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July 1993 <br />-6- <br />923-2515 <br />3.0 GEOLOGY OF THE DR4iNAGES <br />The geologic characteristic;. of each drainage are presented in this section. Each drainage is <br />discussed beginning at the headwaters and progressing to its confluence with a higher order <br />drainage. Figure 1 presents the geology of the drainages as discussed in Sections 3.1, 3.2, 3.3 <br />and 3.4. <br />3.1 Scotchman's Gulch <br />The headwaters of Scotchman's Gulch are situated on the eastern flanks of Iiillberry Mountain. <br /> <br />Typical of most drainages in the area, the headwaters of Scotchman's Gulch occupy narrow <br />clefts between hillocks. These primary, ephemeral streams exhibit a steeper gradient than their <br />higher order counterparts. The slopes adjacent to these streams are mapped as colluvium. The <br />streams aze primarily erosional. Accordingly, only the narrow channels in the headwater area <br />have been mapped as undifferentiated valley fill. <br />Downstream from the headwaters, Scotchman's Gulch is characterized by broad areas of <br />undifferentiated valley fill (approximately 185 acres) with an isolated section of alluvium. The <br />composition of the alluvium is typically silty clays and clayey silts. These areas of alluvium are <br />distinguished from the undifferentiated valley fill by their marked planarity, stream <br />characteristics (e.g. meandering) and to a lesser extent by changes in vegetation with respect to <br />the neighboring slopes. <br />An alluvial fan occurs in this reach of Scotchman's Gulch (SW~kSE~/., sec. 32, T. 6 N., <br />R. 87 W.). This is a relatively lazge alluvial fan covering an azea of approximately 15 acres. <br />Slope gradients of this alluvial fan, approximately 0.5%, aze slightly less than typical of other <br />alluvial fans in the area that range from approximately 1 % to 5%. The slightly irregular surface <br />of this alluvial fan suggests that it may have been reworked since deposition. . <br />r <br />Golder Associates <br />