Deposits of chert, chert gravel, and ganister also are among the variety
of mineral materials found in extreme southern Illinois. Chert consists
principally of minutely crystalline particles of quartz. Some chert is
popularly called flint. In southern Illinois chert occurs in two
principal kinds of deposits, those composed of solid ledges and those
consisting of gravel. The term novaculite is used in southern Illinois
for those solid deposits that are white, comparatively thick, and free
of other interlayered materials. No novaculite is mined at present, but
it is said to have been sold in past years for making sodium silicate
and silica brick.
The chert gravels of southern Illinois are of three kinds—novaculite
gravel, Elco gravel, and “Lafayette” Gravel. The novaculite and Elco
gravels consist of fragments of chert plus lesser amounts of fine silica
particles and clay. The chert fragments of the novaculite gravel are
angular, but the Elco gravel includes both angular and rounded
fragments. These gravels are white, yellow, brown, or reddish brown, the
novaculite gravel usually being the more highly colored. Deposits in
Union and Alexander Counties have been used for road surfacing and other
purposes. Deposits of chert gravel also occur in Hardin and Saline
Counties, and some stream valleys in southern and western Illinois also
contain such gravel.
“Lafayette” Gravel consists principally of brown chert pebbles. Most of
the pebbles are rounded and have a smooth, semi-polished surface. The
sand and clay occurring with the gravel are brown or dark red. In some
places there are deposits of coarse, red quartz sand. The gravel is most
abundant in the four southernmost counties of the state, and deposits
may be as much as 65 feet thick. It is used principally as a
road-surfacing material.