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 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.
