Lake Superior rests in the 1.1-billion-year-old depression left behind when plate tectonics pulled the North American continent apart. Known as the “Midcontinent Rift System, this geologically diverse area holds some of the world’s greatest mineral deposits. Unlike the lower Great Lakes, whose underlying bedrock has largely been buried beneath the sediments of saltwater seas, magma-borne granites and gabbro intersect with volcanic basalts in the Lake Superior basin, creating lodes of copper, veins of gold, silver and platinum, pockets of uranium and vast quantities of iron ore.
From Marquette to Manitouwadge, Houghton to Hemlo, these valuable minerals have drawn prospectors and built communities throughout the Lake Superior basin. As paddlers, we are drawn to Superior’s rocks for aesthetics alone. Here are three features you may encounter on a guided sea kayak trip with Naturally Superior Adventures.
Raised beach – In the wake of the last ice age, the landscape has been slowly bouncing back from the weight of the glaciers in a process known as “isostatic rebound.” On Lake Superior’s shores, this process is manifest on raised beaches, where ascending terraces of baseball- to melon-sized cobbles represent former high-water lines as the shoreline has rebounded since the glaciers retreated 9,000 to 10,000 years ago. Bill Mason’s classic National Film Board production Rise and Fall of the Great Lakes demonstrates this and other ice age impacts on Lake Superior.
Where to see it: Old Woman Bay (Lake Superior Provincial Park), Cobinosh Island (Rossport area), Detention Island (Neys Provincial Park), Otter Island (Pukaskwa National Park).
Diabase dike (intrusion) – Occur where faults in the underlying geology has allowed volcanic material known as “diabase” to flow, creating a formation known as a dike. Sometimes, these formations appear as sinuous lines of jet-black rock snaking through lighter coloured rocks. Differing rates of erosion in diabase (soft) and granite (a hard, igneous rock) often creates sheer-sided canyons, such as the dramatic cleft in the rock near the Agawa pictographs.
Where to see it: Simpson Island (Lake Superior National Marine Conservation Area), throughout Pukaskwa National Park, Agawa Rock trail (Lake Superior Provincial Park).
Jacobsville Sandstone – Like icing on a cake, this reddish, grainy rock (formed when sand sediments fused together 900 million years ago), overlays portions of Lake Superior’s volcanic and igneous foundation, particularly on the southern shores of the basin. Sandstone quarries produced the stalwart “brownstone” used to construct lighthouses in Michigan and Wisconsin and the canal buildings in Sault Ste. Marie, Ontario.
Where to see it: Channel Island (Rossport Islands), Grindstone Point (Lake Superior Provincial Park), St. Mary’s Rapids (Sault Ste. Marie).
To learn more, pick up a copy of Roadside Geology of Ontario: North Shore of Lake Superior by E.G. Pye.