"Gutzon Borglum's design intentionally left three extra inches of granite on the surface of the sculpture so that nature, in the form of wind and water erosion, would finish carving Mount Rushmore for him over the next 20,000 years." - Matthew Buckingham in Cabinet Magazine
|Workmen on face of Geo. Washington, Mt. Rushmore|
Source: Library of Congress
I've stumbled upon claims similar to this a number of times. The amount of stone and the number of years varies. But, I've never seen a footnote or reference to a source that confirms that the meme is true. I found the above quote about 5 months ago in a post titled "Half a Million Years of U.S. History" on the Long Now Foundation Blog. My curiosity compelled me to spend some time digging for an answer. I wrote a reply in the comments section of the blog which was never published. Perhaps I should have waited a bit longer, given the nature of the Long Now project. I'm impatient... So, I'm going to publish what I found here:
I've seen the claim that the sculpture was designed to erode to completion, but after extensive searching I have (so far) had great difficulty verifying if it is true. However, I did locate early references to the erosion of the granite that are enlightening. Former US President Calvin Coolidge was commissioned to write text that was to be sculpted in stone as part of the monument.
"The Coolidge words... will be five or more inches deep, and high enough to be read miles away." "Geologists estimate that the erosion on the granite escarpment will be an inch in 100,000 years. Unless radical earth changes take place, the words on the memorial should endure more than 500,000 years." - Coolidge History of U. S., on Granite, to Last 5,000 Centuries. Chicago Daily Tribune, Jan 17, 1930
This erosion estimate is attributed to C. C. O'Harra, President of the South Dakota School of Mines:
"Dr. O'Harra, geologist, estimates that the monument will withstand the ravages of time and erosion for 500,000 years." - Great Figure on Black Hills Mountain Will Be Revealed With Ceremony Today. New York Times, 04 July 1930
According to the National Park Service the O'Harra estimate was off by a factor of ten. The modern estimate of the erosion rate is 1 inch per 10,000 years.
There was some controversy about the text and it was never included in the monument.
I have no idea if the sculptor intentionally left extra rock on the presidential portraits with the intent that it erode away to completion. But, these quotes confirm that there was at least one person during the time of construction that considered the Long Now implications of the slowly changing granite face of Mount Rushmore.