Ancell Group

In Cook County, the Ancell Group consists primarily of the medium-grained St. Peter Sandstone. The Ancell Group is a moderately important groundwater resource, but its permeability and water yield are not as great as the underlying Ironton-Galesville or Mt. Simon Sandstones (Suter et al. 1959).

The base of the St. Peter Sandstone is a major unconformity. Pre-St. Peter erosion removed hundreds of feet of underlying formations in Cook County. The penetrated thickness of the interval from the top of the Ancell to the top of the Ironton Formation ranges from 351 to 662 feet. The interval's thickness nearly doubles from north to south in Cook County.