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

****:NIAGARA LIMESTONE

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.