The question will occasionally occur to the wanderer by the Wall, whence were the materials obtained for constructing the mounds of the Vallum? With the exception of the fosse, there are no marks of excavation in the neighbourhood, and that the fosse of the Vallum would not yield materials sufficient for the purpose, is abundantly evident.[24] [Sidenote: USE OF THE VALLUM.] The contents of the ditch on the north of the Wall have probably gone to assist in the formation of these lines. This statement of course proceeds upon the supposition that the Wall and the Vallum were contemporaneous works. Upon the same assumption, it may be added that the ramparts of the Vallum are probably indebted for some portion of the stone which they contain, to the chippings of the Wall. Although the distance between the stone Wall and the Vallum is, as already observed, perpetually varying, the lines of the Vallum maintain amongst themselves nearly the same relative position throughout their entire course. No apparent paths of egress have been made through these southern lines of fortification. The only mode of communication with the country to the south, originally contemplated, seems to have been by the gateways of the stations. If we adopt the theory that the Wall and the Vallum exhibit unity of design, a question of some importance arises—With what view was the Vallum constructed? Hodgson, with much probability, conceives that, whilst the Wall undertook the harder duty of warding off the professedly hostile tribes of Caledonia, the Vallum was intended as a protection against sudden surprise from the south. The natives of the country on the south side of the Wall, though conquered, were not to be depended upon; in the event of their kinsmen in the north gaining an advantage, they would be ready to avail themselves of it. The Romans knew this, and with characteristic prudence made themselves secure on both sides. [Sidenote: PECULIAR CONSTRUCTION OF THE VALLUM.] But, whatever we may conceive to have been the design of the Vallum, the peculiarity of its form will excite the attention of the enquirer, though probably without his arriving at any satisfactory explanation. Supposing, according to the common theory, that the Vallum was an independent fortification, erected long before the Wall, to resist a northern foe, why was not the ditch, as in the case of the stone Wall, drawn along the northern edge of the northern agger? I cannot supply an answer. A similar difficulty meets us on the supposition that it was meant to guard against attack from the other side. Again, what part did the smaller rampart on the south edge of the fosse perform? Possibly it may have been intended as a foot-hold for the soldiers when fighting on this platform against the revolted Britons south of the barrier.