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VIl-5 above river level and 20 feet below the Broadway terrace. They are wider and more continuous on the east side of the river where they may be as much as 1,200 feet wide. On the west side of the valley these surfaces are narrow and discontinuous. fine-grained, humic, sandy silt and clay containing very thin beds and lenses of very coarse-grained, sandy, granular, pebbly gravel and organic debris such as sticks and roots. The sand is generally subrounded to well- rounded and composed of mostly quartz, feldspar, chert, and granitic rock fr agment s. Alluvial fan deposits are associated with these straths. The deposits are generally less than 4 feet thick, but may be thicker locally in the fans. The underlying materials generally are different on each side of the South Platte valley. On the east side these deposits represent a period of ero- sion and later alluviation of the Broadway materials; hence the underlying deposits are Broadway sand and gravel. On the west side of the valley, ero- sion of a variety of older surficial deposits, and bedrock, occurred so that the discontinuous straths along the west side are underlain by a variety of materials, The colluvium consists of grayish-orange and light-gray, unstratified, unsorted sand and claystone fragments (derived from the Denver-Arapahoe formations) with occasional pebbles derived from the Verdos and Slocum gravels intermixed with material derived from the older eolian sand and silt deposits. The thickness generally does not exceed 3 feet. In addition to poor sorting and lack of stratification, the colluvium is characterized by a lack of soil development. Younger Eolian Sand and Silt. Extensive deposits of wind blown sand and silt are found southeast of the South Platte valley. The material appears to have formed by the weathering and disaggregation of Denver-Arapahoe sandstones and claystones. The processes of creep and slopewash have moved loosened debris downslope to a greater or lesser degree. The deposits mantle portions of the Broadway terrace. The thickness of these deposits varies from place to place, in part due to their deposition on an irregular surface and in part due to later erosion, since the earlier deposits (at least) were eroded by Second and Third Creeks during the Piney Creek cycle. The younger deposits only partly refilled these eroded valley systems, in spite of a fairly rigorous period of eolian activity. Dune forms in the shallower portions of Barr Lake are visible in aerial photo- graphs taken during a low level of the lake. In general, however, these deposits rarely exceed 35 feet in thickness. Miscellaneous Surficial Deposits. A number of well defined small alluvial fans are being built out onto the Broadway terrace and onto creek floodplains where minor gullys issue onto areas of lower gradient. A great variety of man-made deposits are found in this area. The more important types include: Post-Piney Creek Alluvium. The deposits found within the modern floodplains of the South Platte River and the alluvial materials along the tributary creeks are Post-Piney Creek. (1) artificially placed fills of varying compositions including irrigation canal levees, small earth dams, railroad and highway fills; (2) large volumes of fecal wastes derived from cattle feed lots; (3) sanitary 1 andf ill s; and (4) topsoil fills dumped into abandoned gravel pits. Because the alluvium lies in valleys which were cut into a variety of litho- logic units, its composition and texture vary considerably from valley to valley. Along the South Platte the alluvium is primarily very fine to