South Watuppa Mill Stones

A number of these round stones line the shore of South Watuppa Pond in Fall River, along the former railroad (now a bike path). The extend into Westport also. They appear to be made of a green sandstone. They are about 5-ft diameter and 1-ft thick. They don't appear to be grist mill stones. I'd love to find out what they are, where they came from, and how they got there.