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. {35} [Illustration: Fig. 3.--Three stages in the history of the Vallum.] {36} 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.