The Wisconsin glacial border in northern and eastern Pennsylvania from the Salamanca re-entrant in New York southeast to the Delaware River is of Woodfordian (late Wisconsinan) age. Slightly weathered drift along the border and radiocarbon dates of 12,520 to 14,170 B.P. support this conclusion. These dates are from basal lake- and bog-bottom sediments rich in organic materials. Contemporary and somewhat older dates from sites beyond the border in southern Pennsylvania range from 12,540 to 15,210 B.P.; they indicate that mass movement in this extraglacial region probably ceased, allowing organic sediments to accumulate, about the same time that ice began to retreat. The ice sheet probably began to retreat about 15,000 B.P.

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