American Memory, the Library of Congress' "gateway to rich primary source materials relating to the history and culture of the United States," offers this gem of a site on exploration and land use in the US. These historic and recent maps will be of great interest to ecologists, as they show "changes in the landscape, including natural and man-made features, recreational and wilderness areas, geology, topography, wetland area, vegetation, and wildlife." The site may be searched by keyword or browsed by Subject Index, Creator Index, Geographic Location Index, or Title Index.