Horicon Marsh in eastern Wisconsin is a thirty-two-thousand-acre wetland spanning northern Dodge and southern Fond du Lac counties, including a national wildlife refuge in the north and a state wildlife area in the south. Formed by Pleistocene glaciation, it evolved from Glacial Lake Horicon after the Rock River cut through a natural dam, leaving today’s marsh and its drumlin islands. People have lived around it since the end of the last Ice Age, and Native trails once converged near its southern end. Nineteenth- and twentieth-century damming, hunting, drainage, fire, and restoration reshaped the marsh, which is now a protected international wetland and major migration stopover for waterfowl.