By USGS – http://pubs.usgs.gov/fs/fs-0028-01/fs-0028-01.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4550077
By USGS – http://pubs.usgs.gov/fs/fs-0028-01/fs-0028-01.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4550077