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3.3 Twentvmile Sandstone <br />~• <br />The Twentymile Sandstone is the primary source of rock falls in the study area. <br />l The outer weathered surface is hard due to a thin veneer, or rind, of cement. This rind of <br />case-hardening allows the sandstone to stand as a cliff. The freshly broken sandstone is <br />porous, permeable and friable. Field observations indicate that water flows through the <br />porous sandstone, as evidenced by filaments of white residue characteristic of evaporating <br />mineral-rich waters on the rock surface. <br />The process of case-hardening is currently active and has implications for <br />observing joints in the field. Detailed observations where the cliff face appears to be <br />1 uniform and devoid of joints, indicate hairline cracks with apparent dips approximately <br />parallel to those of well-defined, observable joints. Locally, joints could be followed along <br />the cliff face to points where they could not be seen. However, a short distance beyond <br />these points, hairline cracks could be seen. The rock surface appeared uniform and <br />,• unbroken between the hairline cracks and a joint. We interpret this phenomenon to be a <br />result of case-hardening where the water flowed through the porous rock and deposited the <br />cement in the joint and in the interstices of sand grains along the porous rock face concealing <br />the joint. Extrapolating this interpretation to the cliff face in general, large areas of apparent <br />l uniformity (not jointed) may be misleading. The case-hardened surface may be concealing <br />1 the joints. In effect, the Twentymile Sandstone may contain more joints than those observed. <br />Other implications would be with respect to flow of water in the sandstone. Water could <br />flow between the mineral grains, as well as through joints and fractures. <br />1 The general dip of the Sandstone ranges from about 3' to 10' northwesterly <br />(Figure 6). This dip is into the cliff along the south-facing cliff face and the dip does not <br />have an adverse orientation with respect to the cliff. However, locally, near the western <br />edge of the outcrop north of the curve in the road (see Figure 3), the dip changes to 10' <br />1 southward and is apparently folded with plunge axes southward. These orientations are <br />adverse with respect to the cliff face. If these dips are related to tectonic activity and not to <br />j• 6 <br />