Wisconsin Geological & Natural History Survey » Wisconsin ...
The sandstone and dolomite aquifer, also called the sandstone aquifer, Cambrian aquifer, or Cambrian-Ordovician aquifer, lies beneath the dolomite aquifer and the shale aquitard in eastern Wisconsin, beneath glacial deposits in central Wisconsin, and at the surface in southwestern Wisconsin.It is absent in north-central Wisconsin. These rocks are 425 to 580 million years old.