17MB PDF - Association for Mexican Cave Studies

176unaltered shallow phreatic passage about 1/2 mile long. It seemslikely that when the Los Sabinos area is completely explored therewill be over a mile of large passage for every square mile of surface.Whether this ratio is typical of the Sierra de El Abra as awhole or if this is an exceptionally favorable area is not known.It is possible that the former is true, as there are numerous otherpromising and as yet unvisited entrances in all parts of the range.S6tano de Venadito, located at the end of an arroyo just north ofthe San Luis PotosI state border, has been partially explored, withother larger s6tanos reported in the vicinity. Northeast of Cd.Valles the entrance to Ventana Jabal! opens onto the steep eastface of the range. This cave is essentially one large passage 1200feet long. About 400 feet from the entrance a dome rises 503 feetto intersect the flat surface of the range, forming a skylight andthe deepest free drop in North America. This dome does not appearto be primarily of vadose origin, since, except at the very top,it is quite large and is spanned by a natural arch about halfwaydown. A somewhat similar cave is located in the El Abra pass justabove the Inter-American Highway south of Cd. Mante. This cave,Cueva de El Abra, consists of an entrance passage 60 feet Wide, 70feet high, and 500 feet long leading to a shaft that extends upwardto the surface and downward to the lower level of the cave, a largeparallel fissure. Most of the caves in the Sierra de El Abra appearto be essentially unaltered shalloW phreatic passages. Evencaves that receive large amounts of water are little modified, indicatingthat invasion by surface waters is a relatively recentevent. The well-known Grutas de Quintero, located near the townof Quintero just south of Cd. Mante, appears to be an abandonedresurgence. Lowering of the water table and continued solutionhas drained this cannel. There is no integrated surface drainagedeveloped on the Sierra de El Abraj water falling on the relativelyflat surface of the range sinks into the permeable limestone toreappear at resurgences along the base of the east face of therange. The largest of these resurgences are the Nacimiento delRio Mante just west of Cd. Mante and the Nacimiento del Rio Choyeast of Cd. Valles. The limestone exposed in the Sierra de EIAbra appears to be especially favorable for the formation of caverns.This limestone, partly composed of reef material, has neverbeen deeply buried and it is possible that it has retained some ofits original porosity.A few miles from the southern end of the Sierra de El Abrarise high limestone mountains that contain some of the most spectacularkarst in Mexico. These mountains extend from just northof the Rio Tamuin,pass1ngwest of the town of Aquism6n, to south ofXilitla. This is an area of several hundred square miles completelydevoid of surface drainage except for two rivers. The RioTamuin flows through a narrow canyon that cuts into the northernpart of the karst area and the Rio Moctezuma has cut an even deepercanyon in the south. In this area limestone reef masses over 6000feet thick have been folded into mountains whose peaks rise over9000 feet above the resurgences at the valley level. These rangesrise sharply above the coastal plain and consequently receivelarge amounts of precipitation, with Xilitla receiving over 100 in.per year. Only a small part of this area has been investigated,as efforts here have so far been limited to small areas along thefew roads. Near the town of Tlamaya, northeast of Xilitla, an areaof about 3 square miles has been closel~ examined. This area is1800 feet above the resurgence of the RIo Huichihuay~ and contains