Wisconsin glaciation

Wisconsin glaciation
Hannes Grobe/AWI · CC BY 3.0 · source
Name	Wisconsin glaciation
Caption	Maximum extent of the Laurentide Ice Sheet during the last glacial period.
Time	c. 85,000 – c. 11,000 years BP
Location	North America
Type	Glacial period
Cause	Milankovitch cycles
Effect	Shaped modern Great Lakes, altered Mississippi River drainage, created Driftless Area
Preceded by	Sangamonian Stage
Followed by	Holocene

Wisconsin glaciation. The Wisconsin glaciation was the most recent major advance of continental ice sheets across North America, corresponding to the last glacial period globally. It profoundly reshaped the geography and ecology of the continent, leaving a legacy of distinctive landforms and influencing early human settlement. This glacial episode is named for its well-preserved deposits in the state of Wisconsin, studied extensively by geologists like Thomas Chrowder Chamberlin.

Overview

The Wisconsin glaciation represents the final major phase of the Pleistocene ice age in North America, part of the broader Quaternary glaciation. It was primarily driven by Milankovitch cycles affecting solar irradiance, leading to the expansion of the massive Laurentide Ice Sheet and the smaller Cordilleran Ice Sheet. The period is formally defined by marine oxygen isotope stage 2 and is correlated with the Weichselian glaciation in Europe and the Devensian glaciation in Britain. Its study was pioneered by the United States Geological Survey and forms a key part of North American Quaternary science.

Extent and chronology

At its maximum extent, the ice sheet covered most of Canada and extended into the northern United States, reaching as far south as the present-day locations of New York City, the Ohio River, and Missouri. The Cordilleran Ice Sheet enveloped the Pacific Northwest, while the Laurentide Ice Sheet reached its southern limit near the Missouri River. The glaciation is subdivided into several stadials and interstadials, such as the Farmdalian and Woodfordian, with the last glacial maximum occurring around 26,500 to 19,000 years BP. Final retreat was marked by significant events like the Missoula Floods and the formation of Glacial Lake Agassiz.

Effects on landscape and geology

The advancing ice radically sculpted the underlying bedrock, creating features like the Great Lakes basins, Finger Lakes, and thousands of smaller kettle lakes across Minnesota and the Canadian Shield. It deposited vast sheets of till, forming moraine systems such as the Oak Ridges Moraine and Long Island. Regions like the Driftless Area remained unglaciated, preserving older landscapes. The ice weight caused isostatic depression of the crust, leading to post-glacial rebound evident around Hudson Bay. Glacial meltwater carved massive channels like the Channeled Scablands and altered the course of the Mississippi River.

Flora and fauna

The glacial advance created a shifting mosaic of tundra, boreal forest, and parkland ecosystems, compressing biomes southward. This period saw the presence of Pleistocene megafauna, including the woolly mammoth, mastodon, American lion, and giant ground sloth in regions like the La Brea Tar Pits. Refugia for flora existed in areas like the Appalachian Mountains and the Pacific Northwest. The changing climates and habitats contributed to the Quaternary extinction event, which led to the disappearance of many large mammals as the ice retreated and human populations expanded.

Relationship to human migration

The glaciation played a crucial role in facilitating the peopling of the Americas. Lowered sea levels exposed the Bering land bridge, connecting Siberia to Alaska and allowing Paleo-Indians to enter the continent. An ice-free corridor between the retreating Laurentide and Cordilleran ice sheets may have provided a migration route into the interior, toward sites like the Clovis and Folsom sites. Alternative coastal migration theories suggest travel along the Pacific Coast using resources from the kelp highway. The subsequent warming period of the Holocene allowed these early cultures, such as the Clovis culture, to spread throughout the continents.