Uplift associated with the Laramide orogeny in the late Mesozoic and early Cenozoic created a major fault along which the Guadalupe Mountains were thrust into existence. The range forms the tilted upthrown part of the system and the Salt Flat Bolson forms the downfallen block. Capitan Reef limestone was exposed above the surface with the 1000-foot-high (300 m) El Capitan being its most prominent feature. Other large outcrops compose the Apache Mountains and Glass Mountains to the south. Streams eroded the softer sediment away, lowering the ground level to its current position. Acidic groundwater excavated caves in the limestone of the higher areas and eroded sediment helped fill any remaining Permian-aged caves. Unlike most other caves in limestone, in this case the acid was likely derived from hydrogen sulfide and sulfide-rich brines that were freed by tectonic activity in the mid-Tertiary and mixed with oxygenated groundwater, forming sulfuric acid. Carlsbad Caverns and nearby modern caves started to form at this time in the groundwater-saturated phreatic zone. Due to the semiarid climate, the karst topography that was created lacks the characteristic depressions, sink holes, pits, and solutional fissures on the surface. Mass wasting such as landslides further reduced topographic relief. Additional uplift of the Guadalupe Mountains in the Pliocene and early Pleistocene epochs enlarged Carlsbad Cavern and nearby caves. Parts of the major caves emerged from the saturated phreatic zone into the vadose with temporary periods of repose during which additional solutional excavation occurred in the phreatic zone. These pauses in emergence are thought to be responsible for creating the different levels in Carlsbad Caverns. Precipitation of carbon dioxide-rich water that infiltrated into the cavern created speleothems, especially in the humid parts of the Pleistocene. Speleothems found in the "Big Room" of Carlsbad were dated using electron spin resonance dating and were found to be 500,000 to 600,000 years old. This indicates that the Big Room level was dry by that time. The soft and easily eroded gypsum of the Castile Formation was removed, exposing the Guadalupe Escarpment. Additional erosion intersected the upper part of Carlsbad Cavern and other caves, forming their entrances. Drying of cave air has reduced the growth rate of speleothems and encouraged the development of nodular travertine ("cave popcorn"). See also List of geographical regions in Texas El Capitan (Texas) McKittrick Canyon Chihuahuan Desert West Texas References Earth System History, Steven M. Stanley, (W.H. Freeman and Company;