GRANITE is one of the most widespread intrusive (originating deep within
the earth) igneous rocks. It consists chiefly of feldspar and quartz
with small amounts of biotite, muscovite, or hornblende. Most granite is
light colored, but it can be white, gray, yellow, pink, or deep red. The
texture ranges from medium grained to coarse grained.
Granite pebbles or boulders are the most common igneous rocks found in
glacial deposits in Illinois. They are not native to the state but were
brought here by the great ice sheets or glaciers that advanced from
southern Canada to cover much of northern United States during
Pleistocene time.
Native granitic rock probably lies very deep beneath the entire state.
It has been found in deep oil-test drillings along the western and
northern margins of Illinois.