Earthscapes:
The Red River Valley
Lake Agassiz: Child of Ice
By: Don McCollor
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and additional information
A history of the Great Lakes begins "From the
surface of the moon they are the most instantly recognizable feature on
the North American continent". The Great Lakes--Superior, Michigian,
Huron, Erie, and Ontario--mighty inland seas, the largest bodies of fresh
water on earth. Lake Agassiz was greater than all
of them. Born in an age of ice giants, the waters of Agassiz sprawled across
Dakota and Minnesota and far into Manitoba, nurtured by the melt waters
of the great glaciers of the north. The lake lived four thousand years,
dammed against the towering northern ice, while her overflowing waters poured
south through the glacial River Warren to carve the
Minnesota River Valley. As the ice retreated, Agassiz drained to the northward,
leaving the rich, fertile sediments of the lake bed as its legacy
 Like many things, the history of Lake Agassiz is not
so simple. There was more than one Lake Agassiz, and the fortunes (and size)
of the lake ebbed and flowed. In the process, many of the features that
shape the present Valley were formed. Strand lines (beaches) of sand and
gravel mark stable episodes in lake level. Large alluvial fans mark the
deltas of ancient rivers flowing into the lake. And the advance and retreat
of the glaciers impressed their own marks on the Valley. What follows is
a simplification of the rather intricate story of Lake
Agassiz, with dates which are quite approximate and a subject of controversy.
About 20,000 years ago, part of the Laurentide Ice Sheet (glacier)
covered the eastern Dakotas, Minnesota and extended into Iowa. The Ice retreated
and readvanced several times with advances 17,000, 14,000, and 12,300 years
ago. Probably there were lakes in the Valley before Agassiz, but the ice
scoured away all but vague traces of their presence. The first of Lake Agassiz
began with what is called the Cass Phase 11,700 years ago with a small lake
tucked against the ice at the South Dakota border. About this time, the
predecessor of the Sheyenne river began forming a "delta" (actually
an underflow fan, a subtle difference) on the southwestern side of the Lake.
Drainage probably was through the Minnesota River Valley.
Agassiz expanded north as the ice first melted,
then readvanced during the Lockhart Phase from 11,600 to 11,2000 years ago.
The advance produced the Edinburg moraine in western Grand Forks ad Walsh
Counties. What would become the Pembina river was first sandwiched between
the moraine and higher ground to the west, flowing southward to deposit
the Elk Valley delta. With icemelt the river then turned eastward beginning
to form the Pembina Delta. Further north the Assiniboine delta also began
to form. Drainage was still to the south, with the lake level forming first
the Herman Beach and later dropping to the level of the Campbell beach somewhere
in this time.
The retreating ice opened a path for Lake Agassiz to drain toward Lake
Superior about 11,000 years ago. The southern lakeshore now stood at Fargo,
where the Sheyenne and other rivers began to build the Moorhead delta and
issuing in the (naturally) Moorhead Phase. By 9,900 years ago, the lake
was dry to the Canadian border, with the Red River now flowing northward
in what is more or less its modern position. The glacier appeared to be
tenacious, just like winter is in the Valley. Ice advanced back down in
Minnesota, plugging up the eastern drainage and reflooding the Valley. Lake
Agassiz rose until the water was again at the Campbell beach level and again
draining southward (Emerson Phase). This refilling of Agassiz was not first
recognized, and really does complicate a nice straightforward story as well
as the earlier geological features.
Once yet again, the ice melted somewhat and the lake could again drain
to Superior about 9,500 years ago ushering in the (final) Nipigon Phase.
Apparently, the water kept breaking through a series of ice dams, with the
water level of Agassiz falling like a series of lower and lower drain plugs
were being pulled. As it fell, the lake left a series of strand line beaches
like bathtub rings marking each level. By 8,500 years ago, the southern
part of Lake Agassiz had drained for the last time. Perhaps a millennium
later, the lake was also gone from northern Canada.
Ships Gone Missing, Hemming, Robert J., Contemporary Books, Chicago,
1992.
Quaternary Stratigraphy and History in the Southern Part of the Lake Agassiz
Basin, Fenton, Mark M., Moran, S. R., Teller, James T., and Clayton, Lee,
in Glacial Lake Agassiz, Teller, J.T., and Clayton, Lee, eds., Geological
Association of Canada Special Paper 26, 1983, pp. 49-74
Lake Agassiz was named for Louis Agassiz, perhaps the greatest naturalist
of the nineteenth century. Born in Switzerland, the home of alpine (mountain)
glaciers, he was the first to recognize the characteristic traces of glaciation
on a grand scale across the northern and southern hemispheres, and to propose
the theory of continental glaciation.
The River Warren as well as the Sheyenne and Pembina Rivers of the time
were more what is termed "spillways" rather than the image we
normally conjure up with the word "river". The present Minnesota
River, resting in the valley of the River Warren is a "misfit river"--the
river much smaller than the valley it is flowing through. It is possible
to estimate the speed of a (former) river by the distance between meanders
(bends) and the flow from the speed and size of a channel. These calculations
come to the conclusion that the River Warren carried one walloping amount
of water in its day, as did the Sheyenne and Pembina rivers. The higher
estimates give a flow comparable to the Mississippi in full flood. These
river flows apparently occurred as catastrophic floods, with a large glacial
lake breaking through the retaining ice. By a "domino effect"
the water would surge down a spillway to another glacial lake. That lake
would overflow, with the water pouring into Agassiz via the Pembina or Sheyenne,
and this overflow cascading down the River Warren. These flows happened
fast--with the deluge flowing for only days or weeks. These low channels
would have been a place best avoided by the Pleistocene.
River Warren, the Southern Outlet to Glacial Lake Agassiz, Matsch, C.
L., in Glacial Lake Agassiz, Teller, J. T., and Clayton Lee, eds., Geological
Association of Canada Special Paper 26, 1983, pp. 231-244.
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A note about continental glaciers--of the type that once covered the
Valley and their movements. These glaciers do not flow downhill. When enough
ice becomes piled up, it becomes plastic ( a few hundred or thousand foot
thickness does nicely) and the ice begins to get squeezed out at the edges.
The effect is like pouring stiff cake batter into the center of a pan and
watching it edge out to the sides. And a glacier does not have a reverse
gear to backup. Instead, melting is occurring at the edge of the glacier.
As long as the glacier pushes out faster than the edge melts, it advances.
If the melting just matches the pushing out, the glacier appears to stand
still. Here it acts as a conveyer as all the rock, gravel, clay, and other
debris it has picked up melts out at the stationary edge. Now if the melting
occurs faster than the pushing out, the glacier is retreating, although
the ice is still flowing the same way. Occasionally, a glacier seems to
get tired and lay down in the traces, so to speak, with the ice not moving
at all. This is termed "dead ice" as it melts in place.
Physical Geology, Foster, Rovert J. Charles E. Merrill Publishing Co.,
Columbus, OH, 1971.
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One of the characteristic features left by glaciers, and what cued early
geologists into the idea of continental glaciers is erratic boulders. These
are rocks (the most convincing are very large ones) standing on glacial
drift far (often tens or hundreds of miles) from any bedrock source. The
largest examples are far too large to have been carried by running water.
The rocks have been, of course, picked up by the moving glacial ice and
set down there as the ice melted.
A large example of an erratic boulder used to be a landmark a mile and
one-half west of Hillsboro in Trail County. The rock was twenty feet in
diameter and pointed at the top, partially buried in a sandy ridge near
the Goose River. The rock was easily visible for miles, and marked a stopping
point for early travelers. Sometime after the 1930's, the rock was decided
to be an obstacle to farming of the surrounding field and was buried.
Pillars of Time, Burner, Thea, and Brasel, Merilla, American Yearbook
Co., Visalia, CA, 1980. p.11.
The WPA Guide to 1930's North Dakota, State Historical Society of North
Dakota, Oxford University Press, 1990, p. 193.
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