Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 625-A   January 22, 1977
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

A11 of us have been told that, if we dig deep enough, we come to solid rock. Here in Cook County there are several limestone quarries where we can see solid rock but -- in other regions -- quarries, mines, caves and surface outcrops of bedrock are so scarce that people seldom have the opportunity to see the earth's "bones. " Instead, over a large part of the United States, the only sizable rocks are boulders scattered over prairies and woodlands or piled in stream beds. A large boulder seems so massive and indestructible that it seldom fails to arouse our interest and curiosity. Where did it come from and how did it get here.

The history of these boulders is as fascinating as the unfolding of a detective thriller. The evidence shows that these rocks were carried by the ice sheets, or glaciers, which slowly pushed down from northern Canada at least four times in the last million years. Grinding and plowing as they came, these masses of ice ripped away pieces of the bedrock over which they traveled. Some of it was pulverized into clay and sand and pebbles but, when the ice melted, chunks of granite and other very hard rocks were left as boulders, often many hundreds of miles from their parent rock ledges. In the Chicago area, scratches and grooves scraped in the surface of the local limestone by harder rocks imbedded in the moving ice show that the last glacier moved from northeast to southwest.

Boulders are worn or rounded glacial rocks larger than the cobblestones which were once used to pave city streets -- roughly between the size of a man's hat and his automobile. Those found just where the melting ice left them often show scoured flat faces and are more angular than those along streams and shores which have been rolled and rounded by the action of the water. Most are granite or granite-like rocks formed back when cooling masses of molten rock floated to the surface. Usually they are gray or pink -- occasionally red. black or mottled. A few are metamorphic rocks such as quartzite, schist and gneiss (pronounced "nice"). These show bands which may be regular, or variously bent and crumpled. The youngest of all, sedimentary rocks, contain fossils.

Boulders are common in parts of northeastern Illinois and rather rare in the southern and western parts of the state. Where they are too numerous, they are a nuisance. In the "boulder belts" farmers often spend years clearing them from their fields. The smaller ones are dug up, rolled onto a heavy sled called a stone boat, and dragged into heaps or piled along fence rows. Larger ones are either broken up, buried below the plow line, or endured. Many a former farm boy bears scars because his plow hit a concealed rock.

Since pioneer times, because other rock was scarce over most of the central states, these glacial boulders have had practical uses. In the original land surveys made in the early 1800's, markers were placed at the corners of each square mile. Sometimes they were charred walnut posts but many were boulders buried with just a corner showing above ground. The dams for early waterpower gristmills and sawmills were logs and earth heavily weighted with boulders. Either whole or broken into pieces with sledge hammers, these rocks were used for the foundations of houses and barns, or the masonry walls of cellars, spring houses and chimneys. Today, large colorful natural boulders weighing many tons are sought for memorials and as markers for spots of historic interest.

Here and there in the forest preserves is a rock -- all that remains of an early homestead. The family, the house and the little flower garden are gone. Now it, alone, remains and seems to say, "Keep me company for a moment. Don't just take me for granite."

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