Illinois commercial clays are found in glacial, Cretaceous, Tertiary,
and Pennsylvanian deposits. Pennsylvanian shales also are sources of
clays. Glacial clays, which occur as loess (wind-blown materials),
alluvial (water-deposited) sediments, and glacial till, are widespread
in the state. Cretaceous and Tertiary clays, at the extreme southern tip
of the state, are valuable for the manufacture of heat-resistant bricks
and coatings and for floor-sweeping compounds.
Pennsylvanian clays and shales are the most important source of clay for
manufacturing such products as pottery, stoneware, drain tile, sewer
pipe, flue tile, building tile, brick, and special heat-resistant
fire-brick.