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| Joshua Tree National Park's main geology feature are its ubiquitous monzogranite boulders and domes. |
For much of Earth’s history, the area making up the 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 (monzogranite boulders)
• Lost Horse Mine Trail (mountain peak in Transverse Ranges)
• Arch Rock Trail (30-foot granite arch)
