By Dr. Nat (Drnat) on Tuesday, September 29, 2020 - 08:05 pm:
My advisor when I was working on my M.S. is an expert in concretions, so I hope I learned something while studying with him!
Concretions are places where the cement holding sediments together is more concentrated, making the concretions more resistant to erosion than the surrounding rock. This occurs because there is some nucleus—a unique mineral grain, fossil, piece of organic material, etc.—that makes the cement preferentially precipitate in that area. There are a number of different types of concretions with different mineralogy and different shapes depending on the geologic conditions they formed under.
I happen to like the Moeraki Boulders in New Zealand. They are an interesting group of big concretions.
By jbuck (Jbuck) on Tuesday, September 29, 2020 - 09:30 pm:
Thanks Dr. Nat!
Is this a case where the concretions are continuing to grow even now? Or at least that is the case in some locations?
By Dr. Nat (Drnat) on Wednesday, September 30, 2020 - 10:55 am:
Concretions can grow when the sediments are below the water table. It's the water that is carrying the minerals that precipitate as cement.
By Deb S. (Usedtobeayooper) on Thursday, October 1, 2020 - 06:22 pm:
Gorgeous!