1981-11-13_PERMIT FILE - C1981013 (28)

Home Browse Search

9 review and cites nearly 100 pertinent references dealing with the topic. He provides details of the multiple erosion surfaces, peneplains and pedi- plains dating back to Tertiary. This discussion will focus only on Quater- nary aspects of landform development along the Purgatoire River. The Purgatoire River heads in the Sangre de Cristo Range south of the Spanish Peaks and flows eastward into the Raton Basin eventually joining the Arkansas River. The Purgatoire is a consequent stream whose location is not controlled by lithologic nor structural anisotropies. The head- waters of the Purgatoire are underlain by volcanic rocks and Pleistocene glacial-fluvial sediments. In its downstream course the river flows across the foothills belt of Paleozoic-Mesozoic strata, the terrestrial sediments of Tertiary age including the study area and thence across the marine Pierre Shale. Lithologic control of the geomorphology and channel geometry can be detected in the various terrains and changing water quality should like- wise be expected. Clearly, a natural deterioration of water quality can be expected in streams flowing out of igneous terrains and onto marine sediments such as the Pierre Shale. The high level erosion surfaces that have been previously studied in the region are not represented in the study area. They are situated more than a thousand feet above Purgatoire, are represented by alluvial deposits above and below lava flows and are thought to be mid to late Tertiary in agei Three levels of pediments have been named in the region (Levings, 1951) . They are truncated, gravel-capped surfaces that are slightly concave upward in longitudinal profile and are best represented on the plains east of the study area. The surfaces have been cut on relatively weak sedimentary rock and the gravels are composed of igneous and metamorphic clasts probably derived from the high level surfaces and the Raton formation. It is doubtful that current workers would use the term pediment for these surfaces as they tend to be concordant to bedding and structure. �( The study area contains two levels of rock benches that formerly may have been referred to as Beshoar and Barilla pediments. We interpret these as strath terraces with an alluvial veneer. Both features are shown on Plate 2 with only the lower on Plate 3 (plates are contained in pocket on rear cover).