The Driftless Area spans parts of southwestern Wisconsin, southeastern Minnesota, northeastern Iowa, and northwestern Illinois. It is a distinct landscape in the Upper Mississippi River Basin that was left unglaciated during the last ice-age which ended 10,000 years ago (The term “driftless” indicates it did not receive glacial drift, sediments left behind by glaciers). The underlying geology is known as karst topography: an area of irregular limestone in which erosion has produced fissures, sinkholes, underground streams, and caverns. In the Driftless area, coldwater streams and rivers cut steep canyons prior to joining the Mississippi River.
Source: Trout Unlimited Conservation Succes Index (CSI)
Watersheds in the image are color coded for the quality of brook trout habitat, in order from best to worst: blue, green, yellow, light-brown, red. Dull brown areas are places where Brook Trout have been extirpated (i.e. can no longer be found). Note that Brown Trout can sometimes be found in areas where brook trout no longer survive (and TU publishes a separate CSI index for Brown Trout in the Driftless Area). All of these data can be downloaded as KMZ/KML files to be viewed in Google Earth from the “Midwestern Resources” menu in the sidebar.