Stratigraphic position is a method of designating an aquifer. Aquifers in the Quaternary Sys-
tem, which have been associated with rock-stratigraphic units,consist of one or more members of
the same formation. Two distinct rock-stratigraphic units that are hydro-stratigraphic units are
members of the BannerFormation—the Sankoty and Mahomet Sands. The Sankoty Sand Member
and the Mahomet SandMember are in the same stratigraphic position, but the Mahomet Sand
Member consists of about equal amounts of sand and gravel, contains many silt beds, and lacks the
polished pink quartz grains that distinguish the Sankoty Sand Member (Willman and Frye, 1970).
The Mahomet Sand Member is found in the eastern part of the basin, and the Sankoty Sand Mem-
ber is found in the central and west-central parts of the basin.
The variety of ways that an aquifer has been named is a source of confusion in the nomen-
clature, especially for aquifers that are composed of glacial sand and gravel deposits. The following
are examples of nomenclature with references to the use of aquifer names in Illinois literature:

1. Time stratigraphic—An example is Quaternary aquifer (Bergstrom and others, 1968;
Piskin and others, 1981).
2. Relative position—An example is upper, middle, or lower aquifer (U.S. Department of
the Air Force, 1989).
3. Alphanumeric designations—An example is A1 aquifer and C3 aquifer. This nomen-
clature is often used in ground-water-flow models.
4. Depositional environment—An example is glacial drift aquifer (Suter and others,
1959; Schicht and others, 1976; Zeizel and others, 1962).
5. Depth of occurrence—An example is less than 50 ft aquifer or greater than 50 ft aqui-
fer (Gibb and O’Hearn, 1980).
6. Synonyms for aquifer—An example is hydrofer, aquiformation, or aquigroup (Curry
and Seaber, 1990).
Different nomenclature among reports for aquifers in the sand and gravel deposits make it difficult to com-
pare aquifer properties.
A study of regional ground-water resources in McLean and Tazewell Counties shows that
most water wells are open to the Sankoty Sand Member of theBanner Formation or the lower
sands of the Glasford Formation (Kempton and Visocky, 1992). The altitude of the top of the
Sankoty Sand Member in McLean and Tazewell Counties ranges from 400 to 500 ft. Thealtitude
to the top of the sands and gravels of the lower Glasford Formation generally is about 550 ft within
the bedrock valleys but is higher where it lies over thebedrock uplands (Kempton and Visocky,
1992). Results of aquifer tests performed in McLean andTazewell Counties have shown no evi-
dence of appreciable vertical leakage between the Banner andGlasford Formation (Kempton and
Visocky, 1992). The Sankoty Sand Member in the Mackinaw Bedrock Valley is contiguous with
the Mahomet Sand Member of the Banner Formation in the Mahomet BedrockValley. The mixing
of waters from each aquifer in the two sand members constitutes a water-chemistryconfluence
area (Panno and others, 1994). Ground water in the glacial materials of the buried bedrockvalleys
is mostly confined. Till or other fine-grained deposits overlie the bedrock valley aquifers, impede
vertical movement, and confine water in the aquifer in most areas. Aquifers in the northern half of
the LIRB under unconfined (water table) conditions consist of shallow, sand and gravel deposits of
the Henry and Wedron Formations (Kempton and Visocky, 1992). The water table is in direct con-
nection with streamsand lakes.
The water table usually follows land-surface topography rising under the uplands and inter-
secting the land surface near perennial streams, lakes, swamps, and springs (McKenna and Keefer,
1991). Ground water flows from recharge areas to nearby streams. The shallow, sand and gravel