Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 185

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The history of the Kleifarvatn area, as well as conditions
near Reykjavík, show on the other hand that the dolerites
and the uplift phases can hardly be younger than Middle
Pleistocene; they are of early Mindel-Riss interglacial age
or older.
Among later volcanic events may he mentioned the out-
pouring of breccia and lavas from the top of Rjarnarfell near
Geysir, formation of the volcanic ridges Jarlhettur, Mosa-
skarðsfjall-Rrekknafjöll, Hrúðurkarlar, the volcano Ok and,
still later, the small hreccia ridge of Bræðravirki. Finally
comes the huge production of post-glacial lavas.
As to the pre-dolerite history, we first state that the dole-
rites rest very largely on tuff-breccias. The essentially plane
surface of the latter was in part of erosional nature and pro-
bably corresponds to the Lowest Pleistocene strandplane. The
dolerites rest in some cases on a moraine and are then seen
to be definitely younger than the underlying breccia. This
eroded hreccia most likely corresponds to the main Súlur
layer.
But in other cases the dolerites are a direct continuation
of the underlying fragmental rocks and belong to the same
volcanic cycle. This probably means that these fragmental
rocks were formed in (tectonic) depressions, sunk into the
erosional plain. To such young fragmental rocks belong for
instance those of Hrafnabjörg and Bláfell and the upper main
breccia layer of Ingólfsfjall.
In a number of cases we have been able to study the struc-
ture of these rocks. In Hrafnabjörg the rocks are made up
of bomb-breccias produced in explosive eruptions. The steep-
ly dipping layers, in which slumping was common, were pro-
bably deposited under water. Further we have in many cases
fore-set bedding of the breccias, probably produced by the ad-
vance of “deltas” into basins (e.g. Ingólfsfjall). The fine-
structure of the beds in other cases indicates deposition in
shallow water (Kambar—Núpar, Hestfjall). Even in the high
Bláfell the fragmental rocks seem to have been formed in