Taiga-tundra and the Full-Glacial Period in Chester County, Pennsylvania
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AbstractTwo shallow cores from a marsh in the unglaciated piedmont of southern Pennsylvania reveal a late Pleistocene pollen sequence. Below 70 cm gritty clays contain abundant sedge, grass, and other herbs with a tree-pollen sum of 25 to 50%. In order of their abundance the arboreal pollens include jack pine, spruce, fir, birch, and willow. This flora represents taiga-tundra vegetation of the last glacial maximum, the first pollen evidence of a full-glacial period in North America. Its modern analogue is the subarctic lysotundra, a region of scattered trees in valleys surrounded by bare solifluction slopes. Radioactive samples were collected at 100 cm in gritty clay and at 150 cm in course sand and clay. The C-14 age of each, approxiamtely 13,500 years, is somewhat younger than the accepted age of the last Wisconsin maximum. Surface contamination of sandy layers, indicated by the pollen diagram, may account for the discrepancy. The full-glacial clays and sand are overlain by shallow, more organic late- and post-glacial sediments. A pine zone and a deciduous forest zone resemble the postglacial sequence north of the glacial border. However, a spruce-fir zone is absent and late-glacial forest invasion appears to be of pine rather than spruce. The biogeographical theory that holds that temporate forest occupied favorable habitats close to the Wisconsin ice sheet finds little support in the Pennsylvania pollen diagrams. AccessDownload (10.4 Mb) |