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10 • <br />or K-horizons are unconmwn. B_y co~arison_with ~ils_developed on the <br />high plains of northeastern Colorado ~1alde 1955, Scott 1960, 1963, and <br />Baker 1973) and those of_the Yellowstone_area (Richmond 1972), izt is <br />apparent that soils of the northern high plains are dominantly those that <br />have been forming in situ only during the last 10,000 to 20,000 years. <br />Well developed zonal soils with clay-rich textural B-horizons are developed <br />on some higher river terraces and portions of higher land surfaces but most <br />of these early Wisconsin and pre-Wisconsin soil relicts have been eroded <br />during the latest glacial and/or post-glacial times. Remnants of many <br />of these relict soils can be found today at the bottoms of swales and <br />along river floodplains after transportation and deposition. <br />A common characteristic of regional soils is a concentration_of clay_ <br />sized materials in the top 7 to 15 an of the soil column. Here clay <br />contents may approach 40 percent by weight with little or no clay-bulge <br />below that depth (U.S. Bureau of Land Management <br />1974). Soils on higher terrace~o~ gently slop~_alluvial fans may <br />have a thin oxidized and slightly organic rich surface clay-loam horizon <br />over up to 60-inches of eolian clay-loam subsoil. It is clearly apparent <br />that the dominant mode of origin of clay-sized fractions in the northern <br />high plains soils is wind activity and is also apparent that periods of <br />active wind erosion and transportation have occurred several times during <br />the last 20,000 years of late full-glacial and post-glacial time. Few <br />soils survived pressures of wind erosion during the last Wisconsin glacial. <br />