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4.0 GENERAL GEOLOGY <br />. The general geology of the area has been well covered by the U.S. <br />Geological Survey (Bass, et, al., 1955; and Goffke, T.M., 1979), Peabody Coal <br />Company (1990), and Norwest Mine Services (1991). These reports adequately <br />cover general aspects of the geology in 005 Gulch and vicinity; therefore, this <br />report will be confined largely to a discussion of specific aspects of geology <br />in the gulch as they relate to engineering characteristics of the units mapped. <br />4.1 Rock Units <br />The entire area studied is underlain by the Iilliams Fork Formation, the <br />upper of two geological formations in the Mesaverde Group of Cretaceous Age. <br />In the area mapped the Williams Fork Formation is divided for purposes of <br />this investigation into what are referred to as the "Lower Units," and the <br />"Twentymile Sandstone and Above." The regional distribution of these map units <br />is shown on Drawings 9111-1 and 9111-2. <br />4.1.1 Lower Units. These consist of shale, sandstone and coal. The <br />shales are of marine origin and are the predominant rock type in 005 Gulch and <br />vicinity. Sandstone appears to comprise less than 10% of the total rock <br />section below an upper sandstone immediately below the Twentymile Sandstone. <br />The shales below the Twentymile Sandstone form the least stable rock units <br />in the area. Failures occur in rock sections composed primarily of shale, but <br />with some interbeds of sandstone. In the limited field work that was done, no <br />indication was found that significant amounts of sandstone were involved in any <br />area where failure had occurred or• seemed likely to occur. My estimation is <br />that in areas of failed ground no more than 10;6 of the thickness of the rock <br />section involved is comprised of sandstone layers of up to 6" in thickness. <br />7 <br />