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<br />The most productive ground-water reservoirs consist of un- <br />consolidated deposits of sand and gravel. Other major water- <br />bearing materials consist chiefly of consolidated sedimentary <br />deposits, which generally are much less productive but, in some <br />places, are more extensive. sandstone is the chief consolidated <br />water-bearing material. <br /> <br />Ground water occurs in some of the dense rocks of sedimentary, <br />metamorphic, or igneous origin where fracturing, faulting, weather- <br />ing, and circulating water have created extensive cracks, crevices, <br />and solution channels that lie below the water table. Detailed <br />studies are commonly necessary to determine the extent of such <br />areas, but few have been made in Colorado. <br /> <br /> <br />The four extensive and highly productive ground-water areas <br />in the state, shown in figure 5, are the aquifers of the South <br />Platte, the Arkansas, and the San Luis valleys, and the High <br />Plains. In the South platte and Arkansas Valleys, the aquifers <br />consist of alluvial sand and gravel deposits having a saturated <br />thickness of as much as 200 feet and 125 feet, respectively. The <br />San Luis Valley aquifer consists of a series of beds of sand and <br />gravel interbedded with clay and extends to a depth of several <br />thousand feet. Fresh water extends from near the surface to at <br />least 2,000 feet throughout most of the area. The High Plains <br />aquifer is the saturated part of the Ogallala formation, Which <br />consists chiefly of sand and gravel deposits. Its saturated <br />thickness ranges generally from 100 to 200 feet. <br /> <br />Unconsolidated deposits elsewhere in the State are generally <br />thinner and potentially less productive, although, where present, <br />they commonly constitute the major source of ground water. <br /> <br />In a few places in the State, large yields can be obtained <br />from wells tapping consolidated formations: in most places, a <br />supply adequate for domestic and stock use can be obtained, but <br />in large areas underlain by thick sections of clay, shale, or <br />other dense, relatively impermeable materials, even meager supplies <br />of water of poor quality are difficult to find. The relative <br />availability of water from consolidated rocks is shown in figure 5. <br />Consolidated aquifers are the exclusive source of water for 42 <br />supplies, and are the partial source of water in 18 others. <br /> <br />East of the Front Ranqe.--In eastern Colorado, the oldest <br />major water-bearing unit is the Cheyenne sandstone member of the <br />Purgatoire formation of Early Cretaceous age. The next younger <br />major water-bearing unit is the Dakota sandstone, which overlies <br />the Purgatoire formation. Two younger formations of Cretaceous <br />age, the Fox Hills sandstone and the Laramie formation, are <br />separated from the Dakota sandstone by thick deposits of rel- <br />atively poor water-bearing material, which include the Pierre <br />shale. Still younger consolidated aquifers are the Dawson arkose <br />and the Denver formation of Late Cretaceous and Paleocene age. <br /> <br />17 <br />