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]uly 1993 -4- 923-2515 • <br />2.2 Field Verification and Geologic Units <br />1'he aerial photographic interpretation of the surficial geologic units was field verified by a <br />Golder erigineering geologist between October 4 and October 7, 1992. Field verification of the <br />aerial photo interpretation involved traversing the area on foot and locating geologic units and <br />contacts by direct observation. Geologic contacts located during the aerial photo interpretation <br />were then modified, where necessary, based on field observations. The surficial geologic units <br />described below were observed in the study area; no samples of geologic materials were <br />collected. Photographs representative of the following geologic units are presented in Appendix <br />A. <br />Quaternary alluvium (Qa): Alluvium is broadly defined as stream deposited <br />unlithifted terrestrial sediments comprised of gravel, sand, silt, and clay-sized <br />particles. In the study area alluvial deposits mapped were typically homogenous, • <br />well-sorted silty clays to clayey silts. The alluvial deposits exhibited <br />morphological and compositional characteristics associated with low-energy <br />alluvial deposition. This unit was identified primarily on the basis of <br />morphology. Principally, very flat, broad valley floors and sharp breaks in slope <br />between the valley floor and valley walls were used to define alluvial deposits. <br />In addition, meandering streams were used as an indicator of stream (alluvial) <br />deposition. <br />Quaternary undifferentiated valley fill (Qvf): This unit was mapped where the <br />._ alluvium could not readily be distinguished from colluvium (i.e. no distinct break <br />in slope or lithologic differences were apparent). While genesis of deposits in <br />these areas may have an alluvial component, a clew separation between colluvium <br />and alluvium could not be discerned. Another morphology mapped as <br />. undifferentiated valley fill includes primary tributaries occurring at the headwaters <br />of drainages where stream gradients are typically greater. These primary streams <br />exhibit steep channel sidewalls and little or no terrace development. In addition, <br />very broad valley floors which did not exhibit sufficient planarity to warrant <br />being designated as alluvium were mapped as undifferentiated valley fill. <br />Quaternary alluvial fan deposits (Qf): An alluvial fan may be described as a <br />cone-shaped deposit that forms where primary tributaries of steeper gradient enter <br />- a higher order channel of lesser gradient. In the study area, alluvial fan deposits • <br />were distinguished by their roughly deltaic geometry. Since this depositional <br />Golder Associates <br />