The Driftless Region as it is commonly
called began nearly 2 million years ago during the Pleistocene
Epoch. This area which covers parts of southern Minnesota and
Wisconsin, Northwestern Illinois and Northeastern Iowa, derives it's
name from being unglaciated in a region that had many glacial
episodes. Having escaped the leveling effect of continental glaciers
the ancient land surface has been exposed to essentially continuous
weathering and erosion. Several thousand feet of bedrock strata may
have been removed during this span of some 243 million years. This
erosion carved a series of deep valleys into the gently tilted
bedrock formations with the Mississippi River Valley draining the
entire region.
Massive sheets of ice (continental glaciers) several hundred feet
thick flowed southward from centers of snow and ice accumulation in
the far north and covered parts of present-day Wisconsin and
Illinois several times. The surface topography of most of Illinois
was considerably subdued by the repeated advances and melting of
glaciers, which scoured and scraped old preglacial erosional
surfaces into outwash plains. Although the glaciers did not cover
the Driftless Region , nor completely surround it at any one time,
outwash deposits of silt, sand and gravel were dumped into the
Mississippi River Valley.
In the driftless region the exposed bedrock that was not directly
eroded by the ice was indirectly affected by a drape of fine, wind
blown silt called loess. When these deposits dried out, strong
prevailing winds from the northwest settled out the finer materials
such as fine sand and silt, and carried them across the unglaciated
terrain. Loess up to 35 feet thick can be found in narrow bands
along the uplands adjacent to the Mississippi River, but thins to no
greater than about 12 feet depth in the northwestern part of Jo
Daviess County. The deepest loess deposits are generally on the
moderately sloping ridges and near the base of slopes while in areas
of steep slopes the loess is typically less than 3 feet thick.
At the present time the upper highest bedrock layer in Jo Daviess
County is Silurian age dolomite with Ordovician age shale and
dolomite at the intermediate and lower landscape positions. The
relief from the higher ridges to the valley floors is typically 300
feet or more creating a rugged and scenic landscape. |
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