The stone that stops Lake Superior

A thick layer of stone holds back the waters of Lake Superior and creates rapids where the waters of Lake Superior drop 21 feet at the head of the St. Marys River. Without this layer of stone between Lake Superior and the St. Marys River, the force of flowing water would quickly erode the riverbed and level the two waterways. The Sault and much of the southern shore of Lake Superior rest on the Jacobsville Formation, a bedrock of reddish sandstone. Up to 1,000 feet thick in the Sault and along the shore, its color comes from iron trapped in the rock. Engineers take core samples like this before starting work on each new lock, because understanding the nature of the bedrock is critical when planning new projects.

Sault Ste. Marie, Michigan · United States