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Geology 370: Glacial Geology

GLACIAL MELTWATER IN WISCONSIN

A. Introduction

Deposition:

• supra-glacial and sub-glacial

• ice marginal

• lacustrine

Landforms:

• outwash fans and outwash plains, kames, lake plains, eskers

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B. Meltwater Deposits in Wisconsin

Meltwater Deposits Map

Image credit: USGS Groundwater Atlas of the United States http://capp.water.usgs.gov/gwa/ch_j/J-summary.html

Surficial Deposits Map

Relief Map

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C. Devils Lake Area

Baraboo Range: syncline formed from highly resistant Precambrian quartzite

Devils Lake

• glacial ice blocked both ends of Devils Lake Gorge

• Johnstown Moraine still blocks gorge

Lake basins

• ice dammed drainage

• proglacial lakes formed between retreating ice and Johnstown Moraine

  • Lewiston Basin & Merrimac Basin

  • Lewiston Basin ice dam failed - outburst flood flowed from Lewiston Basin into Merrimac Basin and down Wisconsin River

  • helped form present-day Lake Wisconsin

• flat lake bottom; silty sediment; postglacial gullies eroded along margins

Tunnel channels

• Johnson Ponds

• marginal ice frozen to bed; farther back find thawed ice bed

• meltwater from thawed bed zone carved tunnel channel through frozen bed zone

• sand & gravel deposited in channel along with ice blocks from tunnel roof

• result: deep kettles form lakes, shallow kettles form marshes

• tunnel channels along Johnstown Moraine north of Baraboo: kettles & outwash fans

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D. Glacial Lake Wisconsin

Glacial ice blocked Wisconsin River forming lake

Depending on lake level and ice margin position, lake drained to:

• northwest to East Fork Black River, or

• southwest through Devils Lake Gorge to Wisconsin River

Lake drained catastrophically in a matter of days to weeks

• water broke through Johnstown Moraine & underlying weakly cemented sandstone forming Wisconsin Dells

Broad, flat sand plain; sandy, silty sediment up to 300 feet thick