If you ever wondered what the Upper Midwest might have
looked like before the advent of ice ages, you might travel to this interesting
area of 30,000 square miles (46,000 square kilometers) in southwestern
Wisconsin, southeastern Minnesota, northeastern Iowa, and northwestern
Illinois.
The area is called "driftless" because there is no evidence on the ground
of "drift" (glacial till) from the last few ice ages.
It appears that lobes of the ice sheets advanced on all sides to surround
the region, but never actually covered it. The Driftless Area
is a region of massive limestone and dolomite layers, through which
the rivers in the area (including the upper Mississippi River) have carved
deep gorges walled with steep-sided bluffs. There are no lakes, but
the uplands are a karst landscape, as the underlying limestone and dolomite
is filled with caves dissolved by groundwater, some of which have collapsed
as sinkholes. The region's soil is primarily loess which blew
in from areas in Minnesota and the Dakotas that dried out (desertified)
in proximity to the edge of the ice.
By contrast, the surrounding (glaciated) area is much flatter: the land
was scoured and ground down by ice, then covered by large amounts of Canadian
glacial debris. Tthe land is dotted with lakes. The original Pleistocene
vegetation was worn away, and although glacial till in the soil makes it fertile, boulders make it difficult to plow.
The Driftless Area's isolation during the last few Ice Ages would have
made for specialized evolutionary conditions under any circumstance (
An endangered species of snail has survived there since the Pleistocene).
But in addition, special features of the Driftless Area's topography
create unique microclimates that plants must adapt to:
-
North-facing bluffs that have permafrost
-
Cool breezes blowing out of caves
As a result, the region is full of plant species unique to the area.
Before European settlement, the Driftless Area was covered by expanses
of prairie dotted by oak groves. During the early history of
the United States, the Federal govermnent prohibited farming in the area,
giving the land to lead mining companies instead. Consequently,
much of the Driftless area retains its pre-settlement character; however,
the region contains a large number of endangered species and threatened
species of plants. Over-logging has made cutting the remaining
oak trees unprofitable.
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