Illinois shales are a part of the bedrock—that is, they are associated
with indurated rocks such as sandstone and limestone. Most clays are
surficial rocks occurring in deposits near the surface, where they lie
above the bedrock. Exceptions are certain clays found in extreme
southern Illinois and the underclays, also called fireclays, that occur
beneath coal seams and are part of the bedrock.
The surficial clays are of two principal kinds—till and loess. Till is a
deposit left by glaciers. It is a gray, blue-gray, or brown clay
containing varying amounts of sand, pebbles, cobbles, and even boulders.
Till is found at many places in the state and is used for brick making,
especially in the Chicago area.
Loess is a wind-deposited silty clay or clayey silt and is found in many
parts of Illinois. It is thickest on or near the bluffs of the
Mississippi, Illinois, and Ohio Rivers. It generally is brown and stands
in steep faces in roadcuts and other excavations. It once was widely
used for making brick and tile.
Of major importance in making clay products in Illinois are the bedrock
shales and the clays associated with the coal-bearing rocks that
underlie much of the state. The shales, and the clays to a lesser
extent, are dug at many places for making structural clay products such
as bricks, structural tile, and drain tile. They also are used to make
lightweight aggregate for concrete. The underclays of some of the older
coal seams are used to make buff-colored brick, stoneware, and a highly
heat-resistant brick (firebrick) that is used in industrial furnaces or
in other operations involving high temperatures. Some fireclay, ground
as fine as flour, is added to molding sand to make it coherent enough to
form into molds for metal casting. Sewer pipe and flue lining also are
made from underclays.
Clays unlike those found elsewhere in the state occur in extreme
southern Illinois. One of these has the property of removing color from
oils and was so used at one time by petroleum refineries. Another,
kaolin, was extensively used during World War I for making crucibles.