Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 88

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to the Middle or Upper Pleistocene. Hrafnabjörg may be
considered as an example of the hidden structure of the gra-
ben east of Ármannsfell.
It is possible that this graben contained a lake and that the
breccia masses of Hrafnabjörg as well as the slope formation
in eastern Ármannsfell are subaqueous in origin. This might
also explain the roughly plain surface of the breccia masses
in Hrafnabjörg as marking the level of the lake.
Laugardalsfjöll.
This name is here used for a cluster of tuff-brecCia moun-
tains in a wide area east and northeast of Hrafnabjörg. These
mountains either form elongated ridges with the tectonic di-
rection SW-NE, or more equidimensional table mountains.
The latter have usually a flat cap of weathered and glacially
scoured light-grey basalts, the dolerites. The ridges have as
a rule sharp crests and do not have capping lavas. Usually
the slopes of these mountains are covered with weathering
debris and moraines and few clear sections are available. In
some cases, nevertheless one can see that the ridges are form-
ed of tectonically much distorted stratified tuff-breccias.
The dolerite on Laugarvatnsfjall is naturally understood
as a part of a more extensive sheet. An uplift of Laug-
arvatnsfjall by 300—350 m would explain the conditions
and it seems highly probable that the mountain cluster is
formed in the same way as Hrafnabjörg and Ármannsfell,
and in the same tectonic “storm”. The absence of dolerites
on the narrow ridges is easily understood as a result of de-
struction, e.g. during glaciation. Conditions on the south cor-
ner of Laugarvatnsfjall seem fully to verify this picture. Here
the basalt surface first falls slowly southwards from 600 m to
400 m where it is mostly cut off at the edge of the steep slope.
But at one point it goes down the slope with a dip of 35°,
down to 200 m. Only uplift seems capable of explaining these
conditions. The section of the steep slope is as shown in