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4.3 Surficial Geology <br />The Surficial mapping units used for this investigation include two <br />stream-laid or• fluvial deposits, and three types of deposits that relate more <br />to ground stability than to any other engineering characteristics. The stream <br />deposits have been laid down in the Quaternary Period. These deposits are <br />uncemented fractions ranging in size from silt to boulders and were derived <br />from older bedrock. The other three deposits that relate to ground stability <br />are the result of Quaternary processes active in the weathering and <br />gravitational modification of the lower units of the Williams Fork Formation. <br />4.3.1 Stream Deposits. Descriptions of the mapping units follow and <br />locations are related to stationing for the proposed Haul Road G as shown on <br />Drawings 9111-3 through -6. <br />Alluvium. Stream deposits including primarily clay- to fine sand-sized <br />fractions with fragments of sandstone up to four to six inches in maximum <br />dimension. Locally, sandstone fractions ar•e larger, particularly in the <br />northwestern part of the area beneath the cliffs of the Twentymile Sandstone. <br />Generally the fractions in the alluvium of 005 Gulch are governed by the <br />nature and composition of the nearby bedrock. As a result the fractions are <br />coarsest in the vicinity of the sandstone cliffs at the northwestern end of the <br />gulch near its confluence with Dry Creek. Southeastward from this area the <br />alluvium is composed of progressively larger amounts of silt to fine sand size <br />fractions with scattered blocks of sandstone ranging from less than one inch up <br />to six inches in maximum dimension. These finer fractions and scattered <br />sandstone blocks ar•e weathering products of the predominantly shale composition <br />of the lower units of the Williams Fork Formation. <br />Alluvial Fan. Deposits composed primarily of detrital silt and sand with <br />boulders of sandstone up to two to three feet in maximum dimension. <br />9 <br />