Beaches of Glacial Lake Agassiz

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Beaches of Glacial Lake Agassiz

• Ben Huffman
• Ashley Russell

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Appearance

• 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

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Appearance
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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.
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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)
• Herman Beaches
• Minnesota Beaches
• Norcross Beaches
• Tintah Beaches
• Campbell Beaches
• McCauleyville Beaches

Associated with south outflow
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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)
• The amount of beach ridges associated with each
  set varies from place to place.

Associated with Lake Agassiz northeast outlet
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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
• Stands about 20 to 30 feet above the Herman
• Records that the River Warren outlet was higher
  for a short time

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Associated with South Outlet

• 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

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Associated with NE outlet

• 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
• 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.

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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

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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.

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Timing
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(Morris Phase)
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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

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Equal Postglacial Lift lines
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Elevation Change Table
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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

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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.