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n:\ l' ~, \' IJU .'1...... Although the formations mentioned generally are permeable, they may become useless as aquifers at groat depths beneath the land surface because their contained water generally is highly mineralized. Conversely, they generally become of increasing importance closer to the surface because depth is a major factor determining the cost of developing a grounc:i-water supply. The greatest potential supply generally is concentrated in the areas where the thickest section of water-bearing units lie within the depth zone of fresh water. All the fOX'llJations mentioned underlie the center of the Denver basin where the Denver formation, of Lllte Cretaceous anc:i Paleocene age, is exposed. Only thoGe above the Pierre shale of Late Cretaceous age, however. contain potable water. Radially out from the center, successively older formations are exposed, and the fresh-water-bearing zone above the Pierre shale becomes thinner. The pierre crops out to the west within a relatively short distance from the center of the basin; whereas, it appears at tlle surfaca to the north. south, anc1 east at SOl!lewhat greater distances. Nearly half of the northern twe-thirds of eastern Colorado bas pierre shale exposed or has it underlying the surface at depths generally less than 400 feet. The formation contains little fresh ground water, and, throughout most of Colorado the water in underlying permeable formations is of a quality unsuitable for most uses. Where the Pierre is not overlain by younger water-bearing deposits, grounc1 water is scarce. In the southern one-third of eastern Colorado, the Dakota and Cheyenne are tbe major consolidated aquifers. The younger aquifers are 2lbsent, and the Pierre shale 1s present only in a emall part. 'l'he ];lakota and Cheyenne yield moderate to large quantities of ""ater to wells near the Kansas line and near Pueblo, but their potential yields appear to be generally less west of Prowers and Baca Counties. B~ver, the Dakota sandstone is capable of yielding at least enough water to wells to supply domestic and stock needs in most places in the southern one-third of eastern Colorado. In the southeast corner of the state, rocks of Jurassic and Triassic age contain some fresh water, but the area of occurrence is small. west of the Front Range, the principal aquifers are of cretaceous, Jurassic, or Triaseic ages. These aquifers may yield small t.o moderate supplies of ,,,ater to wells. Older rocks generally are nonwater-bear ing or contain water too mineralized for most uses. The Dakota sandstone of cretaceous age is probably the most extensive and best known aquifer. The !.JanCOS shale of Late Cretaceous age, a thick, generally nonwater- bearing formation similar in character '1;0 the Pierre shale of Late Cretaceous age, overlies the Dakota sandstone and is at or near the surface in large areas in western Colorado. In these areas the underlyi.ng aquifers lie at various depths, and in some areas wells must be comparatively deep (more than 3.000 feet in son~ places) to obtain sufficient quantities of wuter. III some places, hO,","ever, the deep water is salty.