Almost all Illinois limestones were formed in seas that covered Illinois
millions of years ago. The many different limestone formations in
Illinois suggest that oceans covered all or part of the area several
times. Numerous kinds of shell fish, corals, and other marine animals
lived in these oceans and had shells and other hard parts made of
calcium carbonate. Through countless generations, these animal remains
accumulated on the ocean floor and gradually were compacted and cemented
into limestone (fig. 2).
Other Illinois limestones, however, were formed by the hardening of muds
composed mainly of calcium carbonate that accumulated on the floors of
the ancient seas. Still other limestones were formed of a combination of
animal remains and lime mud.
[Illustration: Figure 1—Calcite crystals. Limestone is made up
mainly of calcite crystals, but they are less perfectly formed and
are crowded together.]
The coral reefs of the South Pacific Ocean have their counterparts in
Illinois. The ancient Illinois oceans contained extensive reefs that
were built up just as the modern reefs have been. In northern Illinois,
around Chicago for instance, a number of the ancient reefs are now the
site of stone quarries. In southwestern Illinois such reefs are a source
of petroleum.
The dolomites of Illinois probably were originally limestones, but,
either while the limestones were still beneath the sea or after the sea
had withdrawn, magnesium was exchanged for some of the calcium in the
limestones. If the exchange took place under the sea, the sea water was
the source of the magnesium. If it happened when the limestones were a
part of the land, the magnesium was brought in by water circulating
through the rock. Many of the marine animal fossils became difficult to
recognize after the change, and the texture and general appearance of
the rock also were altered. Some of it became noticeably porous.
[Illustration: Figure 2—Limestone containing fossils. An Archimedes
screw appears at lower left, lace-like bryozoa in the center, and
fluted brachiopod shells at top and center.]