A late Wisconsin buried peat at North Branch, Minnesota

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Abstract
At North Branch peat began to accumulate after the Grantsburg lobe (Mankato substage) started to retreat. The radiocarbon dates on the peat and its contained wood (12,700 and 12,030 B.P.) indicate that the Mankato substage preceded the Two Creeks interstadial rather than followed it. The peat was thus formed during the early part of the Two Creeks interstadial. The pollen and macrofossils from the peat suggest a landscape marked by sedge swamps and probably spruce-tamarack swamps on a broad river floodplain, bounded by oak and ash and prairielike communities on upland sites. The climate seems to have been only slightly colder than today and probably drier. The peat is overlain by sand that contains involuted layers of silt or clay rich in organic matter. These organic layers are believed to have formed as normal fine-grained sediment in quiet water. The pollen analysis indicates a vegetation dominated by pine and oak, typical of the postglacial. A radiocarbon date of 2520 B.P. for the upper part of the series of organic layers confirms the postglacial correlation. The folding of the layers into involutions must therefore have been produced without the aid of a permafrost base and may have resulted from localized annual frost connected with tussock growth of plants.

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