The conspicuous, broad area of eastern Wisconsin that slopes toward Green Bay and the Fox River reflects the slope of the underlying rock that was smoothed and worn by glaciers. The slope into the bay is broken by north-south ridges that are glacial moraines. The flat area extending to the west and south of Lake Winnebago once was part of Glacial Lake Oshkosh, which filled a large part of the lowland as the Green Bay Lobe advanced and later melted away.

Lake Winnebago, the largest lake in the state, is bounded on its eastern margin by an escarpment of resistant dolomite (known as the Niagara Escarpment) that extends northward, forming the Door Peninsula. The dolomite layers dip eastward toward Lake Michigan and are a source of crushed stone and building material in the region.