Abstract

The hundreds of small depressions in the flat crest of the Chuska Mountains of New Mexico are formed in cross-bedded siliceous Tertiary sandstone. The basins are bounded by low rock bluffs and contain intermittent lakes of irregular form and shallow depth. Many are concentrated in broad valley openings along the crest, but others are isolated in the intervening uplands. The clayey lake sediments on the floors of the basins are almost entirely of Pleistocene age, according to pollen analysis and radiocarbon dates.

The depressions cannot be explained by glaciation, volcanism, solution, wind action, or any other of the geologic processes that ordinarily produce depressions. In morphology and distribution, they most resemble collapse depressions, and the hypothesis is proposed that they formed as a result of collapse of cemented sandstone layers above vacuities that had been produced by piping of uncemented sand out to the steep escarpments of the mountains. The pipes originated at springs or seeps along escarpments and extended headward under successive cap rocks, which thereupon became pock-marked. Integration of depressions by lake overflow under Pleistocene conditions of pluvial climate, along with broadening of the basins by slope retreat, produced shallow lowlands on the mountain crest. These lowlands then became dotted with more depressions as the process continued on the next cemented unit. The depressions near the eastern escarpment are truncated by landslide scars which themselves are of Pleistocene age, so the process of lake-basin formation seems now to be extinct or dormant. In fact the entire mountain geomorphology is a relic of processes active in the Pleistocene, when the climate was colder, the water table was higher, and the forest cover was restricted.

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