Abstract:
This map shows the distribution of coarse-grained materials and permeable
bedrock within 50 feet of ground surface in Illinois. This includes bedrock,
sand and gravel, and alluvial units with characteristics that suggest a
potential to store or conduct groundwater and yield potable water to wells and
springs. It was derived from stack-unit map data in the ISGS data set
... STACK-ST.
Aquifers or potential aquifers on this map are defined as sand and gravel units
at least five feet thick, sandstone at least ten feet thick, and fractured
limestone or dolomite at least fifteen feet thick with a lateral extent of at
least one square mile. Minor aquifers typically yield from five to seventy
gallons of potable water per minute. Potable water is defined as water
containing less than 2,500 mg/L of total dissolved solids. For use in this data
set, the following stratigraphic units are considered to be aquifers or
potential aquifers meeting these criteria: (although Cahokia Alluvium is
primarily fine- grained, it is included because it contains numerous sand and
gravel deposits.)
Cahokia Alluvium
Parkland Sand
Equality Formation, Dolton Member
Henry Formation
Sand and gravel within Wedron Formation
Sand and gravel within Winnebago Formation
Pearl Formation (includes Hagarstown Member)
Sand and gravel within Glasford Formation
Mounds gravel and related units
Cretaceous sediments, silts, sands, etc.
Pennsylvanian rocks, mainly sandstones
Mississippian rocks, mainly limestones, some sandstones
Silurian and some Devonian rocks, mainly dolomite
Ordovician and Cambrian rocks, mainly dolomite, some sandstone
Coverage is intended to identify areas of Illinois where there is a potential
for shallow aquifers to exist. These data are appropriate for regional thematic
analysis. 
