2. The Stone Foundation.
Why has the cespiticious wall a foundation of stone ? And
what is to be concluded from the squared kerbs and the periodic
culverts ? That some purpose of strength, security, or dura-
bility was to be served by the stone foundation we cannot doubt.
That this base course of stone made the footing of the wall
firmer, drier, less liable to subsidence and bulges, goes without
saying. It must not only have served to allow the water in the
vallum to pass down through it as through a " rumbling drain,"
but it must also have prevented the vallum from gathering damp
from below by direct contact with the soil. On this phase of its
purpose the periodic culverts afford strong evidence. Although
these are through drains, with an entrance at the one kerb and
exit spout at the other, it is probable that, in addition to carrying
off external water from the higher side of the vallum, they
afforded an outflow for the moisture trickling from the body of
the vallum into and along the stone base below. In modern
earthworks a similar necessity is met by a somewhat similar
One thing seems clear from the squared kerbs considered in
conjunction with the appearance of the sections. The distance
from the outer face of the north kerb to the outer face of the
south one must define the original width of the vallum. No
doubt the terminations of the dark lines appear to pass 3 or 4
inches beyond the kerbs, but it is noticeable that this protraction
is mostly confined to one side, and it has, with some show of
reason, been attributed to a bulging out of the lower courses of
turf induced by the pressure of earth2 and military movements
above. Moreover, it would be hard to understand the careful
laying of squared kerbs if they were to be absolutely covered up.
It is much more natural to believe that when the vallum was
1 " Should the ground rise to the rear . . . drains should be out under the
ground to be covered by the parapet, and filled up with stones and fascines before
the parapet is constructed, so as to let water soak through them. All enclosed
works should be thus drained." Philips, article 280. This, however, appears to
apply only to the drainage of the interior of the work, not to the drawing away
of the excessive moisture from the parapet itself.
2 The same reason is assigned for the occasional depression of the dark lines in
the middle and their concave curvature outwards to the face of the vallum.