On a map, Door County, Wisconsin looks like a simple peninsula. It isn’t. This narrow strip of land is part of the Niagara Escarpment, a hard dolostone ridge that runs through Wisconsin, Michigan, Ontario, and into western New York past Niagara Falls. Door County is one of the clearest places to see how the Niagara Escarpment and the Michigan Basin work together.

The ridge rises above the water on both sides and follows the same gentle inward tilt seen around the rim of the Michigan Basin. Because of that tilt, it forms steep cliffs along Green Bay where the edge of the ridge is exposed. Along Lake Michigan, the same rock dips away more gradually, producing long stretches of exposed bedrock, ledges, and pockets of sand rather than a continuous cliff face.

The escarpment also influences drainage. Most of Wisconsin sends its water toward the Mississippi River, but not here. Surface water in the region surrounding the Door Peninsula flows into the Great Lakes and the St. Lawrence system.

During the last glacial period, moving ice carved deeply into softer rock to the west, helping form the Green Bay basin, while the escarpment remained standing as a resistant spine. At the northern tip, that spine was cut through by glacial ice. The narrow strait called Death’s Door funnels water between Green Bay and Lake Michigan, creating strong currents and hazardous conditions that gave the passage its name.

The orientation and durability of a single ridge of rock determine the defining features of Door County, Wisconsin.

By Brian Calley