Thanks for the ping. These stone walls are an interesting mystery.
Mystery?
To be able to farm, you have to clear rocks. What do you do with the rocks? Stack them up. After a while, you get enough where are you're making a wall...then it becomes a fence.
We make the boys pick up rocks to clear grazing pasture and ones that float up in the wheat field (Rocks float). Use the rocks for erosion control. Then the boys made a three sided structure off in the back...it should last for hundreds of years. *If the cows can't knock it down, it will last.