Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 49

47
explained by yield of the crust only after a certain stress had
been accumulated.
We have now seen that the oldest valleys may be of even
Oligocene age, and then the equality of the 200 m drop of base
level in both countries and in the different parts of the country,
so strongly suggests the drop of sea-level in the great Oligo-
cene regression, that we cannot but accept this solution. The
early valleys are thus of Oligocene age, which morphologically
is highly satisfactory, and the strandflat was largely formed
in the Miocene and Lower Pliocene, which also is satisfactory.
The uplift west of Bárðardalur is then also of middle or
early Oligocene age, and the same applies to the subsidence in
the north, east of Bárðardalur. We ought to find Oligocene
marine fauna at the base of the sediments out in Skjálfandi
Bay, closer to the Bárðardalur fault line, although the lowest
exposed Tjörnes sediments are more likely of Miocene or Lower
Pliocene age, being formed late in the filling up of the subsided
basin.
We can now understand the smooth valley forms of the oldest
generation, as due to forest-clad slopes, and heavy chemical
weathering of the basalts.
The deepening of the valleys in the second generation, by
unchanged base-level, would still be within the Oligocene. We
have no obvious parallel event to compare with, but an increase
in precipitation suggests itself. This might deforest the now
unstable slopes covered with a thick layer of chemically dis-
solved basalt, and an increased main river would most naturally
flatten its course by headward erosion.
The Bárðardalur fault and the extensive uplift of Iceland
in the Oligocene would very likely correspond to the uplift of
a zig-zag formed zone, about 200 km broad and with the present
submarine ridge as axis. The evidence of this uplift and emer-
gence from a shallow ocean is quite clear from the data of
John and Maurice Ewing (35), which they so thoroughly mis-
understood, and from the valuable data of Van Andel and Bo-
win (36), which they refrained from interpreting as an evi-
dence of emergence, in face of the old and wrong dogma that