Since the excavations at Avebury it has been a mystery that the Ditch inside the bank surrounding the greatest stone circle in the world should be thirty feet deep on its south side. A walk round the fosse as it remains to-day reveals the fact that it is deeper on the south side, where the ground-level is higher than on the north; on the latter the ordinary level is 510 ft. O.D., while on the former it is 527 ft. This seems to point to the conclusion that a ditch was planned with a level bottom irrespective of the original level of the ground at any one point, and that the Ditch was not therefore made the same depth all round. The enormous labour of digging this huge trench 30 ft. deep, over 40 ft. wide at top, and 17 ft. at the bottom was incurred for some definite object. Ordinarily the theory of a prehistoric ditch is that it was to keep out man or animals; in this case 10 ft. of depth with fairly steep sides would be impassable for either; therefore to accdunt for the extraordinary exertion of going down 20 ft. deeper than necessary we must adopt another hypothesis, and the fact that the ditch is now deeper on the south, where the ground is highest, gives a clue to the problem.