SP-11 Glacial and Vegetational History of Northeastern Minnesota

Abstract

The broad relief features of the northern Minnesota bedrock permitted the digitation of the margin of the Wisconsin ice sheet into several lobes, and the diversity of the bedrock lithology resulted in the drift of each lobe being of different color, texture, and stone content. Moraines, drumlins, outwash plains, pro glacial lake plains, diversion channels, and other glacial features provide the opportunity to check the contemporaneity of the advances of different ice lobes-or lack of it. Of the four ice lobes identified for the Minnesota area, three affected the northeast. The Superior Lobe moved out of the Lake Superior basin. The Rainy Lobe advanced across the upland north of the Lake Superior basin, and the St. Louis Sublobe of the Des Moines Lobe came from far to the west and almost reached the head of the Lake Superior basin. Four phases of ice movement can be delineated for northeastern Minnesota. In the St. Croix phase, the Superior and Rainy lobes together covered most of the eastern half of the state, forming extensive drumlin fields and terminating at the St. Croix Moraine. During wastage, a great series of southwest-trending tunnel valleys was cut through the drift by subglacial meltwater under hydrostatic pressure, and many were later partially filled with eskers. The Superior Lobe then retreated barely into the Lake Superior basin, and the Rainy Lobe withdrew to near the Canadian border. In the Automba phase that followed, the Rainy Lobe advanced slightly on the upland and formed the Vermilion Moraine. The Superior Lobe expanded out of the Lake Superior basin with configuration different from before, because it was not impeded this time by the Rainy Lobe on its right flank. It sent a long tongue southwestward as far as the Mille Lacs Moraine in the center of the state, and it built the Highland Moraine at the crest of the slope leading up from the north shore of Lake Superior. It also formed drumlins and fluted terrain in its progress toward its terminal moraines. Glacial Lakes Upham I and Aitkin I were dammed on the north side of this long finger of ice. The Superior Lobe then retreated farther into the Lake Superior basin, and a glacial lake in front of the ice received red clayey sediment from the wasting ice. When the ice readvanced once again in a narrow tongue during the Split Rock phase, it incorporated the lake sediments and produced a red clayey till, which formed a veneer over the older glacial landforms. Once again the Superior Lobe retreated into the basin, and then it readvanced a still shorter distance than before-to the Nickerson Moraine, made of more red clayey till. Meanwhile, the St. Louis Sublobe extended eastward from the Des Moines Lobe, overrode the silty deposits of Glacial Lakes Upham I and Aitkin I, and reached within 25 miles of the Lake Superior basin. The Superior and St. Louis lobes then began their retreat together. Glacial Lake Upham II formed in front of the retreating St. Louis Sublobe, and its discharge eastward down the newly formed St. Louis River was blocked by the slowly retreating Superior Lobe and diverted southward to the Moose River and ultimately to the St. Croix. Four diversion channels at successively lower elevations can be identified. Finally, with further ice retreat, the St. Louis River entered Glacial Lake Nemadji and then, when the Superior Lobe retreated still more to uncover lower outlets in Wisconsin, the river drained into Glacial Lake Duluth. During these several epochs of ice-margin fluctuation, which occurred between about 16,000 and 12,000 years ago, the exposed terrain was covered with tundra vegetation, as shown by pollen and plant-macrofossil analyses of the sediment at several lake sites north and west of Lake Superior. The pollen stratigraphy has few features that can be correlated with ice-margin fluctuations, however. Either pollen analysis is not a subtle enough technique to reveal climatically induced vegetational changes in a tundra environment, or the vegetation itself did not respond significantly to climatic change, or the ice-margin fluctuations were caused by some factor not directly or immediately related to climatic change. The spruce forest that covered much of central and southern Minnesota during the time of Wisconsin ice retreat spread to northeastern Minnesota. It replaced the tundra west of the head of Lake Superior about 11,500 years ago, but it moved northward slowly, reaching Weber Lake, about 70 miles farther northeast, 10,000 years ago. Meanwhile the spruce forest in the south was replaced by birch and alder about 11,000 years ago and then abruptly by jack or red pine, which entered the state from the east in very great numbers about 10,500 years ago. The pine spread rapidly northward, but its dominance in the newly established spruce forest developed much more slowly, and it was not until about 7200 years ago that the spruce forest was completely gone. Meanwhile, alder spread abruptly in the area about 9000 years ago, and oak spread up from the south about 8500 years ago. At about the same time, white pine invaded in quantity from the east. By 7000 years ago even prairie openings occurred in the region. Reversal of the climatic trend initiated withdrawal of prairie herbs and oak from northeastern Minnesota, and the westward expansion first of white pine, spruce, and larch, and later of red pine and jack pine. Lakes that had previously been intermittently dry (as indicated by macrofossils) became permanent again, and as the lakes became filled with sediment they developed a margin of marsh or bog in the shallow water, and many lakes became converted completely to wetlands. The changing composition of the forest cover since deglaciation was caused basically by a climatic change to warmer and drier conditions and then the reverse. But the effects were not instantaneous: differential rates of migration of major tree types from Pleistocene refuges resulted in successive arrivals of potential dominants, so that the forest was continually changing. The additional factors of progressive leaching of the soil and paludification of lowlands added habitats that were not previously very extensive. These trends have continued to modern times, when lumbering, agriculture, and fire protection have interrupted the natural successions.