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.
Spanish Cave, above timberline on Marble Mountain in the Sangre de Cristo Range, is probably the nation’s highest limestone cave. It is in thick folded and faulted Pennsylvanian [reef] limestone, at an elevation of over 12,000 feet. The cave has many intricate passageways branching from its main vertical tubes and channels.
Fulford Cave, south of Eagle, is in the Mississippian Leadville Limestone of the northern part of the Sawatch Range. Many other caves are situated south of Fulford, near Woods Lake, where the limestone is widely exposed and highly dissected.
Fairy Cave, northeast of Glenwood Springs, is the best known of the many caverns in the Paleozoic limestones that form the southern flanks of the White River [Plateau].
In Cave of the Winds near Manitou, Paleozoic limestones, cracked and tilted by uplift of the Front Range, have been honeycombed by ground water. Calcite [stalactites] hang from the ceiling, while [stalagmites] grow up from the floor. (Cave of the Winds Company photo)
In the [Plateau] Province another type of cave is formed not so much by groundwater as by weathering of the flat-lying alternating beds of massive resistant sandstone and less resistant, thinly bedded mudstone and shale. Where the resistant layers are undermined, great arching caves develop. These are best observed at [Mesa] Verde National Park, where many of them once sheltered Indian communities. They can also be seen in Colorado National Monument and along the Colorado River and several of its major tributaries.