Wisconsin glaciation
The Wisconsin Glacial Episode, also called the Wisconsin glaciation, was the
This glaciation radically altered the geography north of the
Times
Two related movements have been termed Wisconsin: Early Wisconsin and Late Wisconsin.[3]: 40–75 The Early Wisconsin was the bigger of the two and extended farther west and south. It retreated an unknown distance before halting. During this period of quiet, the glacial deposits were eroded and weathered. This first Wisconsin period erased all the Illinoian glacial topography that its glaciers extended over.[3] The Late Wisconsin ice sheet extended more towards the west than the earlier movements. This may have been due to changes in the accumulation center of the ice sheet, topographic changes introduced by the Early phase or by pressure changes in the ice mass in the north.[3]
Table I | |
---|---|
Table VII – Estimated Age of Glacial Episodes (Leverett) [3]: 74 | |
Age | Years before Present (YBP) |
Culmination of Late Wisconsin | 50,000 |
Culmination of Early Wisconsin | 100,000 |
Beginning of Wisconsin | 150,000 |
Culmination of Illinoian | 300,000 |
Beginning of Illinoian | 350,000 |
Culmination of Pre-Illinoian, i.e., old Nebraskan[4][5] | 550,000 |
Beginning of Pre-Illinoian | 1,200,000 |
Continental ice sheets
Ice caps
Table II | |||||
---|---|---|---|---|---|
Ice Caps[6] | |||||
Keewatin ice sheet | Laurentide Ice Sheet | Nova Scotia Ice Cap | Newfoundland Ice Cap | Greenland Ice Cap |
Labrador Ice Sheet
The Labrador Ice Sheet centered east of Hudson Bay. Expanding towards the southwest, it reached into the eastern edge of
Ice lobes
Table III Laurentide Ice Sheet | |||||
---|---|---|---|---|---|
Glacial lobes and sublobes of the southern Laurentide Ice Sheet during the late Wisconsin Glaciation.[8] | |||||
Major Lobes | Minor Lobes | ||||
Des Moines | Grantsburg | St. Louis | Rainey | ||
Lake Superior[6] | Wadena | Chippewa[6] | Wisconsin Valley[6] | Langlade[6] | |
Green Bay[6] | |||||
Lake Michigan[6] | Delavan | Harvard-Princeton | Peoria | Decatur | |
Minor lobes: Milwaukee, Two Rivers; Straits of Mackinac | |||||
Saginaw | |||||
Lake Huron[6] | East White[6] | Miami[6] | Scioto[6] | ||
Lake Erie[6] | |||||
Lake Ontario[6] | Lake Champlain[6] | Hudson River[6] | |||
unnamed lobe in Quebec – New England | Connecticut Valley[6] | Buzzards Bay[6] | Cape Cod[6] | Georges Bank[6] |
Keewatin Ice Sheet
The Keewatin Ice Sheet began west of Hudson Bay in the Canadian Territory of Keewatin. The ice moved south some 1,500 miles (2,400 km) into Kansas and Missouri. To the west, it reached 1,000 miles (1,600 km) to the foothills of the Rocky Mountains.[7]
Cordilleran Ice Sheet
The Cordilleran Ice Sheet has left remnants throughout the Northern Rocky Mountains. Unlike the other two ice sheets, this one is mountain based covering British Columbia and reaching into northern Washington State and Montana. The Cordilleran Ice Sheet has more of an Alpine style of many glaciers merged into a whole. The striations made by the ice field in moving over the bedrock show that it moved principally to the west through the passes of the coast range.[7]
Formation of proglacial and prehistoric lakes
Whenever the ice sheet melted from the north at a moraine, water would begin to pond in the divide between a moraine and the ice front. The ice would act as a dam as water could not drain through the ice sheet, which in the Wisconsin period covered most of the proglacial river valleys. Numerous small, isolated water bodies formed between the moraine and the ice front. As the ice sheet would continue to melt and recede northward, these ponds combined into proglacial lakes. In areas without an available outlet, the water levels would either continue to rise until reaching one or more low spots along the rim of a moraine, or the ice sheet would retreat, opening access to a lower portion of the moraine.[3]: 40–99 Multiple outlets could form through low spots too until one would become dominant after erosion lowered both the outlet and lake surface.
Running water
Ice melt and rainfall carried large quantities of
Stages of the Wisconsin episode
Table IV | |||
---|---|---|---|
Maxima of the Wisconsin ice sheets[9] | |||
Western Ice | Eastern Ice | Proximate years ago | Position of ice border |
Mankato | Valders | 25,000 | Northern Washington, Idaho, and Montana to the Continental Divide – north of Edmonton – 65 miles east of Edmonton – northwest corner of North Dakota – Des Moines – west end of Lake Superior – Milwaukee – Port Huron – Buffalo – Schuylerville – St. Johnsbury. |
(Great reduction of ice) | Cary | 27,500 | Minneapolis – north Wisconsin – south of Chicago – Central Ohio – 50 miles south of Buffalo – Binghamton - Northampton |
Tazewell | 40,000 | Rockford, Ill. – Peoria – south of Indianapolis – north of Cincinnati – northwestern Pennsylvania – central Long Island | |
Iowan | No known ice | 65,500 | Northern Washington, Idaho, and Montana – northwest North Dakota – east central Iowa - Minneapolis |
Role in human migration
Prehistoric human migration was likely greatly influenced by this last glacial period, as during much of the Wisconsin era, the formation of a
Flora and fauna
North American flora and fauna species were distributed quite differently during the Wisconsin era, due to altered temperatures, surface water distribution, and in some cases coverage of earth surface by glaciers. A number of scientific studies have been conducted to determine species distribution, particularly during the Late Wisconsin and early to mid-Holocene. An example of findings is from the investigation of flora species using pollen core samples in present-day northern Arizona. Here in the Waterman Hills researchers found that Juniperus osteosperma and Pinus monophylla were early to mid-Holocene dominant trees, while Monardella arizonica has been a continuously present understory plant. Celtis reticulata is an example of a plant present in the early Holocene following Wisconsin glacial retreat, a species no longer present at the Waterman Mountains site.[11]
See also
- Driftless Area – Geological region in the Midwestern US
- Geology of Ontario – Geologic features of the Canadian province
- Glacial history of Minnesota
- Glacial Lake Iroquois – Prehistoric lake that became Lake Ontario
- Ice Age Trail – Long-distance hiking trail in the United States
- Ice core – Cylindrical sample drilled from an ice sheet
- Last Glacial Period – Period of major glaciations of the Northern Hemisphere (115,000–12,000 years ago)
- Retreat of glaciers since 1850 – Shortening of glaciers by melting
- Timeline of glaciation – Chronology of the major ice ages of the Earth
- Younger Dryas – Time period with a return to glacial conditions c. 12,900–11,700 years ago
Pleistocene historic names
Region | Glacial 1 | Glacial 2 | Glacial 3 | Glacial 4 |
---|---|---|---|---|
Alps | Günz
|
Mindel | Riss | Würm |
North Europe | Eburonian | Elsterian
|
Saalian
|
Weichselian
|
British Isles | Beestonian | Anglian
|
Wolstonian | Devensian
|
Midwest U.S. | Nebraskan | Kansan | Illinoian | Wisconsinan
|
Region | Interglacial 1 | Interglacial 2 | Interglacial 3 |
---|---|---|---|
Alps | Günz-Mindel | Mindel-Riss | Riss-Würm
|
North Europe | Waalian | Holsteinian | Eemian
|
British Isles | Cromerian | Hoxnian | Ipswichian
|
Midwest U.S. | Aftonian | Yarmouthian
|
Sangamonian |
References
- ISBN 0-7872-5353-7
- ^ SERC Media; Glacial Grooves, Central Park; Image 14884 is a 208 by 173 pixel JPEG; Uploaded: Apr5 09; Wayne Powell, CUNY Brooklyn College; http://serc.carleton.edu/details/images/14884.html
- ^ a b c d e f g h i Chapter II. Glacial History of the Huron-Erie Basin; Geological Report on Wayne County; W.H. Sherzer; Michigan Geological and Biological Survey, Publication 12, Geological Series 9; Lansing, Michigan; Wynkoop Hallenbeck Crawford Co., State Printers; 1913
- ISBN 0-8137-2337-X
- ^ Boellstorff, J (1978). "Chronology of some Late Cenozoic deposits from the central United States and the Ice Ages"(pdf). Transactions of the Nebraska Academy of Science 6: 35–49. Retrieved 2012-04-04.
- ^ a b c d e f g h i j k l m n o p q r s Geological Framework and Glaciation of the Eastern Area; Christopher L. Hill; pg 82-98
- ^ a b c Chamberlin, Thomas C. and Salisbury, Rollin T., Geology, 3 Vols. 1906, Vol III., pp. 330-333.
- ^ The southern Laurentide Ice Sheet; Cavid M. Mickelson and Patrick M. Colgan; Department of Geology, University of Wisconsin Madison; Madison, Wisconsin; 2003
- ^ Correlation of Wisconsin Glacial Maxima; Ernst Antevs; ca 1943; The Quaternary Geology of Southeastern Wisconsin with a Chapter on the Older Rock Formations, Professional Paper 106; William C. Alden; United States Geological Survey; Government Printing Office, Washington, D.C.; 1918
- ^ Harm J. De Blij. 2005. Why geography matters: three challenges facing America, 308 pages, p.69
- ^ C. Michael Hogan, 2009. Elephant Tree: Bursera microphylla, GlobalTwitcher.com, ed. N. Stromberg