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
Wright, H.E. Jr.; Watts, William A.; Jelgersma, Saskia; Waddington, Jean C.B.; Ogawa, Junko; Winter, T.C..
SP-11 Glacial and Vegetational History of Northeastern Minnesota.
Minnesota Geological Survey.
Retrieved from the University of Minnesota Digital Conservancy,