ROCK FORMATIONS AND PRL-GLACIAL HISTORY 



River and Trenton limestones. The Menominee and White- 

 fish rivers cascade over the nortliern edges of these southward- 

 sloping rocks to reach Green Bay and Lake Michigan. When 

 the limestone-making sea became shallower, fine muds were 

 deposited, and through countless ages these also hardened to 

 rock — a soft shale. When exposed at the surface the shale did 

 not resist weathering but gradually was worn away to form 

 the moat behind the Cambrian-Ozarkian-early Ordovician 

 sandstone and limestone rampart. Today we find parts of that 

 moat occupied by Green Bay of Lake Michigan, by North 

 Channel and Georgian Bay of Lake Huron, and by the Lake 

 Ontario basin in the eastward extension of the Ordovician 

 rocks. 



Following the time when the muddy seas of the late 

 Ordovician deposited muds that became the soft shales under- 

 lying the Green Bay-Georgian Bay-Ontario moat, came the 

 Silurian time of genial climates when in the warm clear seas 

 life was abundant. Shelled creatures of many kinds swam the 

 seas or crawled upon their floors, and corals and bryozoans 

 built great reefs. The shelled creatures and corals died, shells 

 and reefs w^ere broken and buried in lime mud, and through 

 long ages the muds hardened to limestone filled with the petri- 

 fied remains of a life long past. So hard and resistant to 

 weathering is this rock that it formed the next great encircling 

 rampart. This limestone is named Niagaran, because of its ex- 

 posure along Niagara River — particularly at the brink of Ni- 

 agara Falls. From western New York south of Lake Ontario, 

 the Niagaran limestone rampart extends northwesterly across 

 Ontario in a broad arc separating Georgian Bay from Lake 

 Huron, rises to the surface as Manitoulin and Drummond 

 islands, continues across the eastern half and forms the south 

 shore of the Northern Peninsula of Michigan (excepting St. 

 Ignace Peninsula) , then circles southward between Green Bay 

 and Lake Michigan along the Lake Michigan shore of Wiscon- 

 sin to the southern end of Lake Michigan, where it forms the 



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