Isolated patches of rough terrain in the unglaciated Driftless Area, primarily in southwestern Wisconsin, stand out from the surrounding, smoothed glaciated ground. Thoroughly dissected by tributaries of the Mississippi River, the topography within these "islands" was engulfed but never overridden by the ice. The appearance of the Driftless Area landscape today probably is much like that of adjacent glaciated landscapes prior to the first Pleistocene ice advance. - USGS