To the visitor exploring Lake Superior, rock structures create dramatic views and intriguing landscapes. A walk along the lakeshore reveals a large variety of rock shapes and sizes. 

How many pictures have we seen (and taken) of visually fascinating rock features from around Lake Superior? Among the oft visited and photographed rock structures are the steep cliffs of Minnesota’s Palisades and Shovel Point, the eroded dike that is called the sea lion in Ontario’s Sleeping Giant Provincial Park and the soft curves of eroded sandstone called Miner’s Castle at Michigan’s Pictured Rocks National Lakeshore.

For Lake Superior’s shores, like the rest of the world, rocks form the framework of the landscape and help set environmental parameters. Each feature tells a story about the region’s beginnings. 

Layers of sediment form cross bedding at Miner’s Ledge in Michigan.

Take, for instance, the Palisades and Shovel Point on Minnesota’s north shore. The impressive precipices reveal the geologic history of the shoreline. It was about 1.1 billion years ago that a specific crystallization pattern of cooling molten rhyolite formed the picturesque cliffs. After complete cooling, the result was a weakened rock mass and row upon row of jointed rock columns (based on rock shrinkage and crystallization pattern). During the more “recent” geological past - 5,000 years ago - the lake’s waves pounded this area and the weakened joints allowed rock to cleave and fall, leaving dramatic cliff faces.

Likewise, Miner’s Castle and adjoining steep cliffs at Pictured Rocks National Lakeshore were formed by powerful water forces of pre-Lake Superior. As the glaciers of 10,000 years ago receded, bedrock released from the weight of the mile-high glacier rebounded, expanding like a squeezed sponge. Meltwater from the retreating ice mammoths was trapped between the glaciers and rebounding rock and formed lakes, whose waves eroded the soft sandstone into popular features seen today.

High pressure and temperatures bake this marble-cake look on stones.

The story of Pictured Rocks goes further back in time. The soft sandstone cliff rock was deposited about 520 million years ago (Cambrian geologic period). Telltale evidence of this period remains. Marine fossils can still be found. Large cross bedding features - layers of sediment built up at different angles by changing current patterns - reveal interaction of a primitive coastal marine system with large river deltas delivering tons of deposited sediment that turned into sandstone. The Michigan highlands to the south and later the Canadian Shield to the north were the source of these sediments.

At Otter Cove in the far southern end of Pukaskwa National Park, Ontario, observation of the Canadian Shield reveals an artistic composition of rose pink, grey and white patterns resembling a marble cake. Technically, this rock is migmatite.

The shield is one of the oldest exposed outcrops of rock in the world. Migmatite is metamorphic rock, resulting when an original rock is subjected to high temperatures and pressures on a regional basis. These rocks partially melted, separating into component minerals of different colors (pink, gray and white). Intense pressure and shearing that accompanied the melting resulted in a marble cake-like mix - cooling rocks twisted and stretched. This major geologic event was the Kenoran Orageny, a regional mountain building that occurred about 2.7 billion years ago.

Rocks and rock formations are nature’s own history books into the past. Take time to investigate the story of rocks. They explain much about the past of the region that we enjoy in the present.

David Lonsdale is executive director of the Great Lakes Aquarium at Lake Superior Center.