May 28, 2002
The diagonal feature crossing this air photo is formed by trees growing on the steep slope of the Niagara escarpment to the south of Green Bay, Wisconsin. The escarpment is the edge of a thick series of resistant Niagara dolomite of Silurian age. The lineament is not controlled by faulting the surface, but by preferential erosion along the edge of a gently dipping sequence of sediments known as a cuesta. The dolomite is very near the surface here as evidenced by the rock quarries visible on the southeast side of the scarp. Softer Ordovician rock underlying the dolomite has been eroded away to form lake basins and lowlands, including Green Bay, during the last period of continental glaciation ending some 12,000 years ago. This same Niagara formation outcrops along the rim of the Michigan Basin to the north and east, and forms the bluffs cut by its namesake, Niagara Falls.