GLASGOW ARCHAEOLOGICAL SOCIETY.
Vegetius states that it was customary to make the width of the
ditch an odd number of feet.
As regards the shape of the fosse defending our vallum, there
can be no doubt that it was fastigate. In other words, it was
V-shaped or triangular in section. In the well-preserved parts
it has this appearance to the eye even on the surface, and the
inference from the outward aspect is fully verified by our
sections. The conditions, of course, preclude the possibility of
determining to an inch the dimensions of the junction of scarp
and counterscarp, but if they did not meet absolutely in a point
—if there was any plane at all at the bottom of the fosse—the
plane can scarcely have exceeded a couple of feet in width.
The angle of the slope of the scarp appears to have been to all
intents and purposes the same as that of the counterscarp. The
number of full sections yet made is insufficient to warrant an
absolute statement as to the prevailing angle of these slopes
throughout the course of the fosse, but approximately it may
be said that they average from about 26 degrees up to about
30 degrees. They do not vary greatly.
These slopes are taken from observations at places where the
nature of the soil was such as to make it possible to form a
symmetrical slope in the making of the ditch. There are other
places as, generally, on Croy Hill, where the presence of great
dolerite blocks in the ground made the regular formation of the
ditch out of the question. Such places as the latter are, of course,
exceptional: the rule is for scarp and counterscarp to slope at
something between 26 and 30 degrees.
Whether the precise slope varies perceptibly with the different
soils through which the fosse passes is a problem not adequately
soluble until a good many additional sections have been made.
The present inference is that under ordinary conditions there
was no very palpable variation. In any view it is certain that
the slope adopted was a safe one, systematically not higher1
1 Professor Rankine's Tables and Rules, p. 180. " The most frequent slopes
of earthwork are those called 1J to 1 and 2 to 1, corresponding to the angles
of repose 33J degrees and 26^ degrees nearly." The angle of repose for dry
sand, clay, and mixed earth is 37 degrees to 21 degrees.
On the other hand, it is to be observed that these figures rather refer to a
structure made of displaced earth than to a case of earth in situ, as it is when it