Silurian Strata in Cook County

The top of the bedrock in Cook County consists almost entirely of Silurian dolomite. These rocks, which consist mainly of pure to silty dolomite, form an important bedrock aquifer supplying residences in Cook County. The porosity and permeability of the rocks are mainly the result of fractures and dissolution cavities in the dolomite. The rock itself has no matrix porosity; therefore, the aquifer does not have adequate capacity for most municipal water wells. The water is recharged locally from precipitation, and, where the overlying glacial materials are thin, the upper bedrock aquifer is susceptible to groundwater contamination.

The Silurian rocks are a major source of aggregate material, and there are both active and closed quarries in Cook County. Thornton Quarry in southeastern Cook County is among the largest in the United States in annual production of aggregate material.

Our map of the topography of the bedrock surface (interpreted from over 5,900 wells) differs from the bedrock maps of Horberg (1950) and Herzog et al. (1995). Our data, taken from many more wells, do not show the east-west-trending channel systems shown on the two earlier maps.