Archaeologia Islandica - 01.01.2010, Blaðsíða 21

ON FARM MOUNDS
varies with climate, acidity of soil and
precipitation, drainage, chemical condi-
tions on site - especially as regards
organic preservation - and a range of
other factors. In addition to degradation
from natural causes material may be
removed intentionally from a site or dis-
tributed in such a way that it does not
form deep stratigraphies.
These basic premises hold true for all
deep stratigraphies, whether they are
Near Eastern tells, medieval towns or
North Atlantic farm-mounds. Farm-
mounds are therefore no unique phenom-
enon; deep stratigraphies can be expected
in all cultures where settlements are sta-
ble and localised. Not all such settlement
sites form deep stratigraphies however
and it is therefore reasonable to look fírst
for the environmental or chemical rea-
sons behind the development of farm-
mounds.
Although figures for volume are not
available Icelandic excavators of farm-
mounds agree that the bulk of the materi-
al comprising their farm-mounds is
building material, i.e. turf and to a lesser
extent stone (e.g. Mjöll Snæsdóttir
1991 a, 118; Margrét Hallgrímsdóttir
199lb, 115-16). It is only at the very
largest mounds, such as Bessastaðir
(which is more than fíve times larger in
volume than Stóraborg) where ash seems
to make up signifícant proportions of the
accumulation (Fig. 5). It seems however
that this is exceptional and relates mainly
to the very highest status sites.
Turf architecture requires vast
amounts of turf and loose soil to be
brought on site. The difference in volume
from pure timber constructions, wattle
and daub or even brick or stone architec-
ture is up to 10 times greater. The timber
in the walls of a hypothetical structure
with the dimensions 5x20x2 (excluding
the roof) has the volume 20 m3 if the
walls are 0,2 m thick. The walls of a turf
house with the same dimensions would
have the volume 168 m3 if the walls are
1,5 m thick or 224m3 if the walls are 2 m
thick. In addition the turf house also
needs a timber frame and quite often con-
tains a whole timber building inside the
turf walls. Of greater importance is how-
ever the fact that materials like timber
and stone, and to some extent brick, can
be reused to a considerable degree, each
building phase therefore leaving only
small volumes of unusable or inaccessi-
ble materials in the ground when the next
is built on top. Turf on the other hand can
as a rule not be reused, except as filling in
the cores of new turf walls, requiring the
bulk of dilapidated turf walls to be either
levelled out on top of the earlier founda-
tions or removed from site.
To confound this even further, turf is
an unstable construction material which
easily erodes, cracks, crumbles or falls
from under its own weight. It therefore
needs constant repairs with small quanti-
ties of new turf being added on an annual
to decadal basis (Guðmundur L.
Friðfinnsson 1991, 207, 209) and neces-
sitating complete rebuilding every 60 to
100 years (see below). This means that
the replacement rate of turf structures is
much greater than of most other building
materials used in Northem Europe.
The external factors contributing to
the destruction of turf walls are precipita-
tion, frost and erosion. Rainwater enters
19