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<br />shale contains a high percentage of salts; the high salt..content is
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<br />home out by the man)' \~hi to patches of alkali on both irrigated anJ
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<br />noni.rir.ated surfaces.
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<br />T!\c type of salts l'resent in the shalo arc
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<br />nppar(ll1t.ly mostly calci\lIn sl.11fn'Co \o/i'th tin\allo1." wnounts or sodium
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<br />chloride, sodium sul fate ,I :md magnosium sul fate. The ovidence that
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<br />calcium sulfate is tho mo.~t common salt is verified by the exist-
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<br />ence of the mineral gypsum co""nonly found in crystal form in open
<br />joints and fractures 'of the Mancos Shale.
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<br />Duo to the compactness of. ::1\e clay and silt particles makine up
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<br />the s~1:l1e, tho formation, is not considereu as water bearing at depth.
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<br />Howevor the weathered zone near the surf:lcc docs transmit small
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<br />.qu:lntities of water alone' joints, fractures, and open bedding planes.
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<br />This zone is undoubtedly'the area from which percolating water,
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<br />often originating from irrieation of croplands, dissolves out salts
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<br />present in the shale. Since the gre.atest amount of the soils fonnine
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<br />the irrigated lands have been derived fl'om ~Iancos Shale, the soils
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<br />are also a source of salinity. A gravel and cobble layer also has
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<br />reportedly been found under some of the irrigated areas in the Grand
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<br />Valley and is believed to .serve as an aquifer for eround I~ater.
<br />Previous studies have iderid.fied areas where the ground water has an
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<br />upward pressure gradient in the cobble a'1uifer due to the confining
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<br />effect of the ~lancos Shale beneath and the tight clay soil above.
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<br />This situation is believed to be responsible for some areas of high
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<br />\Vater tables. Further s'.:udies of the cobble aquifer will be neces-
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<br />.sary to determine its extent and its influence on the. r,"J:ound I,ate..
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<br />1II-21
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