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Corn Constr ion Company 32 Road Pit • May 10, 1979 Page -2- • The gravel deposits along this stretch of the Colorado River are underlain by the Mancos Shale Formation. In general, this formation is a drab gray, dense, silty marine clay which serves as the bedrock for the majority of the Grand Valley. The Mancos Shale is several thousand feet thick in the project area and local weathering and erosion of this shale has produced the majority of the surface soils in the Grand Valley. SITE GEOLOGY: The 23 Road Pit is adjacent to the north bank of the Colorado River and is within the floodplain for the extreme flood flows. A moderately developed soil sequence covers the gravel deposit throughout this area. This soil is a very silty clay, which has been subjected to agricultural working over much of the pit area. These soils are of variable thickness, ranging from 2 to 8 feet thick. The actual gravel deposit is from 9" to 30' feet thick and is somewhat erratic due to the deposition mode of the river regime. Ground water elevations will vary some- . what depending on the level of the Colorado River and local crop irrigation. In general, the water level should normally vary fro m 8 to 14 feet below the ground surface. Generally speaking the seasonal variation will be greater the further north, and away from the river, the measurements are made. Dewatering the pit area will be necessary if loader excavation is used, however large bucket hoes are commonly used for these shallow pits and dewatering is not a factor. If the wet pit extraction method is used, no substantial change in the groundwater flow or elevation characteristics is antic- ipated. AGGREGATE QUALITY: No laboratory i~ ting has been performed by this laboratory, but field perfo rmance indicated the aggregate is sound and of sufficient quality for road base, general fills and for asphalt pavement construction. The topsoil can be used as general fill material or can be used for agricultural soils. Some areas may contain large amounts of organic materials in the upper part of the profile and should not be u~d for structural or general construction fills. • If any questions arise, please do not hes~ate to contact this laboratory at any time. Respectfully submitted, LINCOLN-DeVORE TESTING LABORATORY, INC. EMM/ s j c ~~~~~~-"-~ t-5~n Edward M: Morris