Jökull - 01.12.1981, Blaðsíða 31

but was not found at all further west. These
deviations in the distribution can be explained
by a strong southwesterly wind curving
around the northwestern side of the volcano,
resulting in the tephra either being prevented
from being deposited or more probably in the
tephra being cleared off the ground by a
strong wind especially if the eruption took
place in wintertime as suggested earlier.
The third phenomenon, the consistent
eastward twist of the isopachs of the smaller
thicknesses may be attributed to local effect of
topography on low altitude wind. Bergthorsson
(1980) has studied the effects of mountains on
a>r pressure in Iceland. A relatively high
pressure is observed over the windward sides
°f mountains, both on large and medium
scale. On the other hand there is a negative
pressure anomaly over the leeside. This phe-
rtomenon is mostly determined by a low
altitude wind but the upper wind seems to be
relatively unimportant. This effect results in
right-hand bending of the wind current over
the mountains (see Fig. 2 in Berglhorsson, 1980).
Bergthorsson (1980) has demonstrated such
effects over Iceland as a whole and to a certain
extent the Vestfirdir peninsula displays some
evidence of this. It is not known if small
mountain chains like that of the Snaefellsnes
peninsula can have such effect (Bergthorsson,
pers. comm.). Nevertheless the above-
mentioned effect could explain the twist. In
our case a southwesterly wind will have more
'vesterly direction over the Snaefellsnes
peninsula at low altitudes. Another cause
could be a systematic change in wind direc-
tton during the course of all the eruptions but
that seems rather an unlikely coincidence
which, however, can not be excluded.
black tephra layers
A few black tephra layers were encountered
but individual layers were not traced to their
ongin. The relative age of the layers can be
established from their relation to the acid
layers. By these means a rough estimate of the
age relations of the basaltic and andesitic vol-
canism may be obtained. Black tephra layers
from individual cinder cones have a limited
distribution and usually do not overlap.
In Breidavík there is a 2 — 3 cm thick black
layer (profiles 37 — 39), and in one of the
profiles it overlies Sn-1. Further west in profi-
les 42 and 43 a 5 — 7 cm thick black tephra
layer occurs. In the latter profile (43) the layer
is overlain by Sn-2 and is thus not the same
layer as in profiles 37 — 39. The former profile
(42) is on top of an old lava flow about 1 km
west of Purkhólar.
The black layer separating the two parts of
Sn-3 has been discussed earlier in the paper.
Another layer is exposed in profiles 19 and 20
on top of Sn-2 and in profile 20 it is in turn
covered by a redeposited part of Sn-2.
In most profiles east of Grundarfjördur
there is a prominent black layer which thick-
ens towards the cinder cones in the Berserkja-
hraun lava field. It is usually 5 — 20 cm below
Sn-2 but well above Sn-3. In profile 78 are two
black layers 3 cm apart and in profile 76 there
are three. These are close to the cinder cones
and support the suggestion made by Jóhannes-
son (1982b) that the Berserkjahraun lava was
formed in three separate eruptions.
AGE RELATIONS
Steinthórsson (1967) reports two C,4-datings
of soil samples from below two acid tephra
layers at Fagrahlíd in Fródárhreppur on the
northern side of the Snaefellsnes peninsula. By
comparing the profiles presented by Stein-
thórsson with ours in the same area it seems
obvious that the two layers dated are Sn-1 and
Sn-2. According to these datings Sn-1 and
Sn-2 are 1750 ± 150 and Sn-2 3960 ± 100
years old, respectively. The age of Sn-3 is not
known, but its closeness to the base of the soil
cover, which started forming at the end of the
last glacial period (about 10000 years ago),
suggests an age of about 7000 — 9000 years.
By comparing the relative position of the
tephra from the cinder cones in the Berserkja-
hraun lava field with the acid layers its age
JÖKULL 31.ÁR 29