Fróðskaparrit - 01.01.1998, Blaðsíða 178

184
LATE HOLOCENE CLIMATIC FORCING OF GEOMORPHIC ACTIVITY
IN THE FAROEISLANDS; NORTH ATLANTIC OCEAN
posed sites without thick snow cover (Har-
ris, 1986). From Fig. 7 is also seen that a
MAAT lowering of only 2-3°C, compared
to modern conditions, presumably is
enough to establish meteorological condi-
tions for widespread discontinuous per-
mafrost in the Faroese highlands. As the
weathering rate of bedrock tends to in-
crease as MAAT is lowered and areas of
permafrost is being established (Humlum,
1992; unpubl.), there is empirical reason to
expect an increased production rate of talus
in the highest Faroese mountains during
years with particular low MAAT. By this,
the vertical distance from the summit of
Slættaratindur to the theoretical level for
discontinuous permafrost (Fig. 7) repre-
sents an inverse proxy for the expected
rock-fall intensity.
The periglacial environment
Our investigations on the Faroese terrestri-
al environment indicate a strong coupling
between the distribution of modern geo-
morphic processes, topography and moun-
tain climate. Adopting a geomorphic ap-
proach, the present periglacial boundary is
typically located within a range from 250 to
450 m a.s.l., corresponding to a MAAT of
3.5-5°C; above this altitude vegetation is
sparse and periglacial activity widespread.
Across this range pattemed ground and
sorted stripes become frequent, and impart
a clear periglacial appearance to the land-
scape. This important geomorphic bound-
ary closely corresponds to that proposed by
Williams (1961) as a means of delimiting
the periglacial environment. It is hypothe-
sised that especially the number of growing
degree days and frequency of freeze-thaw
events (Fig. 5) represent important controls
on the lower limit for modern periglacial
activity. A third control is presumably that
of wind exposure. At some very wind ex-
posed sites the modern lower periglacial
boundary may be as low as 100-200 m a.s.l.
In short, the late 20th century periglacial
environment in the Faroe Islands is charac-
terised by extreme humidity and strong
winds, rather than extreme cold. The pre-
sent treeline is close to sea level, the lower
periglacial boundary is at about 250-400 m
a.s.l., and the potential discontinuous per-
mafrost level is situated 300-500 m above
the highest mountains (Fig. 7). Likewise,
the present potential glaciation level is pre-
sumably situated only 150-250 m above the
highest mountains.
During the LIA, assuming a periodical
MAAT lowering of 2-3 °C in the Faroe Is-
lands, as documented by historical sources
(Lamb, 1989), the periglacial boundary
may have shortly approached sea level in
exposed regions. Within this context the
historical documentation of LIA periods
with extensive loss of sheep due to lack of
grazing may be significant (Madsen, 1990).
This occurred several times during the LIA
and as late as AD 1888, when polar ice ex-
tended south on both sides of the Faroe Is-
lands (Lamb. 1977; 1985). Especially the
winters AD 1694-95 and 1716-17 are
known as very hard (West, 1985). During
the last of these winters as many as two-
thirds of all sheeps were killed, especially
due to recurrent late spring snow. Also fish-
ery was very bad at that time, and the Faroe
Islands suffered an economic disaster