Northern Wilds Magazine
Along the Shore

Understanding the Slate Islands archipelago

Terrace BayLake Superior surrounds three unique island archipelagos: Isle Royale, Michipicoten Island, and the Slate Islands. Isle Royale and Michipicoten Island were uplifted by ancient faults, and all three settings include one billion year old Keweenaw igneous rocks.

The Slate Islands is a circular archipelago of 17 islands found 12 km southeast of Terrace Bay, Ontario. They contain an impressive complexity of deformed rocks, including a little slate. The archipelago is a natural environment provincial park with a trail system and camp sites, but no facilities. Visitors often make private boat arrangements in Terrace Bay and kayaks provide a safe, convenient way to travel between islands.

Visitors to the Slate Islands will find a central highland surrounded by a series of lower islands around the edge of the circular archipelago. The gunflint (iron) and rove (slate) formations are present and folded and faulted. Igneous and sedimentary rocks of the one billion year old Keweenaw province are also present and deformed, including the presence of shattercones, which are conical features associated with the shock waves of meteorite impacts. The axes of the shattercones point toward the direction of impact, usually vertical. Some geologists have proposed that the 9-meter-high shattercone at McGreevy Cove is the largest on the planet. The lowest parts of the islands are a complex of fractured older rocks called a breccia, also common in impact sites. The age of the impact is controversial, but it probably happened 450 million years ago by the arrival of a meteorite; 1.5 km in diameter forming an impact crater and 30 km in diameter, interpreted by the chaotic geology, bathymetry and geophysical anomalies around the archipelago.

Recent glacial features and sedimentary deposits are also found on the islands and we now see a southern boreal forest with shoreline arctic-alpine flora in equilibrium with the cold water of Lake Superior. Woodland caribou arrived in the 1900s and the islands support about 200 animals. Other wildlife is present along the Superior shoreline, maintained by the locks at Sault St. Maire at 601 feet above sea level. Logging activities were in operation in the 1930-40s and there is a lighthouse on the south shore of Patterson Island.John P. Craddock

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