On the north side of the present military way, between the 33rd and 34th milestone, two unhewn blocks, one of basalt, about 6 feet high – the other lower, and of sandstone, stand very conspicuously on a ridge of ground which has been artificially heightened for a small distance around them. They are now called the Mare and Foal; but on Armstrong’s map, which was published in 1769, “The Three Stones.”
Found in ‘A History of Northumberland’ by John Hodgson. p291, pt2, vol3. 1840. Online at Google Books.