The rounded rock forms are due to chemical weathering under thick soil. Acid groundwater penetrates along jointing planes and softens the granite into a loose gravel called grus. When climate changes, the soil mantle is stripped away to reveal the bones of the bedrock beneath. The Mojave was once much wetter than today, but as it dried out this distinctive granite landscape emerged. Periglacial processes, related to frozen ground during the ice ages, may have helped remove the overburden of the tors of Britain.
For more pictures like this, see the Joshua Tree National Park Photo-Tour.