The Chiricahua Mountains are an uplifted structural block of the Basin and Range. The mountains contain Precambrian basement rocks, Paleozoic and Cretaceous sedimentary rocks around a caldera complex formed by volcanic eruptions and intrusions 35 - 25 million years ago. The last major eruption, 27 million years ago, created the Turkey Creek Caldera and laid down of volcanic ash which fused into welded rhyolite tuff. Subsequent erosion has created mountain ridges covered in stone spires and stone columns, hoodoos, that rise up out of the forest. These natural features, preserved in the Chiricahua National Monument, are composed of "Rhyolite Canyon Tuff".