"May 1981 4-R-37453-A. Revised October 1976 Base 4-R-33577." "Base compiled from USGS national atlas, 1970 edition." Available also through the Library of Congress Web site as a raster image.
United States. Soil Conservation Service
1981
<a rel="nofollow" class="external free" href="https://www.loc.gov/item/82692895/">https://www.loc.gov/item/82692895/</a>