glaciers do not move continuously across the landscape. The ice
margin may advance and retreat over a given area many times during
an overall advance--a net movement forward. The same is true during
the overall retreat; the ice margin may readvance multiple times
during an overall meltback of the glacier. Advancing glaciers pick
up debris left by previous glaciers, including sands and gravels,
as well as large boulders. They grind across bedrock in areas not
covered by glacial deposits, incorporating rocks stripped from the
bedrock surface into their load as well. When the ice begins to
melt, till -- the unsorted mixture of rock, sand, clays, and
boulders picked up by the glacier -- is deposited. If the glacial
ice is melting as fast as it is advancing, the margin of the ice
sheet remains essentially in the same spot and deposits large
amounts of material in the that area, forming an end moraine.
Glacial debris can also be deposited by moving water within, on top
of, or underneath a glacier. When this occurs, the materials are
sorted, with the heavier rocks settling out of the flowing water
sooner, while the finer sands and clays remain in suspension over a
greater distance. Rocks, sands, and gravels can also be deposited
as outwash -- material that is carried by meltwater streams and
sorted and deposited both along the ice margin and some distance
away from it. When meltwater from the glacier subsides, the wind
picks up the lighter sands, silts and clays from the streambeds and
carries them still further from the ice margin. The term aeolian is
used to describe materials transported and deposited by the wind.
Glacial debris can also be deposited in lakes via meltwater
streams. When this occurs the material is well sorted, the heavier
rocks, gravels, and sands are being deposited near the lake margin,
and the finer clays and silts are deposited in the deeper water.
The tills in the Midewin area were deposited between about
18,000 and 14,000 years ago as the Lake Michigan Lobe (a lake of
glacial ice) melted back out of Illinois. The Marseilles, Minooka,
and Rockdale moraines and the Valparaiso morainic system, all
important in the formation of the proglacial lakes that covered
much of the Midewin site, were forming during this time period.
Proglacial lakes form when the meltwater from a glacier is
impounded between the ice margin and higher ground that lies in
front of it -- an end moraine, for example. When they eventually
drain, by either eroding a new or existing outlet through the
moraine or overtopping the high ground, proglacial lakes leave
behind lacustrine (lake) sediments and a lake plain as evidence of
|Surficial Geology of the Midewin
Midewin NTP outlined in Black
Excerpt from map by Willman and Lineback, 1970, Illinois State
Geolgoical Survey (Willman, 1971).
This map shows the Rockdale Moraine (green, stippled, labled rd)
which was deposited when the glacier was melting as fast as it was
advancing. Ground moraine, till deposited as the glacier was
moving, is labled as mmg (Minooka ground moraine - light green).
The light purple color (lp) represents lake plain or floors of
glacial lakes (such as Lake Wauponsee) with a thin layer of lake
sediment in places that is underlain by glacial till. The dark
purple areas labeled (S) are dolomite. The areas labeled (sl) are
glacial sluiceways - areas where outlets of glacial lakes were cut
into till and bedrock. Glacial sluiceways contain sand and gravel
deposits. Areas labeled (hm) are also sand and gravel. The bright
yellow color along the streams are deposits in floodplains mostly
silt and sand with occasional gravel.