How Glaciers Made Wisconsin

The last ice sheet

Wisconsin is named in the textbooks of geology: the last glacial stage of North America, the Wisconsinan, takes its name from deposits here ¹. The ice sheet that made them was the Laurentide, spreading south from Canada.

The Wisconsin Geological and Natural History Survey gives the timeline ²:

• About 31,500 years ago, the glacier began advancing into Wisconsin, and it kept expanding for 13,500 years.
• About 18,000 years ago, the climate warmed and the ice started to melt back.
• It took another 7,000 years to leave northern Wisconsin, about 11,000 years ago.

The ice moved as lobes, tongues that followed lowlands: the Lake Michigan and Green Bay lobes in the east, and the Langlade, Wisconsin Valley, Chippewa, and Superior lobes across the north ².

“The many lakes and wetlands and the irregular landscape that characterize so many areas of eastern and northern Wisconsin are also a direct result of the last glacier” ².

Under the ice

• Ground moraine: the blanket of till (unsorted clay, sand, and boulders) the ice smeared across the land beneath it ¹.
• Drumlins: streamlined hills of drift, molded by moving ice into the shape of a long half-egg, steepest and highest toward the end the ice came from. Their long axes point in the direction the ice moved. Southeastern Wisconsin has about 5,000; some stretch into ridges 3 to 4 miles long ¹. The Campbellsport Drumlins are one of the reserve units of the Ice Age Trail ³.

At the stagnant edge

As the ice thinned and stopped moving, meltwater did the building ¹:

• Eskers: long, winding ridges of sand and gravel laid down by rivers flowing in tunnels and channels within the ice.
• Moulin kames: conical hills of drift piled up where meltwater poured down a moulin, a vertical shaft in the glacier, dumping its load at the bottom. The Kettle Moraine has one of the most striking groups of moulin kames anywhere ¹.
• Crevasse fills: debris that filled cracks in the ice, left as ridges.

At the ice front

Where the ice edge held still for a while, it dropped a ridge of debris: an end moraine. The farthest one, the terminal moraine, marks the outermost limit of the last glacier as “a conspicuous ridge of glacially deposited debris” ².

Where two lobes met, their debris piled up between them in an interlobate moraine. The Kettle Interlobate Moraine formed between the Green Bay and Lake Michigan lobes, a jumble of knobs and hollows of gravel rising to 1,311 feet at Parnell tower, with smooth, drumlin-covered ground moraine on either side ¹.

Beyond the ice front

• Outwash: meltwater streams spread sand and gravel in broad, flat outwash plains in front of the ice ¹.
• Kettles: where blocks of ice were buried in drift and later melted, the ground collapsed into hollows, leaving pitted outwash and kettle-studded moraine ¹. Some kettles are dry; those that reach the water table hold permanent ponds and lakes. Many have no stream draining them at all, because water sinks through the coarse gravel below ¹.

The Ice Age National Scenic Trail, about 1,200 miles long, follows these features across the state ³.

Try it

Spend a day in the Northern Kettle Moraine (or any stretch of the Ice Age Trail) with a topographic map. Find a kettle pond, a dry kettle, a conical kame, and a long esker ridge, and note what grows on each: dry gravel kames and wet kettle bottoms make very different habitats a few hundred feet apart.

What to take from this

The last glacier entered Wisconsin about 31,500 years ago, began melting about 18,000 years ago, and left the north about 11,000 years ago, moving in six lobes. Under the ice it left ground moraine and drumlins pointing the way it moved; at its stagnant edge, eskers and moulin kames; at its front, end moraines, including the Kettle Interlobate Moraine between the Green Bay and Lake Michigan lobes; and beyond, outwash plains pitted with kettles.