The rock formations of Joshua Tree National Park were formed 100 million years ago from the cooling of magma beneath the surface.
Groundwater is responsible for the erosion that created the spheres from rectangular blocks. These prominent outcroppings are known as inselbergs or a monadnock.
The dominant geologic features of this landscape are hills of bare rock, usually broken up into loose boulders. These hills are popular amongst rock climbing and scrambling enthusiasts. The flatland between these hills is sparsely forested with Joshua trees. Together with the boulder piles and Skull Rock, the trees make the landscape otherworldly.
The erosional and weathering processes of the present are only particially responsible for the spectacular sculpturing of the rocks. The present landscape is essentially a collection of relict features inherited from earlier times of higher rainfall and lower temperatures.