Forest Preserve District of Cook County, Illinois
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The Fossils of Niagara Limestone
Nature Bulletin No. 667-A   February 18, 1978
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

The oceans and their characteristic saltwater life are a thousand or more miles away from this inland part of the country. Yet, here in the Chicago region, not far beneath our feet is the floor of an ancient sea -- a thick bed of limestone containing fossil corals, many kinds of sea shells and other animal remnants.

This bedrock is named Niagara Limestone because it is the same one that forms the hard lip of Niagara Falls. Between there and here it forms a vast saucer under the Great Lakes. It was laid down during an early geological age when this whole interior part of the continent was covered by a warm shallow sea. Then, for millions of years, the dead shells and limy parts of marine animals slowly piled deeper and deeper. Most of these decomposed and were pulverized by wave action to form a lime mud which later solidified into the evenly layered limestone commonly seen in local quarries. Only in favored localities, such as in the crevices of coral reefs, were many of these dead animals covered with sediment quick enough and left undisturbed long enough to make good fossils.

This bedrock is exposed at a number of places in southern and southwestern Cook County -- along Lake Michigan, in stream beds, in road cuts and as hillside ledges. The rock rubble along the Calumet-Sag Canal and the Sanitary and Ship Canal offer good fossil picking. Some of the fifteen larger limestone quarries of the region are breath-taking sights -- a mile wide and 300 feet deep.

Niagara Limestone shows no trace of fishes or other backboned animals because these had not yet appeared on the earth at the time it was formed. When its fossils are compared with the present day life of the sea, we find that several abundant types of that time have disappeared entirely. The surviving relatives of others are now found most abundantly in tropical waters. They have little in common with the aquatic life of our freshwater lakes and streams.

The corals are soft-bodied animals related to the jellyfishes that enclose themselves in stony walls and tubes. Fossils of both simple and compound types are abundant. The former grow singly and are called horn corals or cup corals. The honeycomb and chain corals are compound forms built like apartment houses. These were and are the reef builders.

Mollusks are represented by the spirally coiled shells of snails and those with paired shells like clams. A third type, related to the squid, octopus and the rare chambered nautilus, are the largest fossils to be found -- often two feet long and three inches in diameter. These heavy tapering shells -- sometimes coiled but usually straight -- can always be recognized by their numerous crosswalls.

Among the most frequent of all animal remains are the segments of jointed crinoid stems known as "Indian beads. " Although crinoids are related to the starfishes, they are called sea lilies because the head, with its five feeding arms, is mounted on a tall stem anchored to the bottom.

The fossils of Niagara Limestone include a long list of other types -- some common and some rare. These sponges, lamp shells, moss animals, single-celled animals and several kinds without common names or living relatives. are no fossil worms but their fossilized tracks are common. Trilobites, extinct relatives of lobsters, crabs and crayfish are the prize specimens of amateur fossil hunters and "rock hounds. .

For further information see the 44-page, well-illustrated booklet: Fossils of Illinois, Story of Illinois No. 11, Illinois State Museum, Springfield.

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