For much of Earth’s history, the area making up Joshua Tree National Park lay underwater. But there were many eras when it was at the center of the action. Tectonic plates have collided several times where the park now sits, causing granite called gneiss to form beneath the surface.
The park’s oldest rocks, Pinto gneiss, are 1.7 billion years old. They can be seen in the Cottonwood, Eagle and Pinto mountains. Another type of gneiss formed about a billion years when a vast mountain range rose on the supercontinent Rodinia. Today, that gneiss can be found in Australia and Antarctica as well as the national park.
The park’s most noticeable geological feature – the exposed monzogranite boulders and domes – formed in the same way over the past 180 million years as the North American and Pacific tectonic plates slipped past one another. Erosion has removed the softer rock covering the gneiss. Some of the exposed gneiss boulders are as tall as 20-story buildings and can be seen in the Wonderland of Rocks as well as the Coxcomb, Eagle and Pinto mountains.
Five of the park’s six mountains are part of the Transverse Ranges, in which the peaks generally trend east-west. Those ranges – the Cottonwood, Eagle, Hexie, Little San Bernardino, and Pinto – were lifted and compressed by the San Andreas Fault, which runs just south of the park in the Coachella Valley, where Palm Springs is located. Parallel faults run through the park, so earthquakes do occur there. The parks boasts 10 peaks higher than 5000 feet.
Some great trails to explore the national park’s geology are:
Skull Rock Trail
Families can day hike with their children to Skull Rock – not the one in Neverland of “Peter Pan” fame but one bearing an uncanny resemblance – in the Mojave Desert. The Skull Rock Trail runs 1.75-miles round trip through the famous Jumbo Rocks, a collection of massive, rounded granite rocks that many park visitors often clamber over. Early morning right after dawn and late afternoon just before dusk offer the best lighting. Take Calif. Hwy. 62 to the park’s north side. Use the north entrance, which is in Twentynine Palms, three miles south of the junction of Highway 62 and Utah Trail. The trailhead is on Park Boulevard, which is the park’s main east-west road. Look for the roadside parking area immediately north of Jumbo Rocks Campground.
Lost Horse Mine Trail
Mountain peak in Transverse Ranges
Stark, rock-strewn mountains rise all across California’s deserts thanks to the Pacific and North American tectonic plates colliding. A 2-mile (4-miles round trip) segment of the Lost Horse Mine Trail crosses some of those raised mountains while heading to the base of one desert summit – Lost Horse Mountain, which rises nearly a mile above sea level to 5,278 feet. From Interstate 10 in the Coachella Valley take Calif. Hwy. 62 north through Morongo Valley. In Joshua Tree, turn right/south onto Park Boulevard (aka Quail Springs Road) into the national park. Just before Cap Rock, go right/south onto Keys View Road. Next, turn left/southeast onto Lost Mine Road, which quickly becomes gravel. A parking lot is at the road’s end, and the trail heads out from the lot’s southeast corner.
Arch Rock Trail
Thirty-foot granite arch
Day hikers can head to an arch sculpted by Mother Nature out of granite at Joshua Tree National Park. The 1.3-mile lollipop Arch Rock Trail partially explores the granite rock formations surrounding the park’s White Tank Campground. Taking a few steps off the main trail allows you to get underneath the arch, and the surrounding rocks can be climbed so you can touch it. From the park’s Oasis Visitor Center on Utah Trail just south of Calif. Hwy. 62, take the former south. Once in the park, Utah Trail becomes Park Boulevard. Turn left/southeast onto Pinto Basin Road. In about 2.2 miles, look for a parking lot on the road’s right/west side. Park in the lot.
- The Lost Horse Mine stamp mill: Rob Bignell
- Skull Rock: Rob Bignell
- The Lost Horse Mine Trail: Rob Bignell
- Arch Rock: Rob Bignell