water flow through structural pathways
(Panno and others, 1994).
The Wisconsinan glacial deposits cover approximately half of the LIRB to the east and north-
east. The concentric band of Wisconsinan deposits intersects the Illinois River near Peoria and is
thicker than olderIllinoian deposits in the southern half of the basin. The Wisconsinan deposits
are characterized by concentric bands of moraines that were deposited during the retreat of glacial
ice lobes that formed Lakes Michigan and Erie. These moraines are the dominant landform in the
northeastern part of the LIRB.
The glacial deposits range from 50- to 500-ft thick and are thickest in buried bedrock valleys.
The thick, sand and gravel deposits in the east-west trending Mahomet Buried Bedrock Valley are
major aquifers for public supply in central Illinois. The sands and gravels deposited by glacial
streams are thick aquifers near the bottom of the valley. The overlying fine-grained tills deposited
by glaciers restrict potentialcontamination from reaching the aquifers. Sand and gravel deposits
within the Glasford Formation arelaterally extensive where they are associated with the Vandalia
Till Member and constitute a locally productive aquifer (Panno and others, 1994).
The mineral composition of glacial deposits indirectly reflects the source or age of the sedi-
ments. Carbonate materials are local bedrock, but heavyminerals, such as hornblende, in the sands
commonly are derived from crystalline rocks of southern Canada. The older pre-Illinoian tills have
a higher calcite to dolomite ratio than younger deposits, which is anindication of an eastern source
for the deposits. The Wisconsinan tills have more dolomite than calcite, more than 50 percent less
calcite in fine sediment than the pre-Illinoian tills, indicating a northern source for the deposits.
Johnson and others (1986) studied the heavy mineral composition of sediments to determine the
source for the tills. The tills in the eastern part ofthe LIRB were considered part of the Lake Erie
glacial lobe deposits, however, the mineral composition issimilar to glacial deposits from a Lake
Michigan or Lake Huron source. Therefore, the tills in the eastern part of the LIRB are likely
derived from the LakeMichigan source. Garnet, epidote, and pyrite are abundant in tills in central
Illinois and have a low mean ratio of garnet-to-epidote (Johnson and others, 1986).
Soller (1993) mapped the lithology of surficial deposits in the eastern United States, which
are mostly glacial in origin for the LIRB. The coarse-grained deposits are along streams and allu-
vial valleys, but thick till is found over buried
bedrock valleys (fig. 15). The type of
surficial depos-
its and thickness are theprimary factors affecting ground-water contamination; therefore, the sand
deposits in the center of the LIRB and along streams have a higher potential for contamination.
The stratigraphic relation of glacial deposits is complex because of different modes of depo-
sition, multiple sources, and lack of regional
continuity(fig.
16A). Areal distribution of formations
andmembers within formations is generalized because boundaries are intertongued and often
indistinct (fig. 16B).
The boundaries
between formations are similar to facies changes. The Wedron
Group is the most extensive till unit of Wisconsinan age in the LIRB, and the type section is largely
till with numerous interbedded deposits of outwash gravel, sand, and silt. Other formations with
type sections within the LIRB include the Morton and Peoria Loess, which are silty and sandy for-
mations of Wisconsinan age (Willman and others, 1975). Major deposits of loess wereformed dur-
ing Wisconsinan glaciations as silt and fine sand was blown from the bottomlands and deposited
on adjacent bluffs and uplands. On the bluffs, southeast of the broad lowlands in the Havana area,
loess accumulated to nearly 100-ft thick, but elsewhere it is not as thick (Willman, 1973). Two dis-
