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which is underlain by bedrock formations. The colluvium in densely <br />vegetated areas is usually mixed with, or lies below, humus derived • <br />from the decay of the vegetal matter. This complex relationship <br />has been simplified in the geologic mapping by including the <br />colluvium within the formations from which it has been weathered. <br />Locally where colluvium is well exposed it has been noted on the <br />maps. <br />3.3 Geologic Structure <br />Structural features mapped include strike and dip <br />determinations, folds, faults, and alignments. These features <br />control the stability of natural slopes, cut-slopes, and through- <br />cuts, particularly in areas of more coherent bedrock such as shale <br />and sandstone. In places where bedrock is deeply weathered the • <br />effects of structure usually are diminished. <br />Strike and dip determinations were mapped in the field and on <br />the aerial photographs by means of photogeology. The field- <br />measured dips are limited in number and are of primary use in <br />confirming the dip values estimated photogeologically. <br />Both field measured and photogeologic dips are most easily and <br />accurately mapped in the Williams Fork Formation where resistant <br />sandstones facilitate estimation and measurement of strike and dip. <br />In the outcrop area of the Lewis Shale, reliable strike and dip <br />information generally is obscured because of the relative non- <br />resistance of shale to weathering. Sandstone and relatively hard, <br />dense shale found near the base of the Lewis above its contact with <br />the Williams Fork Formation locally make it feasible to accurately • <br />estimate and measure dips. A good example of this is in the east- <br />14 <br />