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.
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.
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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Update: June 2012