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<br />Ol"irin and f,lcollnt of $:>l.t r.oadinr:
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<br />All sources in the Grand Valley arc estimated to contribute an averace
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<br />of 700,000 tons of sal~ ~nnually to the Colorado River. 1 It is believed
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<br />that much of these salts leached from tho soil and underlyinc M:mcos
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<br />Shale arc Hashed into the river by deep percolation. and water delivery
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<br />system losses.
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<br />~:ancos Shale is a very thick sequence of drab gray fissile shale that
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<br />lies between the underlyinl; Dakota sandstono and the overlying ~lesa
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<br />Vel'de Formation.
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<br />The tilickness of the sll.1le usually varies between 3,000 and 5,000 feet.
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<br />Due to this great thickl~ess and its easy erodibility, the shale forms
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<br />most of the large valleys of western Colorado and eastern Utah and
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<br />thus underlies the greater percentage of the irrigated lands.
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<br />The ~!ancos Shale is of marine origin and contains marine fossils at
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<br />many locations. GeoloG:c studies. suggest that the shale was depos-
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<br />ited as mud in the shallow water of a very extensive Late Cretaceous
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<br />sea and that the region'\~as gradually subsiding \~hich explains the
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<br />great thickness of the formation. Because of its marine origin the
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<br />1 Colorado River Water Quality Ir.;p:::ove;:\ent Program USDI Bureau of
<br />Rccl:1matioll :<epo:::t dat(;u rcbruary 1972.
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<br />III-20
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