MATERIAL AND WALL THICKNESS
It is obvious that the Church of the Plan of St. Gall was
meant to be a masonry structure (figs. 107-113). Semicircular
apses, circular towers, spiral stairs, the columnar
order of the arcades of the nave—which, because of the way
they were spaced must have been surmounted by arches—
the barrel-vaulted corridors of the crypt, the arched galleries
of the abutting paradise—all these are features
germane to stone construction. Although there is abundant
evidence that in the ninth century a high percentage of the
smaller transalpine parish churches were built of timber,[129]
it is equally clear that an abbey church, intended to serve
as a model, could only have been constructed in stone. All
the major Carolingian churches were built in stone.
Some of the more hallowed parts of the Church, such as
the crypt or the interior of the apse and the fore choir, may
have been built in ashlar, but all the principal walls of the
Church were unquestionably built in roughly coursed
rubble. We have good parallels for both these techniques
in the Palace Chapel at Aachen (798-805), the Abbey
Church of Corvey-on-the-Weser (873-885), and the
Church of Germigny-des-Prés (799-818), in all of which
the external work was built in rubble, while most of the
structural parts of the interior were constructed in dressed
stones.[130]
Other examples of Carolingian ashlar construction
are found in the crypts of St.-Germain of Auxerre (841859)
and Flavigny (864-878).[131]
It is reasonable to assume that a church of the dimensions
of that of the Plan of St. Gall rested on foundations
about 5 feet wide. This is suggested by the dimensions of
the bases of the nave columns, and by the dimensions of
the supports which stand at the point where the aisle walls
meet the walls of the transept. It is equally reasonable to
assume that the full thickness of the foundation walls was
not retained in the walls themselves. A thickness of 3¼ feet
or 40 inches (one and one-half standard units) would
appear to be a reasonable assumption for both the aisle
walls and the clerestory walls.