The mountains toward the east are limestone reef beds deposited in the Cretaceous and Permian seas about 135 million years ago and 250 million years ago; respectively. The limestone reefs overlie deformed rocks in the Ouachita Fold Belt, which is visible along the highway to the east, near Marathon. To the south are younger limestone beds deposited about 125 to 135 million year ago. They also rest above the Ouachita Fold Belt (so named for the Ouachita Mountains of Arkansas-Oklahoma, where formation was first studied by geologists). Toward the west, northwest, and southwest are layers of lava and associated volcanic rocks 25 to 35 million years old. These were thrust upward, into and over most of the older sedimentary rock layers. Many of the molten rock masses cooled and hardened underneath, while vast quantities poured over the surface in the form of lava flows and volcanic debris up to a mile in thickness. Subsequent erosion has sculptured these highlands by removing much of the younger volcanic rock, and cutting deeply into the older sea deposits. Out of the lower, ingenuus hills grow yucca and other cacti, contrasting with juniper and pinon on the higher elevations. (1970)