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• A(lavia! Va[f Floor Irtvesli anon <br />of borehole logs and aerial photos have not revealed surficial evidence of any large scale faulting <br />within the Lorencito permit area. The La Veta Syncline bisects the site with the location of the <br />axis located roughly []trough Lorencito Canyon. Dips on the coals average 1.6° toward the center <br />of the syncline. <br />Gravity driven weathering processes ranging from mass wasting to fluvial erosion have occurred <br />subsequently during the Quaternary period. Colluvial deposits have developed adjacent to steep <br />terrain, resulting in poorly sorted deposits of angular materials in a matrix of gravels, silts, and <br />clays close to their source. These deposits have been periodically reworked by surface water <br />runoff, resulting in smaller, more rounded, better sorted materials in a matrix of gravels, silts, and <br />clays at some distance from their source. Unconsolidated, weathered deposits in the Lorencito <br />tributaries and in Cow Canyon are better defined as coiluvial deposits due to their proximity to <br />steep terrain, the short distance of travel from the source and the limited workiag of materials by <br />fluvial forces. <br />Quaternary unconsolidated deposits have filled the Purgatoire valley and Lorencito Canyon <br />(Powell, 1952). During early Pleistocene time, the Purgatoire River and some of its tributaries <br />entrenched channels in the bedrock surface. Later in Pleistocene time, the valleys of these streams <br />were filled with floodplains of sand and gravel derived from mountains to the west to levels higher <br />than seen presently. A period of weathering and erosion followed, removing much of this <br />material. The Quaternary terrace deposits shown on Map 2.04.7-2 are all that remain of the initial <br />depositional period. The stream channels in Cow and Lorencito Canyons are incised two-to- <br />twelve feet. The depth of downcutting is controlled in many places in Cow Canyon and the <br />tributaries to Lorencito by the presence of heavily indurated, resistant sandstone layers. The <br />Purgatoire is also incised two to four feet. <br />Powell's (1952) drilling in the Putgatoire River alluvium noted 24 feet of silty shale and sandy <br />cobbly gravel south of the river at the Cordova site, and S to 12.5 feet of alluvium at the Lorencito <br />access road crossing, Powell (1952) noted the presence of sandy gravelly clay in the Purgatoire <br />River alluvium which was thickest towards the borders of the streamlaid deposits and thinnest <br />adjacent to the river. He attributed this layer to slope wash from the canyon sideslopes (Figure <br />AV-I). In general, the thickness of streamlaid deposits was least on the perimeter and greatesr <br />adjacent to the channel. Variability in thickness (Table AV-1) was noted for drainages in which <br />there were multiple sites. <br />The width of the unconsolidated deposits and sinuosity of the stream were also used to characterize <br />the maturity of the deposits. Table AV-1 documents the width of the Purgatoire River and <br />Lorencito, Cow, Alamosa, Chimney, Little Pine, Puertecito and Jeff Canyons, The Purgatoire <br />River and Lorencito Canyon are significantly wider than the rest, and, qualitatively, show greater <br />sinuosity. Cow Canyon and Alamosa Canyon are narrower, but show different sinuosity from one <br />another. The stream channel in Cow Canyon does not migrate widely, due to its location in the <br />headwaters of a large mountainous drainage of the Purgatoire River and in pan to its bedrock <br />• control. The stream channel in Alamosa Canyon is more sinuous, suggesting less bedrock control. <br />Avt4751December IJ, 1996 3 <br />