The Hueco Bolson, a down-dropped area with an elevation of 4,000 feet (1,200 m) above sea level, with sedimentary fill nearly 9,000 feet (2,700 m) thick, lies between the Hueco and Franklin Mountains. Shallow, stony soils in the Hueco Mountains support oak, juniper, and some mesquite. The mountains were part of the Rocky Mountain trend, forced upward as part of the Laramide mountain-building period during the late Cretaceous, 60 to 70 million years ago.
The word hueco is Spanish for hollow, gap, or hole.[3]