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VII-4 The Pliocene period ground surface elevations at Brighton were about 1,800 feet higher than the present ground surface. Ihe following Quaternary geologic history was dominated by erosion. In the later Quaternary period erosion was represented by alluvial processes causing formations of valley fills. There were eight cycles of erosion and filling, with a ninth cycle currently in progress. The gravels are badly weathered, the pebbles are often cracked and permeable and are usually partially coated with caliche rinds. The gravels include rounded to well-rounded pebbles and cobbles of medium gray, and fine to medium-grained, quartzite; quartz; highly weathered, medium-grained granite; and weathered gneiss. Intermixed sands are subrounded to rounded, equant to elliptical, and smooth to frosted surface textures. The surficial deposits in the Adams County reach of the study area include the following: Verdos Gravels. The Verdos gravels are the oldest surficial deposits. The Verdos deposits occupy the uppermost surfaces on a line of hills lying west of the South Platte River valley, and are also found near Barr Lake. Older Eolian Sand and Silt. The upland surfaces west of the South Platte valley are extensively mantled by Eolian fine sand and silt and some clay deposits. Silt and clay sizes comprise a significant portion of the grain size distribution curve, at least to shallow depths. In this zone the clay properties dominate the soil behavior, especially when the deposits are wet. The Verdos deposits represent pediment veneers which were moved by intermit- tent, relatively high-gradient streams flowing eastward from the Front Range. The chain of gravel-capped hills paralleling the edge of the valley may represent a former course for the South Platte River 120 feet higher than the stream's present level, or channel deposits of other streams (such as Todd Creek) flowing across the old erosion surface, and now preserved because of their course size. Broadway-Louviers Alluvium. Within the Brighton quadrangle a single large terrace, in places over one mile wide, lies 30-35 feet above the river along the east side of the South Platte valley. The Verdos deposits are predominantly gravels, consisting of equant, ellip- tical, and tabular, rounded to sub-rounded granules, pebbles, and cobbles with only a few boulders. Both Louviers and Broadway deposits were produced by braided alluvial streams. The material was derived from exposed Precambrian rocks in the mountains, supplemented by materials eroded from glacial outwash in the mountains, pediment and older gravel deposits within the mountain front, and sedimentary bedrock units of the piedmont. The deposits are characterized by iron and manganese oxide staining which suggests that some of the sedi- ment was deposited in boggy or marshy areas near the river. Slocum Gravels. Thin deposits of gravels mixed with fine grained materials are occasionally exposed at elevations below the Verdos surface west of the South Platte valley. The deposits are thin, generally being only two to four feet thick. The bedforms, grading, and stratification of these deposits are characteris- tic of alluvial valley-fill deposits. Fine-grained facies represent back- water swamps, and abandoned channels, while the coarser facies represent active channel alluvial deposits. Gravel occurrences are restricted to isolated outcrops along gullys where the intermediate levels of the Slocum surface are actively being eroded, and to small areas where the overlying Eolian materials have been either strip- ped off or were not deposited. These deposits represent the prime commercial aggregate deposits. Piney Creek Alluvium. Deposits of humic sand, silt, and Clay with some smaller gravels form terraces and straths along both sides of the South Platte valley. The upper level surfaces of these deposits lie about 15 feet