Glaciers form during climatic episodes when more snow
accumulates during the winter than melts away during the summer;
over time the snow thickens and under the pressure of its own
weight is compressed into ice and begins to flow outward. During
warm intervals, glacier ice melts and the glaciers recede (melt
back). During cold intervals, more snow accumulates than melts and
the glaciers advance. Rocks and soil frozen in the base of the
glacier are dragged along. These materials can act like sandpaper
to smooth the landscape over which the glacier passes. This
abrasion can smooth the face of rocks and round out hills. It can
also gouge out valleys. On some exposed rock surfaces you can find
evidence of this glacial scouring in the form of glacial striations
(parallel lines carved into the rock).
Like rivers, glaciers tend to follow the path of least
resistance. The glaciers that flowed into the northern United
States from Canada during the Ice Age followed the courses of
former river valleys that were eroded into the least resistant
rocks. The glaciers gouged out rock from the base and sides of
these valleys creating the troughs occupied today by the Great
Lakes.
When glaciers melt, they leave behind whatever they were
carrying. This may consist of a mixture of rock debris and old
soils, called till, material that was ground-up and deposited from
the base of the glacier. Some of the most striking reminders that
the landscape was once covered by glaciers are boulders and
cobbles, called erratics, that dot the landscape. Unlike the rock
you find in local quarries, these are exotic rocks. In Illinois,
erratics are often granitic rocks like those you would find today
hundreds of miles away in Canada.
Glacial meltwaters carried ground-up rock debris away from the
glaciers. In the valleys of major meltwater channels, like the
Mississippi and Illinois valleys, this debris settled out as layers
of silt, sand, and gravel, called outwash. On dry, windy days, the
finest particles of this outwash were blown across the landscape in
glacial dust storms. These particles settled across the landscape
to form a blanket of silt-size particles, called loess. Loess forms
the basis of the young, rich soils of the northern plains of the
United States.
Around Illinois are boulders lying alone in the corner of a
field, someone's yard, or as markers at historical sites. These
boulders are glacial erratics. Many of them are colorful and
glittering granites, banded gneisses, and other igneous and
metamorphic rocks. They seem out of place in the rockless prairie
of our state. These rocks came from Canada or states north of us
during The Great Ice Age. The glaciers pushed them along as they
advanced and left them behind when they retreated.
A cubic yard of stone (27 cubic feet) weighs approximately 2.5
tons, estimate how heavy you think this boulder is.
To log this Earthcache:
1)Post a picture of yourself and GPSr with the
Erratic in the background.
2. Email me the answers to these questions
within 3 days of your log: DO NOT POST ANSWERS IN YOUR LOG.
a)Measure or estimate height, width, and length.
b)Estimate weight of erratic.
If these requirements are not met your log will
be deleted.
Congrats FTF-
jennyNjim
Cache placed by a member of CIGA
Link to CIGA Home Page