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A. Introduction

Meltwater Deposits Map

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

Wisconsin Glacial Deposits

Relief Map

B. Glacial Lake Oshkosh

Ice marginal lake

• Green Bay Lobe dammed northward-draining Fox River Valley

• meltwater accumulated along ice margin

• initially existed ~16,500 - 15,500 YBP

• refilled at least two more times

  • ~15,000 -14,2000 YBP (red clayey till)

  • ~13,500 YBP

Lake levels controlled by:

• extent of Green Bay Lobe

• isosatic adjustment

• ~5,000-10,000 km² at its largest

• relatively rapid drainage at some points in time as indicated by changes in lake bed sediment textures

Outlets include:

• Portage outlet to the Wisconsin River to the southwest

• several outlets to the north or east draining into Lake Michigan

Lake bed sediment

• 3 sequences of varved fine silt-clay lake sediments

  • first sequence ~ 25 ft thick (>257 rhythmites)

  • second sequence ~40 ft thick (>213 rhythmites)

  • third sequence ~20 ft thick (>32 rhythmites)

• 2 till layers

  • Silver Cliff till (sandy red till)

  • Kirby Lake till (clayey red till)

• top layer: marl and peat formed during transition to Holocene warm conditions

C. Two Creeks Forest

D. Glacial Lake Wisconsin

Ice marginal lake

• Green Bay Lobe dammed southward-draining Wisconsin River to the east

• Baraboo Hills bounded drainage to the south

  • syncline formed from highly resistant Precambrian quartzite

• meltwater accumulated along ice margin

• initially existed ~19,000 YBP

• drained & refilled several times

Lake levels controlled by:

• extent of Green Bay Lobe

• isosatic adjustment

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

• northwest to East Fork Black River (main outlet)

  • drainage channel obliterated by subsequent geomorphic activity

• southward through several outlets to the Wisconsin River

  • Dells outlet

  • Baraboo outlet (presently occupied by Baraboo River)

  • Alloa outlet

  • Devils Lake outlet

  

Lake basins and drainage

• broad, flat sand, silty plains; sediment up to ~100 m thick

• main basin

  • drained through Dells outlet to Lewiston basin

  • some drainage to SW to other minor basins

• Lewiston basin

  • dammed by Johnstown moraine to west, Green Bay Lobe to east, Baraboo Hills to south

  • drained through Alloa outlet into Merrimac basin when Green Bay Lobe retreated

    • helped form present-day Lake Wisconsin

  • experienced catastrophic drainage through Dells outlet after breaking through Johnstown moraine

    • eroded weakly cemented sandstone forming present-day Wisconsin Dells landscape

  • probaly drained through Devils Lake outlet prior to formation of Johnstown moraine

    • Devils Lake currently dammed by Johnstown moraine

    • Devils Lake gorge ~1 km wide, 5 km long, 140 m deep

    • gorge most likely cut in late Cambrian

    • at some point during Pleistocene gorge completely buried

    • gorge re-cut prior to Pleistocene

E. Summary