Bruce Peninsula Boulders

Provided and copyright by: Steve Irvine
Summary Authors: Steve Irvine, Stu Witmer

The bedrock on the Bruce Peninsula in southern Ontario is dolostone, (many geologists refer to the rock dolostone by its mineral name of dolomite) and it's not unusual to come across a glacial erratic (boulder) transported from Canadian Shield rock, about 60 miles (100 km) to the north. In some locations, however, there are high concentrations of diverse igneous, metamorphic and even sedimentary boulders all jumbled together. Most are rounded, and many show percussion fractures, as the large boulder in the above photo. What forces brought them all to this place? The cliff in the background gives a clue. Transporting large rocks like these requires huge volumes of fast-moving water. Such flows can occur in subglacial sheetflood events. When water from a sheetflood flowed over the cliff its carrying capacity abruptly decreased, and the boulders collected below. Photo taken on July 6, 2008 from McKay's Harbour, Ontario, Canada.

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