Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 186

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water prior to the uplift of the mountain. On the other hand
the fragmental rocks at Stóri Háls overlie a moraine just
above a lignite horizon and terrestrial conditions seem indi-
cated by the structure itself. And it is remarkable that no
fossils indicating subaquatic environment have been found.
Large rivers or floods have possibly played a part in spread-
ing the fragmental rocks and filling up basins but the flow
of primary breccias is also in several cáses seen to have taken
place (Ingólfsfjall, Mosfell, Vörðufell). High explosivity and
air transport also played some role.
The pre-doleritic history of the area may then be summa-
rized as follows. After peneplanation of the Young Plateau
Basalts was formed a more or less general fragmental layer
(Súlur-Grímmannsfell breccia), laid down on the probably
very low peneplane after this had been covered by a general
glacier. The eruptions may in some cases have started sub-
glacially (Suðurmúli) but mostly they took place well after
the glaciation and may in the first place be characterized by
high explosivity.
During the wide-spread uplift of the country, initiating
the valley phases, an area between the two blocks Esja—Súlur
and Hreppar was left low. However, it was faulted and some
differential vertical movements took place. Here developed
subsequently a wide erosional plain during the valley phases,
ending during the strandplane stage with a low plain (a some-
what similar development is found for the Borgarfjörður low-
lands). The plain cut partly Plateau basalts but mainly the
breccias (lower part of Ingólfsfjall-Grafningsfjöll, Vörðufell) •
Early in the Pleistocene further faulting set in with the
formation of some basins. The Vörðufell block subsided a
little and in the corresponding basin was formed and pre-
served a layer of volcanic breccia of reverse polarity, now
capping Vörðufell. But the main basins were formed a little
later in a narrower zone farther west. These were filled with
rocks of normal polarity only. Partly during this volcanic