Chapter IV: The Vallum

The history of the Vallum in detail would appear to be as follows:

The Emperor Hadrian decided that his new frontier should be defended by a chain of new forts, and that a great ditch--the Vallum-ditch--should be dug across the island, along the line of these forts, to mark the boundary of the Roman Province of Britain. This would be quite consistent with his usual policy of limiting the extent of the Roman Provinces in order to strengthen his hold on what it was most important to retain. By keeping to the south of the chain of forts, the ditch would come under their protection.

That the ditch, and not the mounds, was the objective is pretty certain, for the ditch was made continuous at all costs, while the mounds were afterwards subject to trespass by the road, by quarries, by a mile-castle etc.

The mounds are the upcast from the ditch, not thrown up on the very edge of the ditch, for then rain and other causes would soon have combined {34} to fill the ditch again, but carried some 24 feet away, leaving a flat safety platform, known as a "berm," on each side, between the ditch and the mound. Thus the mounds had the effect of making the ditch appear deeper, and yet had no great tendency to fall back again into it. The mounds are not the mere upcast from the ditch; sods, and sometimes stones are laid as kerbs to strengthen them, and keep them from settling down on themselves.

A subsidiary mound to the south of the ditch is often found, covering a portion of the berm. This is now known to be no part of the original scheme, but to consist of a later clearing of the ditch. Sometimes there are two subsidiary mounds, one to the north as well.

In this connection, three important points have been noted by Mr. F. G. Simpson and Dr. R. C. Shaw.

1. That there are ancient causeways across the Vallum-ditch in some parts of its course, where the ditch has been quite filled up level with the berms.

2. That wherever these causeways occur, gaps occur, opposite the causeways, in the Vallum-mounds; and this suggests that a passage-way has been cut through the mounds, and that the earth removed has been used for filling up the ditch at the same point, so as to make a roadway right across the earth-work.

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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 {37} 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.

It has long been a question whether there was not originally a turf-wall right across the island, thrown up hastily to mark out the course of the Stone Wall, and as a temporary defence, and then gradually replaced by the Stone Wall. The presence of a piece of turf-wall, more than a mile long, between Birdoswald and Wallbowers, running north of the {38} Vallum and south of the Stone Wall, has to be accounted for; and for two reasons it seems probable that this was only a temporary structure: first, because there is no carefully-laid stone foundation under the turves, as is the case with the Antonine Wall; secondly, because no military way accompanies it, and the Romans always had a road accompanying their fortifications.

Mr. Simpson thinks this particular stretch represents a mistake in the laying out of the works, because it is so close to the Vallum that a mile-castle could not have been built at the usual position: which mistake was corrected by the Stone Wall, but the fruits of which it was not thought necessary to remove.

Or--another theory--this turf-wall may have been a temporary local barrier, thrown up during some interruption of the building of the Stone Wall. It is certain that the turf-wall is the earlier, for where it ends, at Wallbowers, its ditch goes under the Stone Wall, and had to be filled up when the Stone Wall was built.

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