Ironton-Galesville Formation

The Ironton-Galesville Formation consists of clean, medium- to coarse-grained, partly dolomitic sandstone and consistently has the largest permeability values of all the bedrock units in northeastern Illinois. It is the most important bedrock aquifer in the county, and municipal wells can obtain relatively large water supplies with high flow rates.

The penetrated thickness of the Ironton-Galesville Formation ranges from 94 to 249 feet. Although the thickness pattern is mostly random across Cook County, there is a definite thinning of the unit in the northeastern corner of the county and adjacent Lake County. The most permeable part of this sandstone occurs in the lower 20 to 85 feet of the unit; however, the entire thickness of the Ironton-Galesville is a good aquifer.