430 Million years of history
At the 2013 Sources of Knowledge Forum professional geologist Daryl Cowell, who lives in Tobermory, gave an excellent presentation describing the last 10,000 years of the history of the Great Lakes and the Saugeen Peninsula in particular.
Hundreds of thousands of tourists are attracted every year to the Peninsula, drawn to the stunning scenery and the possibilities of amusement on the clear water of Lake Huron and Georgian Bay. The foundation of the spectacular views, the clear water and the unique flora and fauna are the rock formations shaping the peninsula and the islands scattered throughout Fathom Five National Marine Park.
The Saugeen Peninsula is part of the Niagara Escarpment, which reaches from Niagara Falls to Manitoulin Island and, indeed, in a great arc to Wisconsin. It began roughly 430 million years ago as a barrier chain of sponge-cored reefs that had built up on the floor of an ancient tropical sea teeming with marine life. Over millennia rivers laid down sediment and sea creatures died and were deposited on the sea floor, gradually building up layers of lime-rich sediments. Beneath these lime-rich sediments were older layers of sediments including muds, silts, sands and other lime-rich materials, deposited within a large depression on the pre-existing 2.5 billion year old bedrock. This part of the Canadian Shield forms a gigantic bowl encompassing lakes Michigan, Huron and Georgian Bay, and Erie. It is known as the Michigan Basin and it comes to the surface north of Manitoulin Island. As you drive north of Little Current you can clearly see where the sedimentary rock of the escarpment ends and the Canadian Shield comes to the surface.
In time, magnesium in the saline surface waters replaced some of the calcium in the underlying lime-rich sediments, converting them to the dolomite mineral, eventually hardening to form dolostone, which characterizes the present day Saugeen Peninsula. Dolostone is the dominant rock type on the surface of the peninsula, however shales (from mud), siltones (from silts), and other dolostones appear beneath the escarpment crest on the slopes leading to Georgian Bay. The ancient sea supported sponges, corals and a variety of shelled organisms which gradually built up an enormous reef, which, apparently, would have rivalled Australia’s Great Barrier Reef. The evidence of these ancient life forms can be found in abundant fossils in many parts of the peninsula.
In geological time, the period when the bedrock now forming the peninsula and the islands were formed is known as the Silurian. Throughout that time, climate and ecosystem changes occurred which caused the nature and frequency of the sedimentary deposits to change and these changes are visible today in the differences in the rock layers. North of Wiarton, the dolomite rock generally falls into three main “age groups” although other, older, pre-Silurian formations are visible lower down on the cliffs facing Georgian Bay. The oldest outcrops on the surface of the peninsula mostly occur at Cabot Head, Cape Croker, and Colpoys Bay. These rocks belong to the Cabot Head Formation. Lying above the Cabot Head Formation and spreading west approximately to the Bury Road and south of Cabot Head is a younger layer called the Gasport/Goat Island formations and on the west side of Bury Road to shore of Lake Huron is the youngest rock type belonging to the Guelph Formation.
The dolostone rock is pitted, often with perfectly round holes. These holes, called “pit karren”, have been formed by rainwater opening up pre-existing pores in the bedrock surface which are subsequently expanded by acids secreted from plants colonizing the rock surface. This is a type of karst landform feature which results from the solution of certain types of bedrock by weak acids in rain and from living vegetation. Karst is a suite of landforms created by solution and enhancing the subsurface movement of water
through sinkholes, cracks and caves down to less soluble rock layers (e.g., shales). There are, for example, sinkholes and cracks which transport water accumulating in the area of Cape Hurd Road and Hwy 6 rapidly via a series of sinkholes into Georgian Bay at Dunks Bay.
Dolostone is not the only rock on the peninsula and the Islands. All over the surface of the peninsula one can find rocks which definitely do not belong. These are known as ‘erratics’ and they have been scooped up in the north and carried huge distances to the area by the glaciers during various ice ages. The glaciers also scooped out the lake basins to great depth so that all the lakes with the exception of Erie, have depths which are well below sea level and could not, therefore, have been carved out by the action of rivers.
The iconic landform in the Fathom Five Marine Park are the structures on Flowerpot Island. Wave action and falling water levels over years have eroded away weaker rock underlying thicker, stronger rock leaving behind rock pedestals which have the locally famous flowerpot shape. The large flowerpot on the island started to form roughly around 3,000 years ago.
A detailed account of the bedrock geology of the Saugeen Peninsula called Memoir 360, Paleozoic Geology of the Bruce Peninsula Area, Ontario by B.A. Liberty and T.E. Bolton
is available from the Geological Survey of Canada, at: https://geoscan.nrcan.gc.ca/starweb/geoscan/servlet.starweb?path=geoscan/downloade.web&search1=R=102382
The surface geology of the peninsula including the types of soft sediments can be found in a more recent Open File Report, Surficial Geology of the Bruce Peninsula, Southern Ontario by W.R. Cowan and D.R. Sharpe from the Ontario Geological Survey, at: https://www.google.com/search?client=firefox-b-d&q=surficial+geology+of+the+bruce+peninsula
A more readable and beautifully illustrated account to which Mr. Cowell made a major contribution is: Geology and Landforms of Grey & Bruce Counties by The Bruce-Grey Geology Committee. This can be ordered from the Ginger Press in Owen Sound.