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-, <br />I <br />2.0 GENERAL GEnLOGIC SETTING <br />~•- - The area near the site (Figure 1) is characterized by gently rolling .hills; underlain <br />•~ by shales, inter, upted by near-vertical cliffs composed of sandstone. At the site, .he 8i~-foot . <br />~ • resistait'fwentymile Sandstone cliff dips gently northward. and forms a cuesta jus. north ef. . <br />t <br />a the northeast-flowing Foidel Creek. <br />I <br />2.1 General Stratiaraohv <br />The general stratigtaphy in the area is presented in Figure 2 (Robson and Stewart, <br />1990). The Cretaceous Williams Fork Formation is composed of interbedded shale, <br />siltstone, mudstone, sandstone, and coal. The lower member of the Formation is composed <br />of approximately 350 feet of interbedded coal, siltstone, sandstone and shale. The coal <br />bearing units are, from oldest to youngest: (1) Wolf Creek Seam, (2) Wadge Seam, and (3) <br />l Lennox Seam. <br />1 <br />Overlying the coal units is the middle member of Williams Fork Formation. It is <br />• approximately 600 feet thick in the general azea and is composed of mudstones, shales, and <br />_ sandstones. This stratigraphic unit underlies the talus slope at the base of the sandstone cliff. <br />Overlying the shales and mudstones of the middle member is the Twentymile <br />Sandstone member. The sandstone at the site is a porous moderately-to well-sorted fine- <br />grained quartz azenite. Characteristically, it is composed of thin sandstone beds and locally, <br />it is cross-bedded. In outcrop, the weathered tan-colored sandstone appears to be massive. <br />This massive appearance is a result of the sandstone's resistance to erosion caused in part by <br />the process of case-hardening, a process where the surface of the rock is coated by cement <br />formed by evaporation of mineral-bearing fluids. The case-hardened rind ranges in thickness <br />from less than one-inch to about one foot. A freshly broken surface shows that the sandstone <br />is white to light-gray and friable. <br />1 Overlying the Twentymile Sandstone is the upper member of the Williams Fork <br />Formation. It is approximately 300 feet thick in the area of the site and is composed of <br />~• z <br />