To everyone saying that the walls keep sheep in, I can tell you from first hand experience that this is simply not the case. Sheep will stay within the boundaries of cleared land, simply because their food source, grass/pasture, is a product of that cleared land. Farmers did not give a rats ass where their sheep went to pasture, as they did not even keep them in barns (sheep can sleep out in the snow all winter long). The early barns you see were built to store either hay or wheat, the latter of which was grown on cultivated land. And, jf you've ever had to plow land in Vermont and try to plant grain, you will soon realize how much area is lost to rocks if you do not make a concerted effort to remove them, dragging them along with your plow until you reach the boundary of your field (where you pile them up to get them out of the way). If you pay close attention you will find that land that was too steep to plow will not have stone walls, although it was most certainly cleared and used for sheep.