What are these large, round rocks?

July 9, 2009 11:17 PM

I came home from Sequim Bay here in Washington with several cannon ball sized rocks I found on the beach. They had fallen from the hill-side as it eroded, leaving round capsule like depressions. They are a bit flaky and seem to be made of mud/sand that has hardened (but not like the sandstone I've seen elsewhere). What are these called? What caused them?
posted by pocketlama to science & nature (4 comments total)

These could possibly be a type of concretion, rocks that are formed from a mineral cement in a layer of sediment. I think.
posted by neewom at 11:38 PM on July 9, 2009

Could be mudstone? or siltstone?
posted by girlgenius at 11:51 PM on July 9, 2009

Indeed it will be a concretion, I have a so called "cannon ball concretion" I found many years ago, it's in the garden as part of a rockery.
posted by hardcode at 12:11 AM on July 10, 2009

Yup, I see now it is for sure a concretion. The pictures at Wikipedia of the round rocks are just like mine. Thanks for your answers!
posted by pocketlama at 1:33 AM on July 10, 2009