Rit (Vísindafélag Íslendinga) - 01.07.1962, Síða 194

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these mountains have in part been formed as fillings of basins
and this in itself suggests a wet environment. Furthermore,
the structure suggests sub-aquatic conditions in a number of
cases. Thus the Hrafnabjörg tuff-breccia was probably form-
ed by explosive activity within a water-filled basin; slump-
ing was a common occurrence.
In some other cases (e.g. Mosfell, Suðurmúli) dipping beds
of breccia, where alternation of layers with unworn and
more or less worn material is found, may have formed the
part of a volcano slope, the worn material being scree form-
ing between eruptions and possibly settling under water. But
in some cases (e.g. Ingólfsfjall) this latter interpretation of
dipping beds seems unlikely and we are then led to the con-
ception of fore-set beds in a mass advancing into water with
the frontal steepness (25—30°) of a coarse delta.
Such bedding may be found in the Plateau (cf. Gullfoss
section) but it is especially prominent in some much young-
er rocks (toplayer of Vörðufell and Ingólfsfjall, Kambar-
Núpar).
Characteristically the beds have in many cases been cut
by a horizontal erosion plane (base-level before uplift) and
the loss of a superstructure much hampers a full understand-
ing of the mechanism of deposition. Parts of the dipping lay-
ers are seen to be primary breccias formed during movement
down a slope and the fragmental masses have somtimes flow-
ed nearly horizontally. But the most regular beds have more
probably been cleared into water by sub-aerial agencies. This
is supported by the fact that there is some altemation of some-
what worn and completely unwom material. In some cases
(Núpar in Ölfus) it seems probable that surf attacked a loose
superstmcture and moved the material to settle as fore-set
beds off-shore. In any case the material of these dipping beds
is almost certainly redistributed and has finally settled in
water.
The erosion of the dipping beds, mentioned above, and the
redistribution of the material much impedes interpretation in