Rit (Vísindafélag Íslendinga) - 01.07.1962, Síða 87

85
Faint glacial stríae are found at a few places. On the whole
the surface rises towards the centre of the cap but no traces
of a crater are seen. The surface is cut by a SW-NE fault-
line, with the east side a little uplifted, and the summit is
formed hy the tectonically broken highest part of that wall.
These facts, coupled with the described encroachment of the
lavas on a low tuff-breccia mound, suggest that the lava cap
did not originate from eruptions on the mountain.
We then come to the following picture of the origin of
Hrafnabjörg. On a plain with fluviatile and eolian sands,
some 50 m or less ahove present sea level, was piled up vol-
canic tuff-hreccia by explosive volcanism. Slumping in this
mass was common and led to unstratified masses of hreccia
alternating with undisturbed primary stratified tuff-breccias.
At least some of the material originated within the area of
the present mountain and tuff layers, flattened homhs, and
lava tongues were deposited over the slopes left by slumping.
Perhaps there was no central crater hut rather a number of
orifices fed by the numerous dykes. In the end a roughly
horizontal plain resulted. Before the uplift of the mountain
this plain has been at an elevation of 300 m or less.
In the next stage lava flows cover the surroundings and
encroach on the slopes of a low mound of tuff-breccias, finally
overriding and covering completely the mound. These lavas
may afterwards have been much eroded and scoured by gla-
ciers prior to the last stage in the development: the uplift of
the present mountain by some 300 m or more, and more in
the centre than at the periphery.
It is likely that this lava phase corresponds to that of
Ármannsfell and that both mountains were uplifted in the
same tectonic phase. The lower tuff-breccias of Hrafnabjörg
may possibly correspond to those forming the core of Ár-
mannsfell but it seems more likely that they are younger
and represent the filling material in the graben formed late
to the east of Ármannsfell. AU the primary rocks in Hrafna-
björg are of normal polarization and quite possibly belong