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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Update: June 2012