Fig. 3.—Three stages in the history of the Vallum.
3. That in many cases gaps occur in the mounds where there are no corresponding causeways across the ditch; but it is just in these regions that the subsidiary mound is found. Where there are causeways, there is no subsidiary mound.
The deduction is that when the causeways had served their purpose the ditch was cleared again, and the clearings were cast up to form the marginal mound; but nothing was done to fill up the gaps, because the ditch was the only consideration.
Excavations have all tended to support this theory. The causeways were evidently made not long after the digging of the ditch, because the ditch is not silted up under the cast-up rubbish of which the causeways are formed; and everywhere else in the Vallum-ditch there is a depth of 3 or 4 feet of silt.
Gaps occur in the Vallum-mounds all along the line to beyond Carlisle, with great regularity (as if they had some set purpose) and generally about 45 yards apart. The explanation suggested is that nearly all the stone and building-materials needed for building the Wall and repairing the forts had to be brought from beyond the Vallum; that thousands of men, employed in bringing materials, would be constantly passing over the mounds and ditch of the Vallum. Hence the need for causeways and gaps.
We can imagine the men would get impatient at having to climb a mound, descend into a ditch, and then climb another mound at each journey north or south. And if rough carts were used, a causeway would be a necessity. By degrees continuous traffic would of itself sink a path across the mounds; and what would be easier than to make deliberate gaps and fill up the ditch with the soil?
The Romans were a methodical people, and in undertaking an enormous work like the Wall they would certainly have points, at regular distances apart, to which building materials had to be brought. The only difficulty in the theory seems to be that a gap every 45 yards could hardly be necessary.
Supposing this theory to be correct, then the Vallum is older than the Wall and its contemporary buildings, but very little older. It has already been proved that the Vallum is not older than the original forts, because it always curves round to the south when it approaches a fort, in order to avoid it.
The Stone Wall appears to have been an after-thought, found necessary for the final solution of the defensive problem.