Dear Faldage: I think I remember your telling me you had
lived in New England, so you may have seen wall there. When
the colonists arrived, most fields had so many stones left
by the glaciers that they were not arable. So walls were built often just to get stones out of the way, as well as
to mark boundaries. Most of them were so rounded it was hard
to made a wall that could withstand frost heaves. Remember Frost's poem about mending fence with his neighbor. And for
hundreds of years the plow would keep turning up new ones.
(The Devil below keeps pushing them up.) Every year in Maine,
before any other work could be done, all hands turned out to
load potato sized stones onto the stone boat.

Frost:Mending Wall
http://www.bartleby.com/118/2.html