Galena and Platteville Groups

In the study area, the Galena and Platteville Groups are combined into a single map unit because the rocks have similar physical characteristics. The combined thickness ranges from 274 to 372 feet in a somewhat random pattern; however, there is a slight thickening trend from northwest to southeast across the county.

In Cook County, these rocks consist of relatively pure fine- to medium-grained dolomite with low permeability and porosity. As reserves of aggregate resources in the near-surface Silurian rocks are being depleted, several quarries in the Chicago area have mined, or are planning to mine, the Galena and Platteville rocks beneath the floor of their existing quarry, accessing these deeper resources via inclines through the intervening Maquoketa Group. Many quarries in the Cook County area cannot add to their resources by expanding laterally because of urban development on surrounding land.