Geology
The rocks of Lake Superior's northern shore date back to the early history of the earth. During the Precambrian (between 4.5-billion and 540-million years ago) magma forcing its way to the surface created the intrusive granites of the Canadian Shield. These ancient granites can be seen on the North Shore today. It was during the Penokean orogeny, part of the process that created the Great Lakes Tectonic Zone, that many valuable metals were deposited. The region surrounding the lake has proved to be rich in minerals. Copper, iron, silver, gold and nickel are or were the most frequently mined. Examples include the Hemlo gold mine near Marathon, copper at Point Mamainse, silver at Silver Islet and uranium at Theano Point.
The mountains steadily eroded, depositing layers of sediments which compacted and became limestone, dolostone, taconite and the shale at Kakabeka Falls.
The continent was later riven, creating one of the deepest rifts in the world. The lake lies in this long-extinct Mesoproterozoic rift valley, the Midcontinent Rift. Magma was injected between layers of sedimentary rock, forming diabase sills. This hard diabase protects the layers of sedimentary rock below, forming the flat-topped mesas in the Thunder Bay area.
Amethyst formed in some of the cavities created by the Midcontinent Rift and there are several amethyst mines in the Thunder Bay area.
Lava erupted from the rift and formed the black basalt rock of Michipicoten Island, Black Bay Peninsula, and St. Ignace Island.
During the Wisconsin glaciation 10,000 years ago, ice covered the region at a thickness of 1.25 miles (2.01 km). The land contours familiar today were carved by the advance and retreat of the ice sheet. The retreat left gravel, sand, clay and boulder deposits. Glacial meltwaters gathered in the Superior basin creating Lake Minong, a precursor to Lake Superior. Without the immense weight of the ice, the land rebounded, and a drainage outlet formed at Sault Ste. Marie, which would become known as St. Mary's River.
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