 The Hueco Bolson, a down-dropped area with an elevation of above sea level, with sedimentary fill nearly 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.