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<br />5 <br /> <br />thickness of this oldest alluvium is 150 feet, counting the soil <br /> <br />profile on top. It is estimated to be between 10,500 and 500,000 <br /> <br />years old. <br /> <br />Lying above this oldest alluvium is a pink alluvial sand <br /> <br />(Broadway). Mechanical analyses of grab samples indicate that it <br /> <br />contains about 10% pebbles, 15% granules, 45% sand, and 30% silt -- <br /> <br />all well sorted. The maximum thickness is about 40 feet. A bed, or <br /> <br />beds, of clay may separate these two alluvial deposits. <br /> <br /> <br />A third alluvial deposit.of light brown silty sand (Bijou Flats <br /> <br /> <br />tongue) overlies or is partially interfingered with the pink alluvium. <br /> <br /> <br />These upper beds could have been deposited as. late as 6500 years ago. <br /> <br />These Bijou Flats beds are thought to have been derived from the <br /> <br />drainage areas of Kiowa and Bijou Creeks, in contrast to the older <br /> <br />alluviums derived from the Rocky Mountains. The permeability of <br /> <br />these brown silty sands is much less than that of the coarser alluvial <br /> <br />deposits below. <br /> <br />In most places, all of the deposits mentioned above are <br /> <br />covered with windblown sand and silt. <br /> <br />WINDBLOWN DEPOSITS (LOESS) <br />A yellowish brown sand is the most widespread surficial deposit <br /> <br />along the South Platte River in northeastern Colorado. It overlies, <br /> <br />unconformably, all ~~e older alluvium and the Pierre Shale. On <br /> <br />Bijou Flats this windblown sand is generally several feet thick but <br /> <br />can range up to 50 feet in ~~ic~ness. Elsewhere in the area surrounding <br />