1876 Map of a reconnaissance from Carroll Montana Ter. to the Yellowstone National Park and return by Captain William Ludlow, Corps of Engineers, Chief Engineer Dept. of Dakota, in July, August and September 1875 - commonwealth 4m90fb53. From the Digital Commonwealth (<a rel="nofollow" class="external free" href="https://www.digitalcommonwealth.org/search/commonwealth:4m90fb53n">https://www.digitalcommonwealth.org/search/commonwealth:4m90fb53n</a>).
US Army Corps of Engineers
Digital Commonwealth