Arch Rock is a natural arch worn into the old granite that occurs throughout Joshua Tree National Park.
In the northern Mojave section of Joshua Tree, the landscape is dominated by vast piles and mounds of weathered granite, the result of the intrusion and slow cooling of magma within the earth's crust in this region eons ago. Piles of boulders occur where weathering of the solid granite took place underground prior to exposure of the rock to the surface. Above ground, the granite not worn into boulders exhibits great cracks, thought to have formed by expansion as the weight of the overlying rock was removed by erosion. Subsequent further erosion of the granite smooths and opens these cracks, creating the rounded features and eventually cutting off slabs from the larger body of rock.