As you explore the trails and parks of the Illinois and Michigan
Canal National Heritage Corridor, you'll become aware of the beauty
and variety of the landscape along the canal route. But you may not
realize that the distinctively shaped hills and ridges, the bedrock
gorges, the marshes, and the lakes are all evidence of the activity
of glaciers and glacial meltwaters.
The modern landscape you now see records the retreat of the last
major ice sheet that extended into Illinois from 25,000 to 14,000
years ago. This invasion took place during the most recent or
Wisconsinan Glaciation, which geologists estimated extended from
75,000 to 10,000 years B.P. (before present time). During that
time, the Laurentide Ice Sheet covered much of Canada and the
northern United States. Nurtured by a continental climate colder
than today's, the ice sheet grew as snow accumulated and the
pressure of its own weight caused it to change to ice and spread
outward from its Canadian center.
The tongue of the ice sheet that flowed into Illinois came from
the north. It became known as the Lake Michigan Lobe because it
flowed as a river of ice through the Lake Michigan Basin before it
spread out into central Illinois. When the glacier reached its
southernmost limit, the ice was a mile thick at Chicago--an
enormous weight that depressed the land beneath. Later as the ice
retreated and the glacier's weight was released, the Earth's crust
began to rebound. The crust is still rebounding today, especially
from Milwaukee northward.
When glaciers extended into Illinois, the climate was much
different from today. At the ice margin, long-haired mastodons
browsed among spruce forests. The average yearly temperatures were
near or just above freezing, and most of the year's precipitation
was in the form of snow. In the summer, enormous volumes of
meltwater and sediment flowed away from the glacier, but during
winter, river volumes were reduced to a relative trickle.
Geocache is placed on Department managed property with
permission. It is the visitor's responsibility to orient themselves
with policies and rules pertaining to this Department managed
site.
To log the cache, please email your answers to the
following:
A) How deep is the canyon? (take a picture of a person of
known height and then count how many 'persons' deep the canyon
is)
B) Starved Rock and the St. Louis Canyon are erosional
remnants of what type of sandstone?
C) What factors influenced the shape and size of the canyon
in its present condition?
