Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 10

8
Stages, which must also have been characterized by easterly
winds as these bring the main precipitation to the country.
This connection between easterly winds and eruptions are in
agreement with the normally E-W direction of the larger
horizontal principal crustal stress, for easterly wind stress
then increases the volcanically important maximal shear stress.
Thus, there must be a way to transform a trifling part of the
exogeneous energy of insolation origin into an endogeneous
one, and wind stress and the plasticity at a shallow depth,
indicated by the overwhelming shallowness of foci in the vol-
canic zones and in the areas of the ridge system in general
lead the way to a solution of that problem.
Zonal volcanism in Iceland began in the Upper Pliocene.
The untilted lavas of this time have been divided into magne-
tic groups and traced over wide areas. They testify that there
has been no such process as “spreading” in Iceland since the
Upper Pliocene. Their age is demonstrated by the fact that they
were dissected by shallow valleys at a time before there existed
glacial rivers characterized by gorges. These pre-Pleistocene
lavas overlie the Breiðavík deposits of Tjörnes.
The uppermost zone of the Tjörnes deposits proper, the Car-
dium Groenlandicum Zone, was for a long time taken to be
of Upper Pliocene age, and recently it was even postulated
to herald the beginning of the Pleistocene, but now it is found
not to be younger than Lower Pliocene. The still lower Tjör-
nes deposits are of special interest, as they take us at least
so close to the Oligocene, that the very marked drop by 200 m
of base level of topographic development in the Icelandic Terti-
ary Plateau Basalts and in Norway indicate most likely the
great regression at the end of the Oligocene. Thus, most of
the erosional work in the uplifted Tertiary Basalts was carri-
ed out during the Oligocene. These basalts consist of a lower
division with many indications of a prolific flora in a warm
climate, and an upper division devoid of lignite, but with
clear signs of frost and generally severe climate. Intercalated
tillites also abound, for which reason Pjeturss claimed a Plei-
stocene age for this division. Actually, the tillites were form-