The northeastern corner of the Mexican state of Coahuila contains some of the most extensive limestone outcrops in the country,
yet the karst and groundwater have seen relatively little investigation. The western (recharge) portion of the area contains ridge
tops of Lower Cretaceous rocks over 1500 m in elevation, which slope down to the east and south to plunge underneath less
karstic Upper Cretaceous outcrops. This represents a confined aquifer zone where wells can flow under artesian pressure. In this
respect it resembles the Edwards Aquifer across the Rio Grande in Texas, with similar carbonate lithologies and karst
components.
Over 60 caves have been explored in the area, most since the year 2000. Upland portions of the recharge zone contain vertical
caves that likely contribute to aquifer recharge, although numerous seep-spring caves in canyon walls disgorge some water
prematurely. The canyons themselves are major rechargers, exemplified by El Abra, a horizontal stream cave that is the longest in Coahuila at 1841 m in length. In the lower part of the recharge zone close to the artesian zone, there are a number of caves which are estavelles. These have large funnel-shaped sinkhole entrances that slope down to pits, dropping up to 90 m to flowing streams. These normally take water, but during times of heavy rains in the recharge zone they can become springs.Integrative Biolog
