Maquoketa Group

The type section for the Scales Formation is in Jo Daviess County, Illinois.

The difference in terms between Iowa and Illinois has made the present nomenclature all but impractical.

Present studies suggest that the Maquoketa should remain a group and not a formation for practical purposes.

The variable thickness is due to an erosional unconformity caused by an ice age at the end of the Ordovician, which lowered global sea level.

It exists in small patchy areas and was extensively eroded at the end of the Ordovician.

It exists in large isolated patches in the subsurface in Lake, Cook, Will and Du Page counties in Illinois.

It is possible that it was originally deposited as a calcareous oolite and hematite later replaced the calcite.

[citation needed] The Maquoketa has almost no economic use since it is dominantly a weak shale interbedded with thin dolomite, it is not good for track ballast or concrete aggregate and is often stripped and piled as waste in open-pit quarries.

The main exception is at the Neda's type section in Wisconsin, where it was once mined for iron ore.

Recently legislation was passed in Illinois allowing horizontal drilling in the shale.

Kolata & A.M. Graese, Facies analysis of the Ordovician Maquoketa Group and adjacent strata in Kane County, Northeastern Illinois 1991, A.M. Graese, Facies analysis of the Ordovician Maquoketa Group and adjacent strata in Kane County, Northeastern Illinois 2007, S.D.J.

Baumann & T. Arrospide, Paleozoic Geology of the Fox River from Batavia to Oswego Illinois

This table shows equivalent rock units within neighboring states around Illinois.
A piece of the Fort Atkinson along Aux Sable Creek in Illinois, showing the coarse texture and multitude of colors.
Waubonsie Creek in Oswego, Illinois . The greenish rock under the concrete block at top center is the base of the Brainard and is dominantly a sticky green shale with thin beds of purplish gray dolomite.
At Kankakee River State Park . The black pencil near the right center of the picture is near the Silurian-Ordovician contact.
Tentaculites oswegoensis