I posted this week about rocks being carried long distances by ice. When I read The Pebbles on the Beach by Clarence Ellis last year, he mentioned that sometimes you can be fooled by rocks that were carried as ballast by ships and then dumped. This section of the Tyne near Wallsend was one such place. Thames dredgings mean that some of the rock could be chalk. I didn't go searching because of warnings about chemicals from old lead and tar works. In Physics, the extra mass carried in the bottom of the boat makes it harder to rock from side to side. It would give it a moment of inertia.
