http://www.megalithomania.com/show/image/63
http://www.megalithomania.com/show/image/1762
http://www.megalithomania.com/show/image/2427
This stone (one of many at Castleruddery) shows that they can't be for splitting the stone as they run down two sides. You justt wouldn't split such a small stone in that way: http://www.megalithomania.com/show/image/55
Today's stones: http://www.megalithomania.com/show/image/6244