Pleistocene open-system pingo scars have been identified within the Ridge and Valley province in central Pennsylvania. The features are elliptical basins, averaging 20 by 50 m in plan and 4 m in depth, bounded by low ramparts. Located beyond the limit of Pleistocene glaciation, they closely resemble features reported as pingo scars in England, and they are dated at 12.8 ka by radiocarbon analysis of their basal fill. These scars are clear evidence of the existence, depth, and continuity of permafrost in the eastern United States at the end of the Pleistocene. They show that perennially frozen ground extended to at least 5 m in some places, at the same time that the ground nearby was unfrozen.
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