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<br />w <br />~_.. <br />en <br />f-- <br /> <br />i <br />, <br />I <br /> <br />Prominent pediment remnants dominate the 800k Cliffs piedmont. They <br />radiate away from canyon mouths in roughly fan-shaped patterns. The <br />highest remnants tend to have a long, narrow, sometimes sinuous shape in <br />plan view. The lower-lying remnants, which may not have been subject to <br />dissection for as long as the higher remnants, retain a fan shape. At <br />places near Grand Junction and Price there are as many as 5 levels of <br />pediment remnants preserved above the streams draining from the Book <br />Cliffs. The average gradient of pediments in the Price area is 4%. <br />The smooth gently sloping surfaces that sweep away from the Book <br />Cli ffs convey an impression that the bedrock-gravel contact is also a <br />smooth plain, but in fact the gravel thickness is highly variable and <br />the bedrock surface is very irregular (Fig. 3-8). Apparently the <br />irregular Mancos Shale surface was either buried by debris flows that <br />were triggered by mass failure of the 800k Cliffs during periods of <br />higher precipitation or stream capture rejuvenated drainage basins which <br />provided large amounts of gravel that buried the irregular topography <br />of the piedmont badlands to form the smooth gravel surfaces of the <br />pediments. <br />Up to 40 m of partially cemented calcitic alluvium caps the pediment <br />remnants. This alluvium contains some extremely coarse gravel which is <br />derived from the Mesa Verde Group, and it is mixed with finer sediment <br />from the Mesa Verde and younger formations. There are boulders as large <br />as 5 m in maximum diameter and in one sample locality, the mean diameter <br />of the ten largest clasts was 1.95 m. One or more zones in these gravel <br />caps are cemented by calcite, so that the gravels act as a caprock <br />protecting the underlying Mancos Shale from erosion. 9Jrrounding and <br />encroaching upon the pediment remnants are active pediments that are <br />currently forming. Rills, sheetwash and creep also erode the pediment- <br />remnant side slopes undercuting and reducing them in area. <br /> <br />,;\ <br /> <br />,i: <br />:~ <br /> <br />':1 <br /> <br />':1 <br /> <br />"l <br />, <br /> <br />Geomorphic Stability <br /> <br />Pediments are relatively stable surfaces. Their surface slopes are <br />too gentle for rill formation, and the gravel cap acts as a protective <br />armor. They support a much greater vegetative cover than the badlands, <br />due in part to deeper soil development and higher infiltration rates <br />than in the badlands (U.S. Bureau of Land Management, 1978). However, <br /> <br />49 <br />