North Shore (Lake Superior) - Geology

Geology

The North Shore lies on the north side of the Midcontinent Rift System which ran 1,300 miles (2,000 km) northwest from Michigan, through what is now Lake Superior, and southwest again into Kansas. As the granitic crust was torn apart by continental drift, lava flowed out from 30–60 miles (50–100 km) under the crust and formed a basalt crust instead. Tectonic forces were not strong enough to continue to separate the two sides of the continental rift; when the rifting stopped, the lava cooled and the heavy crust sank and was filled with sediment.

During recent glaciations, a large amount of the basalt and sandstone, which erode much more easily than granite, was removed by the glaciers. This formed the rough, rugged shoreline on the North Shore today. As the glaciers retreated, they left behind eroded igneous material, much of which covers the rocky beaches on the North Shore. The Wisconsin side of the basin and the bottom of the basin filled with the residue from the eroded sandstone and thus are sandy beaches today.

The melting water from the retreat of the glaciers ran into the basin and began to fill, forming the Great Lakes. The shoreline at its maximum reached over 500 feet (150 m) above its current height, and at its minimum fell to 250 feet (75 m) below its current level. When the levels of Lake Michigan and Lake Huron fell 2,000 years ago, it created rapids at Sault Sainte Marie, which restricted the release of water from Lake Superior and brought the shoreline to its present level.

The modern shoreline is composed of basalt lava flows. In the south, near Duluth, other materials, such as slate, greywacke and sandstone, are found a short distance inland; however in the north, the entire bedrock made of basalt and gabbro is exposed in patches miles from shore. When the bedrock hits the surface of the lake at a shallow enough angle, the beach is covered with washed up rocks. When it hits the lake at a steep angle, it breaks off and makes sharp cliffs ending at the lake.