Much of the unglacial or driftless portion of the state is embraced within its limits, and although the streams now have a gentle or even sluggish flow, they have greatly broken the surface of the country.
South-western Wisconsin and parts of the adjacent states of Illinois, Iowa and Minnesota are known as the driftless area, because, although bordered by drift sheets and moraines, it is free from glacial deposits.
The reason for this exemption from glaciation is the converse of that for the southward convexity of the morainic loops; for while they mark the paths of greatest glacial advance along lowland troughs (lake basins), the driftless area is a district protected from ice invasion by reason of the obstruction which the highlands of northern Wisconsin and Michigan (part of the Superior oldland~ offered to glacial advance.
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