[williecoyote]

Around here we call them "ballast rocks." They were used as ballast in sailing ships, and pitched overboard at the entrance to the bay, or harbor to lighten the ship. This let the ship float higher in the water so that it wouldn't run aground while trying to dock. After supplies were exchanged, the ship could reload ballast after leaving the bay, and continue on out to sea.
Most of the ballast rocks around here are about the size of a cantaloupe. I never thought, but they would be great for cobble stones. Hard work though!
I met a geology professor once who collected, and studied them. He said that you could tell where the ships came from by the type of stone used for ballast. Ireland , Africa, etc.
Worthless bit of information, but sorry about your rocks walking off Vash.