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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.)\nCahokia 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\n\nThe scale of these data is 1:250,000.", "geometryType": "esriGeometryPolygon", "sourceSpatialReference": { "wkid": 102100, "latestWkid": 3857 }, "copyrightText": "Illinois State Geological Survey", 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