In some of the lakes and ponds left by the glaciers lived numerous small
mollusks with calcium carbonate shells. As the animals died, their
shells formed a deposit on the bottom of the lakes and ponds. Certain
plants, especially algae, may have added a mudlike precipitate of
calcium carbonate to the deposits, and varying amounts of clay washed
from the shores mixed with both these materials. The resultant deposit
is called marl. Some marl deposits have peat mixed with them, and peat
also overlies some marl deposits.
Only comparatively small amounts of marl are known to have been dug in
Illinois. One deposit containing many shells and shell fragments, some
of it associated with peat, was worked in southeastern Livingston County
as a source of agricultural liming material. Other deposits have been
reported at other places in northeastern Illinois. The available
information indicates that the marl deposits are likely to be
principally of local importance.