glaciation of wisconsin

8/13/2019 Glaciation of Wisconsin

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Glaciation of WisconsinEducational Series 36 | 2011Fourth edition

W I S C O N S I N G E O L O G I C A L A N D N A T U R A L H I S T O R Y S U R V E Y

For the past 2.5 million years, Earths climate has uctuatedbetween conditions of warm and cold. These cycles are theresult of changes in the shape of the Earths orbit and the tiltof the Earths axis. The colder periods allowed the growth ofglaciers that covered large parts of the worlds high altitudeand high latitude areas.

The last cycle of climate cooling and glacier expansion inNorth America is known as the Wisconsin Glaciation. About100,000 years ago, the climate cooled and a glacier, theLaurentide Ice Sheet, began to cover northern North America.For the rst 70,000 years the ice sheet expanded and con-tracted but did not enter what is now Wisconsin.

During the last part of the Wisconsin Glaciation, theLaurentide Ice Sheet expanded southward into the Midwestas far as Indiana, Illinois, and Iowa. The ice sheet advancedto its maximum extent by about 30,000 years ago and didntmelt back until thousands of years later. It readvanced anumber of times before nally disappearing from Wisconsinabout 11,000 years ago. Many of the states most prominentlandscape features were formed during the last part of theWisconsin Glaciation.

The maps and diagrams in this publication show the tim-ing and location of major ice-margin positions as well as thedistribution of the glacial and related stratigraphic units thatwere deposited in Wisconsin.

GreenBay

Madison south shore ofLake Superior

E x

t e

n t

o f

t h e

G r e e n

B a y

L o b

e

10,000

15,000

20,000

25,000

30,000

A

B

C

D

EF

N

calendaryears ago

LaurentideIce Sheet

Graph followsextent of icealong this line

Tracking the glacierMaps showing the extent of the Laurentide Ice Sheet and changes to glacial lakesat several times. This set of maps, keyed to the graph below, shows when ice rstbegan its advance into Wisconsin (A), ice near its maximum extent (B and C), the initialretreat (D), when ice left Wisconsin (E), and the nal major readvance (F).

A B C

D E

29,000 years ago 21,000 years ago

18,000 years ago 14,000 years ago 13,250 years agoF

31,000 years ago


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Area glaciated duringlast part of WisconsinGlaciation, betweenabout 35,000 and 11,000years before present.

Area glaciated priorto about 35,000years before present.

No evidence of glaciation.

A phase is a geologic event rather than a period of time. Most phasesrepresent at least a minor advance of the edge of the Laurentide IceSheet. Each line marks the edge of the ice sheet during a phase ofglaciation. For example, during the Johnstown Phase of the WisconsinGlaciation, the southern edge of the Green Bay Lobe (see back page forlobe locations) of the Laurentide Ice Sheet advanced to the line markedJohnstown in southcentral Wisconsin. Only the most recent phase isshown at any location.

Phases of glaciation

Ages of glaciation. In this map and the oneon the last page, areas are distinguished byage: older or younger than about 35,000 years.Ages are determined using geochronology(radiocarbon and other dating techniques)and by studying features in the landscape.Younger glacial features are relatively fresh anduneroded; older glacial features are mostly orcompletely worn away.


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C o r r e l a t i o n o f I c e A g e e v e n t s a n d l i t h o s t r a t i g r a p h i c u n i t s a c r o s s W i s c o n s i n


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Distribution of Pleistocene lithostratigraphic units

Published by and available from:

Wisconsin Geological and Natural History Survey3817 Mineral Point Road Madison, Wisconsin 53705-5100(608) 263-7389 www.WisconsinGeologicalSurvey.orgJames M. Robertson, Director and State Ge ologist

ISSN: 1052-2115

Lobes of the Laurentide Ice SheetArrows indicate the direction of ice movement.

For additional information, see the Lexicon of PleistoceneStratigraphic Units of Wisconsin , Wisconsin Geological andNatural History Survey Technical Report 1.

Contributors: John W. Attig, Michael Bricknell, Eric C. Carson, Lee Clayton,Mark D. Johnson, David M. Mickelson, and Kent M. Syverson

(Map does not show loess units.)

glaciation of wisconsin

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