Glaciers shaped much of Wisconsin’s landscape, and with it, much of Wisconsin’s natural history. Even the area unshaped by the most recent Ice Age is defined by the lack of glaciation – The Driftless Area.
Glaciers shaped the land both as they formed and grew, but also as they melted and retreated. The glaciers scraped the landscape, removing softer sedimentary rock from the surface. This formed basins which would later fill, forming the Great Lakes and other large lakes in the region. The removed material was deposited hundreds or thousands of miles away when the glaciers finally melted and retreated.
This material, depending upon how it was deposited, forms the local landscapes in the areas once covered by the glaciers. This article will investigate some of these land formations.
Glacial moved sediment is generally called till. Glacial till is unsorted and includes all sizes of deposited glacial material from fine sands to erratic boulders weighing hundreds of tons. Till was carried within the ice of the glacier to the locations where it was finally deposited as the glacier melted and retreated. The specific way that the till was freed from the glacial ice resulted in various landforms as the till piled up or was spread out by the water of the melted glacier.
The glacier moves till forward, pushing the till out in front of the glacier and pulling the till forward through the ice of the glacier. The till piles up from these processes at the furthest extent of the glacier, forming a terminal moraine. The glacier doesn’t retreat all at once, but rather in a series of retreats and readvances, which can form recessional moraines. The size of the moraine depends generally upon the length of time that the glacier paused there.
The meltwater from the receding glacier continued to shape the landscape after the glacier retreated, including the formation of lakes behind moraines and rivers which broke through the moraines, and in many cases eroded the moraines away.
As the glacier melts, rivers and lakes form in the interior of the glacier and on top of the glacier. Sediment of glacial till builds up within these features. Eventually, the melted ice of the glacier cannot sustain the weight of the sediment and water and locally collapses. This releases the water and deposits the collected sediment onto the land. The sediment built up within a glacial lake forms a kame, while the sediment built up within a glacial river forms an esker.
The glacial meltwater form a delta of sediment laden rivers fanning away from the glacier. Typically the sediment moved by these rivers is small in size, and forms a sandy outwash plain over the bedrock previously exposed by the now retreating glacier. The formation of this outwash can be punctuated by the collapse of the glacial ice and the sudden release of the water from the glacial lakes.
When the ice of the glacier breaks up, large ice chunks calve off of the glacier. These chunks of ice could be up to miles in size and crashed into the wet, newly exposed sediment. The melting glacier further mounds till around the ice, even insulating the ice from further melting. This provided time for sediment to engulf the ice chunk and for the ground to solidify around it. The land eventually collapses into the space previously held by the ice, forming a deep depression. These depressions are called kettles. The water from the melted ice and from the glacial melt-water may fill these kettles forming lakes, while other kettles dry out.
Most all areas of the northern half of North America have been shaped in part by glacial action. An understanding of our geologically recent glacial history and the mechanisms of glacial action gives context to these features which we call the landscape.
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