Forest Preserve District of Cook County, Illinois
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Niagara Limestone
Nature Bulletin No. 282-A   November 11, 1967
Forest Preserve District of Cook County
Richard B. Ogilvie, President
Roland F. Eisenbeis, Supt. of Conservation

Chicago stands at the crossroads of America -- the heart of the Middle West -- and one of the most important natural resources upon which it depends is the Niagara limestone beneath it. The bedrock in this region consists of layer upon layer of limestones, shales and sandstones stacked almost a half mile thick on top of the ancient granite, once molten, that formed the original surface of the earth before oceans formed and life appeared. The Niagara limestone is the uppermost layer here but few of us are aware of it because it is covered with soil and ground up rock -- glacial drift -- ranging from a few feet to a hundred or more feet in depth.

The steel skeletons of the skyscrapers, in and near the Loop, rest on huge concrete "legs" that extend down to rest upon this thick layer of Niagara limestone. Farther out, huge industrial plants have been built where this bedrock lies just beneath the surface. Formerly, many industries, including the stockyards and meat packing plants, and many suburbs, depended upon the supply of underground water obtained from deep wells into this limestone or the other layers of sedimentary rocks below it. Thousands of outlying homes, and our forest preserves, still depend upon wells tapping the Niagara formation. Around the city, in its suburbs and as far away as Kankakee and Joliet, there are several great limestone quarries in operation: some of them hundreds of acres in area and some over 300 feet deep. Many more have been abandoned and several, including some very deep ones within Chicago itself, have been filled with refuse and excavated material.

Years ago, these quarries supplied blocks of limestone for the buildings and sidewalks of this region. Miles and miles of such blocks protect our lake front. Today, crushed limestone is used in making the concrete that goes into the construction of buildings, streets, sidewalks and highways. Limestone dust is spread over the fields of Illinois farms as "soil sugar" to sweeten the soil and help increase its fertility. The cement used in making concrete is manufactured at mills in Gary, Indiana, and near LaSalle and Dixon in Illinois, where crushed limestone is burned with slag or with clay and then ground into fine powder. Further, crushed limestone is an essential ingredient, a fluxing material, that makes it possible to remove impurities from the iron ore in the smelters which produce pig iron for the great steel mills at South Chicago and Gary.

This limestone is called Niagara because it is the same layer that dips downward and reappears as the ledge of hard rock that forms the lip of Niagara Falls. Around Chicago it varies in thickness from about 450 feet to 200 feet or less. Its composition is more than 90 percent calcium carbonate and magnesium carbonate. .Where the latter is present in sufficient quantity, it is called dolomite. It was formed beneath a warm shallow salty sea that must have covered almost half of North America, a few hundred millions of years ago. The shells of countless billions of many kinds of marine animals disintegrated and formed a limy mud upon the bottom, which became thicker and thicker and gradually hardened into rock. All this went on very, very slowly. It is estimated that several thousand years were required to build a single inch of this rock, and many millions of years to lay down the entire layer of Niagara limestone. There was no hurry, then.

In the quarries we find ancient reefs of corals and the fossil remains of the ancestors of our modern octopus, starfish, snails, clams, the chambered nautilus and the horse-show crab.

They lived, they died, and now they help build Chicago.

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