Middle and Upper Ordovician formations are well developed in Minnesota, where the following classification is usually recognized:
Saint Peter sandstone
The Glenwood beds are generally of slight thickness and rest disconformably on the Saint Peter sandstone. Although these beds are not very fossiliferous, their contained fauna is sufficiently distinctive to cause them to be included in the Mohawkian series.
Kay has established a three-fold division of the Decorah formation 2 in Iowa. These divisions can be recognized to some extent in Minnesota as well, but they are less distinct than farther to the south. In fact, the formation in Minnesota is more truly a shale, and many of its limestone layers are lenticular. Faunally, however, subdivision is possible. The Decorah shale is easily separated from the Platteville limestone, but it grades upward . . .