[Illustration: In the San Luis Valley, runoff from the San Juan and Sangre de Cristo Mountains sinks into layers of sand in the Alamosa Formation. Flowing along the sand layers toward the center of the valley, it provides artesian water for irrigation of valley farmlands.] SAN JUAN MOUNTAINS LIMIT OF FLOWING WELLS HUBBARD’S WELL OTTOWAY’S WELL ALAMOSA WELL GEORGE NEWSOM’S WELL CALKIN’S WELL LIMIT OF FLOWING WELLS Moraine Alluvial Slope SANGRE DE CRISTO MOUNTAINS Sands, lava beds, gravels, conglomerates, etc. Alamosa formation Granites WEST SANTE FE FORMATION SANTE FE FORMATION EAST Groundwater is extremely important to Colorado, especially in the Prairie Province and the San Luis Valley. Below these two areas lie a number of distinct and productive groundwater aquifers, several of them artesian. In Otero County, for example, there are five major aquifers: three separate Quaternary gravel deposits, the Cretaceous Dakota Sandstone, and the Cheyenne Sandstone Member of the Purgatoire Formation, also Cretaceous. All these aquifers are characterized by their high porosity and permeability, which allow water to flow rapidly through them. Wells in the younger, shallower aquifers produce as much as 2,000 gallons per minute; those in the older, deeper aquifers produce about eighty gallons per minute, some of it with an artesian “head.” The San Luis Valley supports intensive agriculture, made possible by a great artesian water supply. A thick series of soft interlayered clays and sands, the Alamosa Formation, slopes down toward the center of the basin from the surrounding mountains. Water entering the sandstone beds at the mountain edges flows through the sand layers held there by the impermeable clay beds. By the time it reaches the center of the valley, it has developed considerable hydrostatic head, and the water rises in wells without pumping. Unfortunately, both the irrigation water and the soils in the San Luis Valley are highly alkaline. Constant evaporation from the irrigated fields has concentrated the alkali near and on the surface, rendering some of the land less usable than it was originally. Caves Colorado has many caves, most of them carved by underground water in Paleozoic limestone. The Cave of the Winds at Manitou is the only one in the state which has been developed as a tourist attraction. It is in highly faulted Ordovician and Mississippian limestone near the mountain front, where the faulting, coupled with the high relief, has accelerated solution of the rock by allowing groundwater to percolate downward rapidly. The cavern was probably carved during the Pleistocene Ice Age, when surface water and groundwater were much more abundant than at present. Deposition of stalactites and stalagmites has occurred within the last few thousand years, as supplies and movement of water have decreased.