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The Parks / Ontario / Fathom Five National Marine Park

The bedrock underlying Fathom Five and the Bruce Peninsula is limestone - sedentary rock formed by warm inland seas that covered the area 400 million years ago when this area was near the equator. As the sea dried up, calcium deposits made from the shells of countless marine creatures that had collected on the ocean floor, absorbed the water’s magnesium content and became dolomite - a rock somewhat like limestone but harder. The upheaval of this ocean floor created the escarpment. Evidence of geological forces that shaped the islands includes the underwater caves, the pitting and scouring caused by the water action on the soft limestone, and the erratics - huge boulders moved far from their original location in the north by receding glaciers. Submerged geological features include pop-ups, such as the one found to the northeast of Echo Island, a 5 metre-high pressure ridge 1750 metres long. A fissure along the top of the ridge is 6 metres at its widest - a measure of isostatic rebound. An even more exciting, and possibly more confusing, discovery is the ‘submerged waterfall’. 5000 to 10 000 years ago, what lies underwater now between Manitoulin Island and the Bruce Peninsula was dry land cut by raging rivers and gigantic waterfalls. As glaciers continued to melt and the Great Lakes were formed, waterfalls were lost under the waves. The spillway south of Middle Island was probably larger in volume than Niagara Falls. Now located 30 - 35 metres beneath the surface, it falls 40 metres over the 800-metre length of rapids that have eroded 20 metres of the escarpment bedrock. Along the ancient, now submerged land bridge between Manitoulin and the peninsula, archeologists search for evidence of the aboriginal people who used this corridor for trading, fishing, hunting and camping for at least 3000 years.