The Maquoketa Group (Ordovician) is composed of impermeable to low permeability shales and dolomites that form an important aquitard between the locally recharged Silurian aquifer at the top of the bedrock succession and deeper bedrock aquifers.
There is a significant unconformity on top of the Maquoketa that has caused wide variations in the thickness of the top unit across the county (Kolata and Graese 1983).
The Maquoketa Group is up to 260 feet thick in southeastern and central Cook County. In southwestern Cook County there was significant erosion before the overlying Silurian rocks were deposited, and, because of this erosion, the unit thins to about 100 feet.
Updated 10/23/2009 SLD