How firm a foundation

How firm a foundation

By By Robert A. Hedeen, Naturalist

The Rock River Valley is the heart of the Midwest, and one the important natural resources which supports it is the Niagara limestone beneath it. The bedrock of this area and northeastern Illinois consists of layer upon layer of limestone, sandstone, and shale stacked almost a half of a mile thick on top of ancient granite. The granite base was once molten and constituted the original surface of the earth before oceans formed and life appeared.

The Niagara limestone is the uppermost layer of the bedrock here, but few are aware of it because it is covered with soil and ground up rock—glacial drift—ranging from a few feet to about 100 feet in depth. Actually the limestone underlying our area is technically called dolomite. Limestone is composed primarily of calcium carbonate, and dolomite is composed of this compound plus magnesium carbonate. Limestone and dolomite are terms that are frequently used interchangeably. Geologists refer to this massive stratum of rock as the Niagara Formation because it outcrops again at Niagara Falls in New York and hold ups the lip of the cataract.

During the Ordovician and Silurian periods of geological history (some 450 million years ago) a warm tropical sea covered this part of Illinois and almost half of North America. This vast ocean swarmed with a wide variety of living creatures: clams, snails, brachiopods, trilobites, sea fans and sea pens, cephalopods, protozoa, sponges, corals, and a host of other invertebrate types. Many of these animals absorbed minerals from the sea and secreted shells composed primarily of calcium carbonate. With the passage of millions of years and the gradual receding of the sea, untold trillions of these animals died and formed a massive layer on the floor of the sea. As this massive layer of carbonate mud increased, the weight of the water and other shells on top compressed this mass into limestone and dolomite. Thus, the Niagara Formation was formed. We know this is a true account of the events that occurred so long ago because one has only to study the rock making up this formation to determine that it is crowded with fossils of the species that lived during that ancient time in the earth’s history.

Chalk is a variety of porous, fine-grained limestone composed mainly of the shells of one-celled organisms called foraminifers. Few people realize that the famous chalk cliffs of the coast of Dover, England, (The White Cliffs of Dover) are composed almost entirely of untold trillions of skeletons of these tiny animals.

Around Rockford, Chicago and other locations in northern Illinois, limestone quarries exist to harvest this valuable mineral. Some of these quarries are hundreds of acres in area, and some more than 300 feet deep. When permission can be obtained, these quarries are a fossil hunter’s paradise.

The Niagara limestone forms a firm foundation for the huge concrete legs of large buildings in this area, including the skyscrapers of Chicago’s Loop. The supporting legs of the structure extend downward to rest upon the firm bedrock.

The Niagara Formation is honeycombed with aquifers containing an almost infinite supply of pure, fresh water for homes and industry. Years ago, blocks of dolomitic limestone supplied the building material for many homes and buildings, and the lakefront in Chicago is protected by miles and miles of large blocks of this limestone. Today, limestone is mainly used in making concrete that goes into the construction of buildings, sidewalks and highways. When limestone is “burned” or calcined (raised to a high temperature), it produces lime. Lime is sometimes called “soil sugar” and is routinely spread over the fields of Illinois farms to increase their fertility.

When dolomite is treated with sulfuric acid, it yields calcium sulfate (gypsum) and magnesium sulfate (Epsom salts). Calcined dolomite is employed as a lining for Bessemer converters in the production of steel from pig iron. Man has found many other practical uses for this mineral.

The countless animals that lived and died in that ancient sea eons ago left a priceless legacy that we enjoy today.

Dr. Robert Hedeen is a former resident of eastern Maryland, with degrees in zoology and botany. He is a former professor of biological science. He has had 30 scientific papers published, has written numerous magazine articles, and is the author of two books on marine biology of Maryland.

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