We called them "slave fences," having been told that they had been built by slave labor before the Civil War. They're dry-stack stone walls, built without the use of mortar between the stones, and are common in Kentucky's Bluegrass region. There they originally were "clearance walls," built from stones cleared from the fields to make plowing easier. In Kentucky, they date mostly from the nineteenth century, although they have a much older history in Great Britain.
While dry-stack stone walls may come in various designs using different materials elsewhere, in Kentucky they tend to be similar wherever found. The fields of the Bluegrass are underlain by limestone, so that's the stone available. The first several courses are lain flat, wide enough to provide stability. The final course is placed near the vertical, which both increases height and anchors the top. It happens to be esthetically pleasing, as well.
Today the stone walls are mostly decorative. On working farms they tend to be backed by electric fences that do the real work of keeping the stock confined. Often they are in disrepair and additional fencing is essential. But where they are found, there's always a tug to get the camera out and shoot some pictures.