Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 39

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very much greater than that of pure lake water. Thus we con-
clude that the Helluvað channel began to take shape and finally
lowered the level of Mývatn soon after the beginning of the
first Pleistocene glaciation of such a magnitude that at least
the Vatnajökull came into being as a glacier. But the indicated
advance- and retreat stages during the whole Pleistocene time
created the Helluvað channel and kept it open.
During fullfledged glaciations the Helluvað channel could
have been filled with moraines, and also the slopes towards
it might be covered with such masses to form a temporary dam
and a high Mývatn level. This probably took place during the
last glaciation as the formation of the ringmountain Hverfjall
suggests (23).
The Kálfborgará — Seljadalur N-S river valley belongs to
the generation of the shallow N-S striking valleys, and the
river cuts the reverse lavas from Kálfborg (22). This river
was later captured by the Skjálfandafljót, as the Bárðardalur
grew deeper and deeper during the Pleistocene. The two main
magnetic groups, our main normal one (N3„) and the over-
lying reverse Kálfborg lavas (R2„), have thus both been sub-
jected to this pre-glacial N-S valley formation.
As we can trace the lower group right to the fault which
separates the uplifted and northwest-wards tilted Tjörnes block
from the mainland, and as we find two groups in Tjörnes which
very reasonably correspond to our mainland groups, we seem
to be able to bridge the fault. The main basalt group covering
the Breiðavík deposits, is of the reverse magnetization, and
extends over all the western half of the peninsula with constant
dip rising to a height of 760 m in Búrfell (24). We accept this
group to correspond to the Kálfborg lavas, which have a rather
wide extension north of Kálfborg where it has survived. This is
further supported by the fact that there is no visible origin of the
reverse group in Tjörnes itself, and the group must have come
from the mainland, as the lavas overflowed marine sediments
in Breiðavík. The tilting of Tjörnes preserved the reverse
group there, while it would not survive easily the formation
of Aðaldalur by the Laxá during those intervals, when it was