The rock formations of Joshua Tree National Park were formed more than 100 million years ago from the cooling of magma beneath the surface into monzogranite, with roughly rectangular joints. Groundwater then filtered through the joints to erode away the corners and edges to create rounded stones, and flash floods washed away covering ground to create piles of boulders. These prominent outcrops are known as inselbergs.
I remember going out to Joshua Tree not long after I moved to LA, with the specific intent to re-create the cover photograph of the U2 album.
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