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<br />0::"' <br />C'-J <br />"- <br />~ <br /> <br />C> <br />((C) ,2. <br /> <br />(c <br /> <br />lc <br /> <br />2 <br /> <br />'- ~-' <br /> <br />REGIONAL GEOLOGY <br /> <br />2.1 Site Physiography/Topography <br /> <br />Regionally, t~e .site is located within the Colorado Plateau <br />Physiographic Province. More specifically, the site is located <br />in the Grand Valley just north of the Uncompahgre Plateau and <br />south of the Book Cliffs as shown on Figure 2. The Grand Valley <br />is characterized by several levels of long, deeply dissected <br />pediments which sweep down from the Book Cliffs towards the <br />Colorado River (Sinnock, 1981). <br /> <br />The physiography of the site is typical of the Grand Valley. The <br />site is characterized by two broad, relatively flat pediment <br />deposits that have been dissected by several intermittent <br />streams. The pediments to the east appear older than those to <br />the west since they are located at a higher elevation and <br />distinct bluffs over 100 feet high border this eastern pediment <br />on the north, east, and west (Plate 1). It appears that numerous <br />changes in the drainages entering the site left the higher <br />eastern pediments with a very small watershed, hence protecting <br />them from the larger flows that ultimately resulted in both <br />deposition on the western pediments and erosion of pediments <br />adjacent to these major bluffs. <br /> <br />Presently, rainfall in the area is relatively low averaging 8 to <br />10 inches per year (U.S.D.A., 1978). Stream flow is <br />intermittent, generally occurring in the spring. However, sudden <br />and sometimes violent summer thunder storms can cause larger <br />flows which can cause erosion. This has been observed primarily <br />in the major drainages on the western part of the site, where a <br />number of stock ponds have been constructed. <br /> <br />2.2 Description of Stratigraphic Units <br /> <br />The site is covered by recent Quaternary deposits that consist of <br />pediment materials, alluvium, colluvium, and residuum. These <br />deposits range in thickness from 2 to 31 feet in the boreholes <br />drilled on-site. Typically, the pediment materials are thicker <br />as one moves away from the streams dissecting them. Whereas the <br />residuum, alluvium, and colluvium deposits are typically only 2 <br />to 10 feet thick. <br /> <br />Beneath these recent deposits is a considerable thickness of <br />sedimentary rocks. The following descriptions of these <br />sedimentary units and the crystalline basement beneath them is <br />taken from Lohman (1965) and is given in order of increasing <br />age: <br /> <br />o Mancos Shale--grey to olive black marine shale with a few <br />thin beds of sandstone near the base and containing a few <br />thin beds of limestone. The thickness is variable <br />between 3,000 and 4,000 feet. In the area of the site <br />the thickness varies between 500 and 1,400 feet. <br />