Section 55-901 of the municipal code establishes the North Hills environmental resources overlay district. The ordinance was created because the North Hills area is characterized by “steep slopes and valleys with heavy forestation and ridge tops of forests or uncultivated grasslands.” The ordinance requires a special permit for grading and excavating, removing vegetation, and disturbing sensitive features which include native prairies, intermittent and perennial streams, loess bluffs and glacial tills, and extremely steep slopes.