As the glacial ice edged slowly southward from Canada, it froze fast to
and picked up soil and loose pieces of rock, with enormous force tore
away huge chunks of bedrock, and mixed and ground these materials
together (fig. 17).
Into Illinois the glacier carried rock materials from Canada, Wisconsin,
Minnesota, and Michigan; other rock fragments were picked up in Illinois
as the ice front advanced. When the glacier melted, it left behind its
load of rock flour and rock fragments, much of it as a gray clay
containing pebbles, cobbles, and boulders. Geologists call such deposits
glacial till.
[Illustration: Figure 16—Extent of the exposed deposits of the
Wisconsinan, Illinoian, and Kansan glaciers in Illinois, and the
unglaciated areas of the state.]
UNGLACIATED
WISCONSINAN GLACIER
Freeport
Fulton
Peoria
Decatur
Charleston
ILLINOIAN GLACIER
Kewanee
Waterloo
Carbondale
Harrisburg
KANSAN GLACIER
UNGLACIATED
Hardin
The ice in the continental glaciers usually crept forward, sometimes
slowly, sometimes more rapidly. Whether the front of a glacier moved
forward or back depended on the balance between the rate of forward
motion of the ice and the rate of melting. When the ice advanced faster
than it melted, the front of the glacier moved forward. When the glacial
ice melted faster than it moved forward, the front of the glacier
receded. When the rates of melting and advance were about equal, the
front of the glacier stood still or moved back and forth in a narrow
zone.
[Illustration: Figure 17—Striated boulder. Scratches and flattened
surfaces were caused by abrasion by other rocks while boulder was
embedded in glacial ice.]
When such a more or less stationary front existed, an enormous amount of
clay, silt, sand, pebbles, and boulders was deposited in a belt only a
few miles wide along the front of the glacier, creating a line of hills
and ridges that extended for many miles. Such belts, called end
moraines, can be seen today in many parts of Illinois.
The building of end moraines often was accompanied by the release of
great quantities of water (meltwater) from the melting ice. The water,
laden with rock debris, flowed from the front of the glacier in many
streams.
As the meltwater flowed away from the glacier it sorted its load,
although the sorting was rarely perfect. The heavy boulders and pebbles
usually were dropped first, then the sand, next the silt, and finally
the clay. In general, the farther the deposits were from the glacier the
finer they were. The major streams frequently carried pebbles 50 to 100
miles from the glacier and it was many more miles before all the sand
was dropped. They carried some of the fine silt and clay as far as the
Gulf of Mexico.
Sometimes the floods of glacial water were greater and flowed faster
than usual and so were able to carry coarse rock materials farther. As a
result, gravel was laid down on top of earlier sand deposits. Later
there may have been further sand deposition.
The debris-laden meltwater that flowed into valleys often deposited in
them a considerable filling of sand and gravel. Some valleys were filled
to a depth of as much as 100 feet. Such deposits are called valley fills
or valley trains. Modern streams have cut their courses into many of
these fills and even worn away large parts of them. Remnants of valley
train deposits are now large terraces or benches along streams, many of
them well above the present stream channels.
Where many small streams flowed from the glacier, they deposited sand
and gravel as a large apron in front of the glacier. Such deposits are
called outwash plains and many of them extend for miles.
Two other types of sand and gravel deposits made by glacial meltwaters
also are significant. One was formed where water issued from the front
of a glacier or poured into holes or crevasses in the ice. The sand and
gravel in the water formed a deposit that now appears as a rounded hill
associated with a terminal moraine and is called a kame. The second type
of deposit was laid down in beds of streams flowing under, through, or
on the glaciers and was left as a more or less continuous ridge of sand
and gravel when the ice melted. Such a deposit is called an esker. Some
eskers in Illinois are about a quarter of a mile wide and several miles
long. Typical are the Kaneville Esker northwest of Aurora, the Adeline
Esker south of Freeport, and the Exeter Esker west of Jacksonville.
The deposits of both the Illinoian and Wisconsinan glaciers are widely
distributed throughout the state. Melting of the Illinoian glacier
caused comparatively little flooding; consequently, extensive gravel
deposits were formed in only a few places. The ice of the Wisconsinan
glacier, however, melted rapidly and produced great floods laden with
sand and gravel. Thus, most major gravel deposits in Illinois are
related to the Wisconsinan glacier.
Wind sweeping across the sand and gravel deposits blew the sand into
hills or sand dunes near such places as Havana, Prophetstown, Kankakee,
and Watseka. Even today the wind shifts sand of long-forgotten glacial
floods.