Transcript Beaches of Glacial Lake Agassiz

Beaches of Glacial
Lake Agassiz
Ben Huffman
Ashley Russell
Appearance
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Old beaches are traceable
Continuous, smoothly rounded ridge
Most lie 3-10 feet above till side, and 10-20
above lake side
Variations in height are due to unequal
currents and wave power
Some beaches are cut through from old
streams
Marked by gravel and sand sloping into water
level and till
Provide strong evidence for the existence of
glacial lakes
Appearance
Formation
Storm waves moved gravel and sand from
erosion of till deposits transported into
Lake Agassiz
 Fines settled to the middle of the lake
 Interactions with wind, waves, and
currents formed bars, spits, hooks, loops
and terraces
 Best preserved on moderate slopes.
 Successions of beaches can mark pauses
in uplift after ice retreat, subsidence with
outlet erosion, and lowering of lake levels
with new discharge routes
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These locations marked
in red have well
preserved beach ridges
from many stages of
Lake Agassiz.
Beach ridges can be
described through
Minnesota, North Dakota,
and Manitoba.
Beach Successions
There are many beaches that describe the
levels of Agassiz through time.
 The main ones are as follows (starting
with the oldest and highest):
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Herman Beaches
Minnesota Beaches
Norcross Beaches
Tintah Beaches
Campbell Beaches
McCauleyville Beaches
Associated with south
outflow
Beach Successions (cont.)
– Blanchard Beaches
– Hillsboro Beaches
– Emerado Beaches
 (2 main series)
– Ojata Beaches
 (2 main series)
– Gladstone
– Burnside
– Ossowa
– Stonewall
– Niverville
 (2 main series)
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Associated with Lake
Agassiz northeast outlet
The amount of beach ridges associated with each
set varies from place to place.
Herman Beaches
Uppermost beaches
 Doubles in northern part of Agassiz
 Gravel with pebbles 2-3 inches in diameter
 Highest stand of Agassiz at 1,055 feet
above sea level
 Some places show the Milnor stage
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– Stands about 20 to 30 feet above the Herman
– Records that the River Warren outlet was
higher for a short time
Associated with South Outlet
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Minnesota, Norcross, Tintah, Campbell, and
McCauleyville beaches
Hard to match with northern beaches due to the
progression of Agassiz to the north.
Campbell stage is the most conspicuous below
the Herman
– Shows Agassiz at a much lower level
– Sand and gravel swept southward from the
Pembina Delta
McCauleyville beaches show that the outlet from
Agassiz eroded below Lakes Traverse and Big
Stone
Associated with NE outlet
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14 total shorelines
Lie below the McCauleyville
River Warren was no long receiving drainage from ice, and
thus Agassiz obtained a lower NE outlet
Beach sets are separated by 10-45 ft.
Blanchard stages are the oldest
– Show three levels or three pauses in uplift
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Hillsboro beaches show spits formed from currents
associated with the fall of lake level
Emerado beaches are very traceable, and only have one
beach in MN and ND, and two beaches in Manitoba
Niverville Beaches show 2 or 3 stages, caused by northward
uplifting of land.
Timing and Epeirogenesis
 Epeirogeny
- Uplift or depression of
the Earth's crust.
 Relationship
– Lake levels are associated with the
outlet level
– Rebound contributed to outlet shifts
Epeirogenic Dependence
 It
has been estimated that the lake
bottom of Lake Winnipeg may have
only been about 100 feet above sea
level.
 Now it is ~ 600ft above sea level
 Elevation changes were affecting
lake outlets.
 Talk overlap.
Timing
(Morris Phase)
Epeirogenic Dependence
 Today
we see shorelines that must
have been level at one time, but now
display vertical changes in elevation.
 South to North
 Elevation change show some latitude
dependents
Equal Postglacial Lift lines
Elevation
Change Table
Take away
 Beaches
are dependent on Lake level
which are dependent on outlet
elevation.
 Epeirogenesis was a driving factor is
shifting lake outlets
 Measurements of uplift can be
attained by measuring vertical
change in beaches
Sources
Teller, J.T., and Clayton, L., 1983, Glacial
Lake Agassiz, The Geological Association
of Canada, Special Paper 26.
 Thorleifson, L.H., 1996, Review of Lake
Agassiz History, Geological Survey of
Canada.
 Upham, W., 1895, Glacial Lake Agassiz,
U.S. Geological Survey, Monographs XXV.
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