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r," . i (I' U \J.L 1. J , 20. " Although the nam~d.formations generally are permeable wherever found, they become unimportant as aquifers when 'they occ,ur at great depths beneath the surface because the water in them is highly mineralized. Conversely, they generally become of increasing importance the nearer they are to the surface because depth is an important factor determining the cost of developing a ground-water supply. The greatest potential supply generally occurs in the areas where the thickest section of water-bearing units lie within the depth zone of fresh water. All of the named formations underlie the center of the Denver Basin where the Denver formation, the youngest of the group, is exposed at the surface. Only those above the Pierre shale, however, contain potable water. Radially out from the center, successively older formations are exposed at the surface and the fresh-water bearing zone above the Pierre becomes thinner. The Pierre surfaces to the west within a relatively short distance from the center of the basin, whereas, it appears at the surface to the north, south, and east at somewhat greater distances. Nearly half of the northern two-thirds of eastern Colorado has Pierre shale exposed at the surface or has it underlying the surface at depths generally less than 400 feet, The formation contains very little fresh ground water, and throughout most of the area, the water in underlying permeable formations is of a quality uhsuitable for most uses. Where younger water-bearing deposits do not overlie it, ground water is scarce. . .' In the southern one-third of eastern Colorado the Dakota and Cheyenne are the most important consolidated aquifers. The younger aquifers are absent and the Pierre is present only in a small part. The Dakota and Cheyenne are known to yield moderate to large quanti- ties of water near the Kansas line but their potential yields appear to be generally less west of Prowers and Baca Counties. However, the Dakota sandstone is capable of yielding at least enough water to supply domestic and stock needs in most places in the southern one-third of eastern Colorado. In the southeastern 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 Creta- ceous, Jurassic, or Triassic age. These a,quifers may yield small to moderate supplies of water. Older rocks generally are non-water- bearing or contain water too mineralized for most uses. The Dakota sandstone of Cretaceous age is probably the most extensive and best known. The Mancos shale of Cretaceous age, a thick, generally non- water-bearing formation similar in character to the Pierre shale overlies the Dakota sandstone and is at or near the surface in large areas in western Colorado. In these areas the underlying aquifers occur at varying depths and in some areas welts must penetrate a considerable thickness of ma- terial (more than 3, 000 feet in some places) to obtain - 4 - , I .