Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 63

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level of about 3 km, behaves like an elastic solid, when seismic
waves pass through it. Under the small stresses of winds last-
ing for climatic periods, it does flow, and thus by deformation,
allows the accumulation of stress up to the breaking point in
the thin crustal layer above it.
Thus we have seen for one case, and will see it for more of
the endogeneous agencies, that they correspond to the old story
of not seeing the significance of the simple things before your
eyes, because all the remarkable things must be behind the
mountains.
This digression into seismicity was necessary, for obvious
reasons, but we now proceed with the main theme of this
chapter, by considering the correlation of climate and volcanism
in Iceland in the prehistoric postglacial time.
In Table I we have put side by side the climatic periods in
Sweden (Magnusson, N. H., Lundquist, G., and Regnéll, C.:
Sveriges geologi, 1963) and in Iceland. Ht to Hr> are acid
Hekla tephra layers, which are extensive, as traced by Sig-
urður Þórarinsson. They have been dated by the C-14-met-
hod, where they rest on organic material. This tephrochrono-
logical dating has been found to be most valuable back to 7000
B.P. The glacial ages rest on C-14 dating of shells. A number
of geologists have collected the material for these datings,
but it would carry us here too far to acknowledge the share
of each individual.
It should be pointed out that the used half life of C-14 has
changed a little since the first datings, here included. Further-
more, Dendrochronology has led to the insight that the higher
postglacial and lateglacial C-14 ages are too low. Using a cor-
rection table given by Þórarinsson in a paper in Icelandic, I
have put Þjórsárhraun I at 9000 B.P. instead of 8000 B.P.
according to the rather old original C-14 dating. Other dates
I have not corrected in that way. The limits of the climatic
periods are copied from (4), whose author did all the palyno-
logical study, and thus characterized the climate of the periods.
He used tephrochronological dating.
There may be some small inconsistences in the time scale