The Driftless Region covers parts of southern Minnesota
and Wisconsin, Northwestern Illinois and Northeastern Iowa. This area derives
its name from being unglaciated in a region that had many glacial episodes,
going back nearly two million years to the Pleistocene Epoch.
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 an
overall 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.
An Icy Neighborhood
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.
Dirty Wind
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
three feet thick.
Where We Stand Now
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.
