|The sand and gravel deposits in Boone and McHenry counties are also attributed
to the glaciers. These are some of the most valuable and productive in the
state, supplying tons of construction aggregates to the Chicago area. As
the meltwater from the glaciers gushed off the top, front, and from beneath
the ice sheet, sand and gravel were flushed out or carried beyond the glacier
by the meltwater torrents. The material is called outwash.
The geology of the area, however, is not confined to what is visible to the naked eye. One hundred feet below the surface lies another landscape, complete with rolling hills, flat plains, and valleys. The Troy Valley and its tributaries traverse most of the assessment area. A major valley that drained parts of southern Wisconsin and northern Illinois during the Pleistocene, most of it is entrenched in bedrock and is covered with till. It is impossible to tell from the surface topography that a deeply incised bedrock valley lies below. The Troy Bedrock Valley can still be seen in southwestern Boone County, where it merges with the Kishwaukee Valley. As the Troy became blocked by glacial ice, meltwater was diverted to the west across a dolomite ridge. The major concentration of flow cut the narrow 150-feet deep gorge through which the present Kishwaukee River flows.
Several Natural Areas and a quarry highlight the geology of the Kishwaukee area. Flora Prairie in Boone County has dolomite outcrops. Harvard West Geological Area in McHenry County has an example of pitted outwash plain while Harvard East Geological Area is an example of a moraine protruding down a valley. On Illinois Route 20, near the Boone/ McHenry County line, is an abandoned quarry. This area has the distinction of having the only bedrock exposures in McHenry County and is a 'bedrock high' along the margin of the Kishwaukee River; it stands about 25 feet above the general valley level. When the first geological surveys of the county were conducted during the 1860s, stone was already being quarried there.
James Shaw, 1873
During the waning stages of the Ice Age, the first Native Americans began
to arrive in Illinois. Settlement first occurred along many of the state's
rivers and streams, and the Kishwaukee was no exception. The archaeological
data base for the Kishwaukee River basin indicates that the region was
continuously occupied for the last 12,000 years, in spite of major changes
in both social and physical environments. A total of 560 archaeological
sites have been recorded for the Kishwaukee area and range in age from
Paleo-Indian (10,000 B.C.) through the Postwar Industrial period (1946