GLASGOW ARCHAEOLOGICAL SOCIETY.
at Birrens and Ardoch.1 The non-prevalence of this broad
berm is evident also from sundry incidents in Roman warfare.
It is repeatedly on record that, when an attack was made on
an entrenched camp, the attacking force strove to tear down
the rampart and push it forward into the ditch, and Livy
tells how an ill-rammed agger tumbled into the fosse.2 This
is scarcely compatible with the existence of a berm of any
magnitude. These are cases of camp walls, however, and
it would seem that even in the case of the camps on
our wall a distinction prevailed. Thus, at our sections in
Roughcastle there is. in Nos. 1 and 2, which are part of the
vallum, an unquestionable berm of the usual dimensions, whilst
in No. 3, which is a cutting through the vallum of the camp
only, there is either no berm at all or a berm of the smallest
dimensions, which is now externally not visible. On what
principle does the existence of the broad berm depend ?
At an early stage in our investigations we were strongly
impressed by an argument3 that there was such a correlation
between the vallum and the fosse as tended to render the problem
of the berm considerably less difficult. It was contended as
probable that the top of the wall was in line with the angle of
the scarp of the fosse, so as to have the bottom of the ditch fully
in view and under the fire of the soldiers on the wall, without
their unduly exposing themselves. Without a berm this would
not have been so; the bottom of the ditch would not have been
under effective fire from the top of the vallum at all; in other
words, the bottom of the ditch would have been what in
military language is termed " dead." By setting the wall some
distance back from the ditch this disadvantage would be
obviated, whilst, at the same time, the perfect stability of the
structure would be ensured.
1 The sections of the main rampart at Birrens, made in 1895 by the Society of
Antiquaries of Scotland, strikingly resembled in most of its details our Rough-
castle section No. 3, which on this point of the berm differed so widely from all
our sections of the vallum proper.
2 Livy, x. 5, quoted supra, p. 33.
3 Set forth by Mr. Chalmers in a letter published in the Glasgow Harold, and
dated 31st March, 1891.