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mudstones, silts!ones, and calcareous shales. Some.of the cement for the case-hardened '~1• sandstone prohably is derived fr ~m this unit. Localiy, the member contains coal seams near its top; some of the coal has bee.i. striF-mined nearby. • This; upper member overlying the 1 sandstone at the site constitutes t+e-Founded, gentler slopes at the top of the cliff. 2.2 General Structural Geology The major reverse faults and northwest-southeast plunging asymmetrical folds in the general area occurred during the late Cretaceous Laramide Orogeny. Eocene deformation modified these Cretaceous faults and folds and produced normal faults. 1 The Twentymile Sandstone at the site has a generally uniform dip of 3' to 10' northwestward. Locally, near the western edge of the study area, the base of the sandstone outcrop appears to be moderately distorted with dips approximately 10' southeastward and small folds that plunge a maximum of 30 degrees southeastward. The rocks in this • anomalous area may have been structurally affected by an arcuate rock failure along the sandstone cliff. Alternatively, the distortion may be a manifestation of deformation associated with an inferred northwest-striking buried fault near the location where County • Road 27 passes through the eroded notch of the sandstone. The sandstone in the azea is cut by several northwest-striking faults or zones. Locally, the faults or zones are sites of erosion that form re-entrants into the cliff face. Elsewhere, re-entrants do not occur. In some locations a relatively high frequency of joints and fractures are associated with these faults or zones, whereas, in other areas, the high joint frequency is not obvious. Seven strong joint sets and other irregular fractures have been developed in the sandstone. They will be described in the sections on Engineering Geology (3.0) and Joint J Mapping (4.0). l • 3