Wisconsin glaciation

The Wisconsin glaciation, also called the Wisconsin glacial episode, was the most recent time when huge sheets of ice covered much of North America. It happened more than 20,000 years ago and lasted until about 11,000 years ago. During this time, massive glaciers grew and moved across the land, changing the shape of the earth.

Maximum glaciation of the Northern Hemisphere (black) during the Quaternary climatic cycles

Big ice sheets grew in many places. One was the Laurentide Ice Sheet, which covered most of Canada, the Upper Midwest, and New England. Another was the Cordilleran Ice Sheet in the western mountains. These glaciers were part of a worldwide cold period that affected many parts of the earth.

The glaciers changed the land a lot. They helped create the Great Lakes and left deep scratches in rocks that we can still see today. As the ice melted, huge floods reshaped areas far to the south. During this cold time, sea levels were lower, and people and animals could walk between North America and Siberia using a land bridge called Beringia. The Wisconsin glaciation played a big role in shaping the landscapes and environments we see today.

Timeline

The Wisconsin glaciation had two main parts: Early Wisconsin and Late Wisconsin. The Early Wisconsin period was larger and reached farther west and south than the later one. After this first part, the glaciers pulled back for a while, and during this quiet time, the land changed and old ice marks were worn away.

The Late Wisconsin period saw the ice push farther west than before. This might have happened because of changes in where the ice built up, changes in the land from the first part, or shifts in the weight of the ice up north.

Table I
Table VII – Estimated Age of Glacial Episodes (Leverett) ^: 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	550,000
Beginning of Pre-Illinoian	1,200,000

Continental ice sheets

Laurentide ice sheet

Main article: Laurentide Ice Sheet

Cordilleran ice sheet

Main article: Cordilleran ice sheet

The Cordilleran Ice Sheet left marks in the Northern Rocky Mountains, covering British Columbia and reaching into northern Washington State and Montana. It was made of many glaciers that joined together. The lines carved by the ice show it moved mainly west through mountain passes.

Innuitian ice sheet

Main article: Innuitian ice sheet

The Innuitian ice sheet was centered on the Queen Elizabeth Islands.

Table III Laurentide Ice Sheet
Glacial lobes and sublobes of the southern Laurentide Ice Sheet during the late Wisconsin Glaciation.
Major Lobes	Minor Lobes
Des Moines	Grantsburg	St. Louis	Rainey
Lake Superior	Wadena	Chippewa	Wisconsin Valley	Langlade
Green Bay
Lake Michigan	Delavan	Harvard-Princeton	Peoria	Decatur
Lake Michigan	Minor lobes: Milwaukee, Two Rivers; Straits of Mackinac
Saginaw
Lake Huron	East White	Miami	Scioto
Lake Erie
Lake Ontario	Lake Champlain	Hudson River
unnamed lobe in Quebec – New England	Connecticut Valley	Buzzards Bay	Cape Cod	Georges Bank

Formation of proglacial and prehistoric lakes

Meltwater

As the ice melted and rain fell, it carried clay, sand, and gravel from the ice. Clay could travel far in the water, but sand and gravel stayed closer to the ice. These materials formed different shapes near the ice, like long ridges called kames, mounds called moraines, and winding ridges called eskers. Beyond the edge of the ice, wide flat areas of sand and gravel called outwash plains formed. When the ice melted completely, it left behind a layer of mixed dirt and rocks called ground moraine or till plain, which can hold a lot of water and is useful for people building things.

Stages of the Wisconsin episode

Table IV
Maxima of the Wisconsin ice sheets
Western Ice	Eastern Ice	Estimated years before present	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
(Great reduction of ice)	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

The last glacial period, called the Wisconsin glaciation, likely had a big effect on how people moved long ago. During this time, a land bridge called Beringia formed across the Bering Strait. This land bridge may have let some of the first humans travel between North America and Siberia in Asia. Other paths for people to move also opened up during warmer times in Europe and Asia.
Main article: Settlement of the Americas

Flora and fauna

During the time of the Wisconsin glaciation, plants and animals in North America lived in very different places because the climate changed and large glaciers covered the land. Scientists studied where species lived, especially toward the end of the Wisconsin period and the beginning of a warmer time called the Holocene.

One study in California, at the Waterman Hills, looked at tiny bits of plant material called pollen core to learn about trees and plants. They found that trees like Juniperus osteosperma and Pinus monophylla were common during that time, while a plant named Monardella arizonica was always there underneath. After the glaciers melted, another plant called Celtis reticulata appeared, but it is no longer found in the Waterman Mountains today.