Quartz monzonite (a very rough-textured granite) rock formations formed 100 million years ago from the cooling of magma. These prominent outcroppings are known as inselbergs or a monadnock, are very popular among rock climbing enthusiasts. Joshua Tree National Park, created 1994 with passage of California Desert Protection Act (previously National Monument Est. 1936). 790,636 acres (319,959 ha) or 1,235 sq. mi. Mojave Desert, Riverside County, CA.