It was formed in the Ordovician period and has three rock members: Galena, Decorah, and Platteville formations.

It lies on top of the platteville limestone formed by the shallow sea that covered central North America.

It also serves as release of phosphate ions into the ground water system if acid rain is introduced to it.

The Sinnipee Group covers around 25% of Wisconsin,[5] including part or all of the following counties: Brown, Calumet, Dane, Dodge, Fond du Lac, Grant, Green, Green Lake, Iowa, Jefferson, Lafayette, Marinette, Oconto, Outagamie, Rock, Walworth, Waukesha, Winnebago.It is a belt approx.

twenty miles wide that stretches from Marinette County south through the Fox Valley to the Beloit area, and west to the Mississippi River[5].

It is believed that this group formed from a shallow sea that once covered much of North America.

The Sinnipee Group over burden consists of glacial till from the Laurentide Ice Sheet that is around 11,000 years ago.