Jökull - 01.12.1977, Blaðsíða 42

Fig. 19. Changes of wind direction during the
Plinian phase of the eruption.
Mynd 19. Breyting á vindátt meðan á aðal-
gosinu stóð.
Hi was erupted 1104 A.D. The tephra layer
is heavily eroded in the Hekla area and its
uppermost part is nowhere preserved. Tlie
lower part consists of a layer of rather coarse
pumice overlain by a finer, well stratified —
and sometimes cross-stratified — deposit thought
to be formed by base surges. The colour is uni-
form throughout, white, but the tephra sector
is two-coloured in certain places outside the
Hekla area.
H3 is the biggest tephra layer produced by
Hekla. It is very poorly stratified and, by far,
the most coarse-grained. The colour changes in
the pumice are almost identical to those al-
ready described for H4, but the two lowermost
units have a pinkish hue that, together with
the more coarse-grained nature of the layer,
makes it easily distinguishable from H4. Lithics
are similar to those found in H4 but less abun-
dant. Maximum thickness of H3 in the Hekla
area is more than 6 meters.
The Selsund pumice was once called H2 as it
was thought to be younger than H3, but later
it was found to be between H3 and H4. The
Selsund pumice was presumably erupted in
calm weather during winter some 3500 years
ago. Most of the hot pumice and bombs appear
to have fallen on the snow-covered NW flanks
of Hekla and were carried by the resulting melt-
water down to the lava fields SW of Hekla
(Figs. 20—21). A 2—3 meter thick layer of the
Selsund pumice was deposited in a lake then
existing at the mouth of the valley between
Selsundsfjall ancl Bjólfell, and pumice was also
carried farther southwestwards to the river Ytri
Rangá. The airfall deposit in this area is fine-
Fig. 20. The Selsund
Pumice. Distribution of
the airfall deposit accord-
ing to present defective
knowledge.
Mynd 20. Bráðabirgða-
kort af útbreiðslu Sel-
sundsvikurs. Orvar sýna
hlaupfarvegi, lárétt strik-
un vatnsstœði, sem fyllst
hefur af vikrinum.
40 JÖKULL 27. ÁR