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<br />10 <br />These straths are located approximately 90 and 170 feet above the river. • <br />l 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 occasions] 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 />I 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 />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 overbook 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 around 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 />