Glacial Features (Large)
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Glacier
Lakes (Tour
Map symbol 'L')
Glacier lakes, locally called
'Prairie Potholes', abound in Eastern SD (Space
Shuttle photo) and were formed when the stagnant ice and meltwater
were left behind in small depressions as the glaciers melted. Most of these
shallow, localized bodies hold some water most of the time. Smaller
lakes undergo periodic filling
and drying. Most of these occur in depressions
in hilly landscapes and are usually not
connected to a larger drainage pattern. Officially, the largest of these
glacier lakes is Lake Thompson
in Kingsbury county. Their maximum depth is seldom greater than 7m in South
Dakota.
Former
Lake Dakota Shoreline
(Tour
Map symbol 'T')
A large volume of water from the melting glacier collected in east central South
Dakota to form Lake Dakota, a very large lake whose depth exceeded 30m. Glacier
Lake Dakota had beaches and terraces
marking several high water levels and occupied most of the James River valley
north of Hitchcock. The lake drained when the ice dam holding the water eventually
broke. The terrain (Tour
Map symbol 'Lp') of the former
lake bottom forms a very very flat
landscape.
Ice-Walled
Lake Sediments
(Tour
Map symbol 'I')
Water from the melting glacier could have been trapped between ice bodies before
draining off the landscape. These ice-walled lakes may have existed for decades
and collected sediments from the melting glacier. After the ice walls completely
melted, the profile of the former lake bottom appears as an large
elevated area or mound with a distinct
oval or circular surface shape.
Moraines
(Tour
Map symbol 'M')
Rock debris moved by glaciers was deposited as large ridges or piles near it's
terminus as the ice melted. Their numerous locations indicate a complex series
of ice advances and retreats during the glaciation period. The landscapes marking
the end of the melting ice sheets are hummocky,
hilly, and can be steep.
Moraines are often seen as prominent
ridges along the horizon out
from which large boulders have weathered.
The large Bemis moraine oriented
in a northwest-southeast direction near Sisseton has a vertical
relief of 250m in some places relative to the Minnesota River valley to
the east. It marks the western position of the Des Moines ice lobe and the "Prairie
Coteau".
Loess
(Tour
Map symbol 'Ls')
Before vegetation was established on the exposed till after the glacier melted,
it was subjected to significant wind erosion. Clay-sized materials were blown
far away and the silt-sized particles were carried a few, to tens of miles away.
The deepest silt deposits
or loess (pronounced 'luss') occur on the east, west, and south of major river
floodplains (James River, Big Sioux River etc.). Loess also occurs as a thin
mantle over some glacial till and forms very productive soils. Roadcuts
through loess form stable walls even when steep, since the angularity of
the individual grains resists slumping. Streambank cuts can expose loess
deposits, associated till, and buried soils in one profile which can foster
interesting discussions about the geological events of a region.
Sand
Dunes
(Tour
Map symbol 'D')
Sand-sized particles were piled into dunes during the post-glacial wind erosion
events. Sand dune areas are identified
in several places in the state. The largest height of the dunes is about 6m.
Eskers
(Tour
Map symbol 'E')
A melting
glacier can form an internal drainage channel as water moves out from the ice
body. When the ice is completely melted, this former
channel of gravel and sediment appears as a sinuous
ridge on the landscape.