A Unique Glacial Past

During the past 2 million years, massive sheets of ice – continental glaciers – flowed from the far north, advancing and retreating several times, covering portions of Illinois (Reinertsen, 1992).  An area of approximately 20,000 square miles (circled in red on the map to the right) appears to have escaped numerous bouts of glacial advances, thus it has been named the Driftless Area (Schuberth, 1986).  The term “driftless” reflects the lack of evidence of all glacial deposits, or drift, that are sometimes classified as poorly sorted tills (Schuberth, 1986).  Sometimes referred to as the Wisconsinan Driftless Section, the Driftless Area includes portions of southwestern Wisconsin, northwestern Illinois, and a thin slice of eastern Iowa that were untouched by the Wisonsinan, Illinoian, and perhaps earlier glacial stages as shown in.    Most of Jo Daviess County falls within the Wisconsin Driftless Section, which makes it part of this tiny, yet unique, portion of the continent that repeatedly missed out on the direct effects of glaciers.

The exact reasons for the existence of the Driftless Area are not fully understood.  Some believe its leeward location on the south side of an upland area in Wisconsin may have deflected the ice away from the region (Schoenberg, 1988; Schuberth, 1986).  The nearest location of the ice boundary to the field trip area is 3 miles east of Apple River Canyon State Park, our final destination (Wiggers, 1997).  Others believe that channelization of ice into deep river channels and basins left little remaining ice to spill into the Driftless Area (Schuberth, 1986).

Absence of glacially derived sediments and glacial striae in the region support theinterpretation that this area was unglaciated (Schuberth, 1986).  Some glacial sediment was introduced to the region via meltwater, but most of those materials were washed toward the Mississippi River where they now exist as deposits less than 12 feet thick (Reinertsen, 1992).  Glacial striae are defined by Schuberth (1986) as “scratches on bedrock surfaces produced when rock debris carried within and beneath the glacier acts as grit, much like sandpaper rubbed over balsa wood.”  These features are noticeable in outcrops surrounding the Driftless Area, but have not yet been discovered within the region.