Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 149

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tilting in the area. However, the dip of N2 under Súlur is
slight and the stratification of the tuff-breccia not a safe guide
to distinguish between original and tectonic dip. It would also
seem strange if the Súlur tuff-breccia were older than the
Kjölur-Skálafell group. It therefore seems most natural to as-
sume that Súlur rests on the peneplaned Plateau and is a
normal group following immediately the Kjölur reverse group.
The latter has been thin and not of general occurrence, as we
shall also find indicated in Kvígindisfell (below).
Some general remarks. It is striking that in the mountain
cluster Mosfellssveitarfjöll the summit heights are in the
neighbourhood of 300 m (Úlfarsfell 295, Reykjaborg 286,
Reykjafell 273). This suggests that the cluster was eroded
out of a block that had previously been cut by a 300 m ero-
sion plane, and this would most likely be the same plane on
which the breccia cap of Grímmannsfell rests. It is not ex-
cluded that this plane was cut during the 300 m valley stages,
in which case it would be younger than the uplift of the Esja
massif. But as the caps of Grímmannsfell and Skálafell-Kjöl-
ur most likely correspond to each other it now seems more
likely that the 300 m plane is older than the uplift of Esja
and represents a stable block, unaffected by the uplift.
In any case, we find on the Esja massif an uplifted pene-
plane and on this near-horizontal layers which are older than
the valley topography.
We have previously mentioned that Ármannsfell and
Laugardalsfjöll probably contain cores of these rocks. Simi-
larly it is likely that Vörðufell and the lower part of Ingólfs-
fjall and Grafningsfjöll belong to them. Also the group r,
of Heljarkinn in Hreppar may belong to the same category.
And now we are confronted with the impórtant question
whether also such an occurrence as the Young rocks of the
Búlandshöfði area belongs to this category. The fossiliferous
sediments and the covering reverse group clearly rest on an
erosion plane on Old basalts and it would be most natural to
assume that these rocks are older than the Older and Younger