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The eastern escarpment of the N.-trending Chuska Mountains in northwestern New Mexico is capped at an average elevation of 8900 ft by a well cemented unit of the cross-bedded Chuska Sandstone (Miocene?). Three areas of landslide debris mark the flank of the mountain below the escarpment for 25 mi from the Toadlena area to the southern end of the range at the successively lower elevations of 8400, 8000, and 7600 ft, The highest area of debris consists of distinct ridges and troughs parallel to the main escarpment. Remnants of individual ridge-blocks as large as 100 ft wide, 100 ft high, and 1000 ft long extend along the ridges. Many of the ridge-blocks, since coming to rest, have been broken into smaller blocks that have migrated downslope into the troughs. The troughs are also partially filled with sand washed from the adjacent ridges or from the main escarpment. On the second level of landslide debris, the ridge pattern is disconnected and subdued as a result of longer downwastage. On the lowest level, the landslide debris has been downwasted until it occurs as discontinuous hummocks of Chuska Sandstone blocks on the underlying Cretaceous shale as much as 8 mi from the present mountain front; no ridge pattern is apparent on this level. The lowest landslide mass is deeply indented at its terminus by a younger pediment which extends far to the E., where it grades into a terrace that can be traced N. along the Chaco and San Juan rivers into the Wisconsin glacio-fluvial terraces and moraines of the Animas River in the San juan Mountains. Individual elongate blocks of the capping sandstone of the mountain range have separated along vertical joints and have slid down and out by the process of block glide, without the backward rotation that is found in Toreva blocks and similar slump features. Sliding took place on poorly cemented sandstone and on the underlying Cretaceous and Triassic shale. The separation of ridge-blocks and the gliding may have been aided by piping of loose sand from beneath the sandstone caprock; collapse subsequent to piping may also have formed some of the closed depressions on the flat top of the Chuska Mountains. Block glide is not now occurring on the E. flank of the Chuska Mountains. It is suggested that the process was active during the Pleistocene when lower temperatures and increased precipitation provided more effective moisture for the increase of pore-water pressure. Evidence for periods of lower temperature and greater precipitation comes from pollen-analytical and geomorphic studies of lakes on the crest of the Chuska Mountains; these studies suggest that the late Pleistocene there was characterized by a subalpine vegetation and higher lake levels. Elsewhere in the Southwest, glacial features below the present snowline similarly indicate past periods of decreased temperature and/or increased precipitation. The ridged pattern of landslide debris on the E. flank of the mountains is interrupted E. of the volcanic center at Washington Pass by a thick tongue of debris that is interpreted as a mudflow deposit. The flow extended out 5 mi from the mountain front along a preexisting broad transverse valley that had been cut through the downwasted block-glide debris. The flow may have started as a result of sudden drainage of a lake in the valley by a dam of landslide ridges.
This record provided courtesy of AGI/GeoRef.
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