Jökull - 01.12.1977, Síða 32

Fig. 2. Isopach map of the tephra layer H3. (From Thorarinsson et al. 1960).
Mynd. 2. Þykkt og útbreiðsla gjóskulagsins H3.
ed by a postglacial layer, as only H3 has a greater
volume.
As seen from Fig. 3, H4 has two thickness
axes, one running due N from Hekla to the
Skagi peninsula in North Iceland, the other
and main one running NNE to the Langanes
peninsula. The thickness distribution of the
light and dark parts of H4 are shown separately
on Figs. 5 and 6. The volume is calctdated in
Table I on p. 33.
As seen from the isopacli maps four of tlie
Hekla layers were spread mainly towards N antl
NE. H4 covers approximately the same parts of
Iceland as H3 and is in many places similar in
thickness. In East Iceland and North Iceland
east of Héradsvötn a conspicuous characteristic
of H4, when compared with H3 in soil sections,
30 JÖKULL 27. ÁR
is that its uppermost or upper part is greyish-
brown to brownish-black in colour and this part
of the layer increases in thickness in relation
to the lower ancl light coloured part, towards
E and SE, so that in the farthest southeast it
is brownish to brownish-black almost all
through. The shift in colour makes it easy to
distinguish between H4 and H3. In North and
East Iceland H3 is uniformly light-coloured.
In the western part of North Iceland the colour
of FI4 is unifonnly white, whereas the colour
of H3 is light greyish-yellow. In North Iceland
Hi is of similar colour as the light lower part
of H4. H5 is of a light yellowish-grey colour.
It is on the whole more fine-grained than the
others and tlie upper and lower limits of the
layer less distinct. In loessial soil it is more
easily overlooked than the other three.