Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 154

152
the eruptions probably began under an ice cover, this must
soon have disappeared. The regular alternation of primary
sheets and scree material could not be reconciled with sub-
glacial conditions.
Farther north, at an E-W fault marked by Hrúðurkarlar,
the basalt surface is uplifted by 200 m. On it rests here a
nearly 200 m thick cover of breccias, topped by horizontal
lavas of cube basalt. This mass forms the socle of Thórisjök-
ull. It is the uplift that raises the surface of the socle above
the snowline.
Hrúðurkarlar is a row of tuff heaps, usually of fine grain,
that formed on the fault between the low and high level,
while the socle of Thórisjökull and Suðurmúli are erosional
remnants of a much older breccia mass, possibly a general
breccia layer, some 200 m thick. These remnants seem quite
comparable in age and other respects to the main Súlur
breccia, which, as we have seen, is a remnant of an extensive
thick layer. That these scattered remnants were originally
parts of the same extensive layer cannot easily be proved
but seems more and more likely. Lyklafell, Fanntófell, and
Oköxl are probably further remnants of such a layer.
It may finally be stated that Thverfell is a breccia volcano
that was formed within the fully developed valley Lundar-
reykjadalur and dammed up the lake Reyðarvatn. As to age
it should therefore be close to Hvalfell. The bottom of Reyð-
arvatn fits very well into the drainage pattem around it and
must have changed very little since the lake was formed.
(I am indepted to Sigurjón Rist of the State Electricity Autho-
rity for making a bathymetric map of the lake available to
me). This is an important fact when we consider a small
ridge, Drangshlíð, to the east of the lake. On a 400 m stretch
we have a section of breccia layers dipping 25° SE or from
the centre of Thverfell. The layers are cut off by an erosion
horizont about 90 m higher than the lake and closely similar
to the surface of the basalts of the surroundings. The breccia
consists of unporphyritic bombs and bomb fragments bu> no