on the ground explore such places as Hidden Valley, which offers a mile-long loop trail that leads you through a massive boulder field that once served cattle rustlers as a hideout.