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10 <br /> These straths are located approximately 90 and 170 feet above the river. <br /> They have been eroded from shale and siltstone containing thin interbeds <br /> ' of fine to very fine, silty sandstone. The overlying alluvium has a maxi- <br /> mum thickness of approximately 35 feet and is composed of predominantly <br /> silt and sand containing pebbles and occasional clasts up to 20 inches in <br /> diameter. Deposits of caliche are common. The coarse clasts are predominantly <br /> igneous and metamorphic lithologies. The overall color of the alluvium is <br /> a medium dark reddish brown and is considerably darker than the modern al- <br /> luvium found below. Drill holes and backhoe pits encountered no ground <br /> 1 <br /> water in the strath terraces and prominent springs near the alluvium-bedrock <br /> ! contact were not apparent. These deposits are not hydrologically connected <br /> �] to the alluvial terraces juxtaposed to the Purgatoire. <br /> Two levels of alluvial terraces were identified adjacent to the river. <br /> These terraces are located 5 to 10 feet and 40-50 feet above the Purgatoire <br /> j in the study area and have also been investigated by Powell , 1952. They are <br /> coMposed of relatively unweathered, gray to light tan alluvium overlain by <br /> medium to dark brown sandy overbank deposits. Powell (1952) has found <br /> buried channels that were incised into the underlying Raton formation and <br /> then filled with alluvium. These terraces support typical hydrophytic vege- <br /> tation characteristic of floodplains and contain ground water hydrologically <br /> connected to the river. Although absolute dates on the alluvial deposits <br /> are not available, the use of analogs from the Rocky Mountain Region suggest <br /> that the strath terraces are of Pleistocene age and the alluvial terraces <br /> of Holocene age. <br /> From the perspective of this report, the significance of the terrace <br /> materials is relative to their hydrologic function within the study area; <br /> a subject discussed in more detail in the section dealing with ground water. <br /> HYDROLOGIC SETTING <br /> The Purgatoire River, tributary to the Arkansas River, is the major <br /> surface drainage feature of the area. The headwaters of the Purgatoire <br /> are located on the eastern slope of the Sangre de Cristo Range a few miles <br /> west of the study area. The three major branches of the river, the North <br /> Fork, Middle Fork, and South Fork, all converge within or just below the <br /> I <br />