2021 changes:

Park Statistics: NPS

Last card: "After about 500,000 years, the faces will likely have lost some of their definition. But at this rate the basic shape of the presidents’ heads might last up to 7 million years"

From October 2018:

• Carving History:
  https://www.nps.gov/moru/learn/historyculture/carving-history.htm
• Author of 4 books on Mount Rushmore:
  http://www.jeanpatrick.com/mount-rushmore-qa-part-3.html
• The Workers:
  https://www.nps.gov/moru/learn/historyculture/the-workers.htm
• Lincoln Borglum Dies; Aided at Mt. Rushmore:
  https://www.nytimes.com/1986/01/29/obituaries/lincoln-borglum-dies-aided-at-mt-rushmore.html
• How the mountain got its name:
  [https://rapidcityjournal.com/how-the-mountain-got-its-name/articlee0fc2daf-280c-5831-b984-eb7359c901f2.html](https://rapidcityjournal.com/how-the-mountain-got-its-name/articlee0fc2daf-280c-5831-b984-eb7359c901f2.html)
• Mt. Rushmore National Memorial:
  http://memory.loc.gov/diglib/legacies/loc.afc.afc-legacies.200003524/