F. 4.--The same decay is probably the key to the problem of the "Vallum," that standing crux to all archaeological students of the Wall. Along the whole line this mysterious earthwork keeps company with the Wall on the south, sometimes in close contact, sometimes nearly a mile distant. It has been diversely explained as an earlier British work, as put up by the Romans to cover the fatigue-parties engaged in building the Wall, and as a later erection intended to defend the garrison against attacks from the rear. Each of these views has been keenly debated; the last having the support of the late Dr. Bruce, the highest of all authorities on the mural antiquities. And excavations, even the very latest, have produced results which are claimed by each of the rival theories.[289] F. 5.--Quite possibly all are in measure true. The "Vallum" as we now see it is obviously meant for defence against a southern foe. But the spade has given abundant evidence that the rampart has been altered, and that, in many places at least, it at one time faced northwards. Though not an entirely satisfactory solution of the problem, the following sequence of events would seem, on the whole, best to explain the phenomena with which we are confronted. Originally a British earthwork[290] defending the Brigantes against the cattle-lifting raids of their restless northern neighbours, the "Vallum" was adapted[291] for like purposes by the Romans, and that more than once. After being thus utilized, first, perhaps, by Agricola, and afterwards by Hadrian (for the protection of his working-parties engaged in quarrying stone for the outer fortifications), it became useless when the Wall was finally completed,[292] and remained a mere unfortified mound so long as the Roman power in Southern Britain continued undisturbed. But when the garrison of the Wall became liable to attacks from the rear, the "Vallum" was once more repaired, very probably by Theodosius,[293] and this time with a ditch to the south, to enable the soldiers to meet, if needful, a simultaneous assault of Picts in front and Scots[294] or Saxons behind. Weak though it was as compared to the Wall, it would still take a good deal of storming, if stoutly held, and would effectually guard against any mere raid both the small parties marching along the Military Way[295] from post to post, and the cattle grazing along the rich meadows which frequently lie between the two lines of fortification.