An examination of glacial gravel deposits in Illinois reveals pebbles
and larger pieces of many kinds of rock. Some are gray, others white,
pink, brown, or black. They commonly include limestone, dolomite,
granite, and many rocks with less common names such as quartzite,
schist, and basalt. The limestone and dolomite were picked up by the
glaciers from outcrops in northern Illinois, Wisconsin, and Michigan.
Some of the granite pebbles resemble outcrops in Wisconsin; others look
like granite that crops out in Canada. The quartzite probably came from
Wisconsin, and black shale fragments found in some gravel deposits came
from the floor of Lake Michigan or from western Michigan. Occasionally
pieces of metallic copper are found that probably had their source in
the Lake Superior copper-bearing area.
In addition to the sand associated with gravel deposits, extensive
deposits of sand alone are found at many places in Illinois. Most of the
sand grains are pieces of minerals that were constituents of rocks until
weathering, the grinding action of the glaciers, and other erosive
agencies broke the rocks into sand. The principal mineral in glacial
sand is quartz, but many others occur in lesser amounts, including
calcite, dolomite, feldspar, pyroxene, tourmaline, garnet, magnetite,
and hornblende. Most of these are foreign to Illinois, although the
calcite and dolomite may be native.
[Illustration: Figure 18—Moraines left by Wisconsinan glacier.]
Modified from George E. Ekblaw, 1960
_ILLINOIS STATE GEOLOGICAL SURVEY_
[Illustration: Figure 19—Gravel dredge, with plant for processing
the gravel in background.]