These potato-shaped prizes in the Alabama Hills are mostly made of biotite monzogranitean, quite a sexy mineral in its own right. But what really makes them stand out from the landscape is their spheroidal weathering pattern.
As the old bedrock approached the surface over millennia of overburden pressure (one of the processes behind Yosemite’s towering monoliths), geometric fractures in the rock allowed water to cyclically penetrate and react with the surface of these hidden blocks. These cycles caused the formation of ‘onion-layers’ of interesting minerals on the blocks’ surface; a chemical weathering process. Once these structures find their way to the surface, the layers erode sequentially, exposing these oddly-spherical solid monuments. These iconic shapes of exposed rock are especially common east of the Sierras, famous in places like Joshua Tree, and make boulderers even happier people.