Paleobotanical analyses of basal sediments in the Spider Creek drainageway, southern St. Louis County, Minnesota, suggest a late-glacial tundra vegetation which was rapidly invaded by boreal forest at the beginning of the postglacial period. The tundra is inferred mainly from plant macrofossils which include leaves, seeds, or fruits of such arctic plants as Dryas integrifolia, Salix herbacea, Vaccinium uliginosum var. alpinum, Juncus balticus, Arenaria cf. humifusa, Rhododendron lapponicum, Stellaria humifusa type, Potentilla nivea, and Carex capillaris. The pollen diagram (Pl. 1) shows a late-glacial sequence typical of northeastern Minnesota, with high abundances of nonarboreal pollen, Picea, and Cyperaceae.
The beginning of the postglacial period is marked by a sudden disappearance of all arctic macrofossils and a concomitant appearance of Picea and Larix needles. On the pollen diagram arboreal pollen and Picea increase in abundance, Cyperaceae decreases, and Betula forms a prominent peak.
Correlation with other pollen diagrams from northeastern Minnesota indicates that the late-glacial sediments were deposited between 12,000 and 10,500 years ago. Pollen and plant-macrofossil analyses indicate that boreal forest covered all but northeastern Minnesota during this time; environmental conditions apparently were not suitable for a forest cover there during the early stages of glacial retreat.