Trilobite Detritus

The Craigleith area of Ontario is well known for its abundance of fossils. In 1992 I visited this locality on the southern shore of the Georgian Bay to search for them. There was a railway cut through the shale near Collingwood. The first slab of rock that I cracked open revealed a cluster of trilobite carapaces. They date to the late Ordovician which is about 445 million years ago.

Closeup of trilobite tail pieces.

Half a Million Years of U.S. History

"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:

Shatter Cone

Shatter Cone from the Sudbury Basin.

In 1992 I attended a conference on Large Meteorite Impacts and Planetary Evolution in Sudbury, Ontario. The highlight of the visit was the field trip that we took to view the geological evidence of a massive meteorite impact nearly two billion years ago. The impacting object was likely in the range of 10-15 km (about 1/2 - 1 mile) in diameter. It left an oval basin which is 60 by 30 km (40 by 20 miles) containing breccia, a rock formed from the broken fragments of pre-impact bedrock. There are also rocks that have been shattered by the shock wave from the impact. These are only found in impact craters and are called shatter cones.