Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 47

45
able that also during the intense volcanism of the Lower Ter-
tiary, the magma also had such or a similary shallow origin,
with the inference that much higher lava volcanoes than the
post- and Late-Glacial ones could not be formed, and thus could
not carry glacier caps to leave any vestiges in the plain country
around.
This was quite different during the formation of the Upper
part of the Plateau Basalts. Signs of frost action and storms
(dreikanters etc) are very clear indications of severe climate,
but the glacial signs are certainly due to local glaciers, creep-
ing down volcanoes or tuffbreccia heaps, as we have point-
ed out.
The Age of the Topographv in the Tertiary Basalt Areas
and other Dating.
We have spoken of the subsided east side of the Bárðardalur
Fault. That is true for the northern part. But we should at the
same time speak of the uplift of the western side. This uplift,
which we can now consider to be of Miocene or older age, ex-
tends far to the south on the west side of Bárðardalur, and in-
creases gradually on the east side, so that both sides of the
fault are of equal height in the Krókdalur region. This uplift
led to the formation of the present topography west of Bárðar-
dalur. The present author has described the development of this
topography in several papers (33), and has come to a geo-
morphological age of at least 15—20 million years. This can
be in good agreement with the age of the Tjörnes deposits, and
is also in agreement with my interpretation of the K/Ar-dat-
ing of the oldest rocks of Iceland (20), according to which the
date is that corresponding to a shallow position of the now cold
samples after or during the excavation of the valleys. The date
cannot be that of the rocks themselves, as argon would have
been lost during the early position of deep burial, when zeolites
were being formed.
We must now consider closer the morphological development