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• observed in the fractures, and there was no indication of bedrock. <br />Based on other observations, the Lewis locally weathers even more <br />deeply than indicated by these fractures. <br />3.2 Surficial Units <br />Alluvium and colluvium are the only Surficial deposits that <br />occur in the area. <br /> <br />3.2.1 Alluvium. Alluvium includes primarily detrital material <br />derived from the weathering and transportation of bedrock. <br />Alluvial deposits are mapped in the major drainages where map scale <br />permits; although, virtually all streams that do not flow on <br />bedrock have developed alluvial deposits. Locally the alluvium as <br />• mapped includes other geomorphic forms such as terrace deposits and <br />small alluvial fans. The relatively small size and lack of <br />importance of these deposits makes it unnecessary to map them <br />separately. <br />The general size range of alluvial materials is silt, clay, <br />sand, and gravel. The composition of these size fractions is <br />controlled mainly by the bedrock type through which a stream flows, <br />although this may vary considerably along a given drainage. In the <br />Alluvium mapped throughout the project area the finer fractions <br />predominate. <br />3.2.2 colluvium. colluvium generally includes loose, incoherent <br />deposits that accumulate as rock weathers in place and with the <br />• weathered material generally moving slowly downslope in response to <br />gravity. colluvium is present throughout most of the mapped area <br />13 <br />