Rit (Vísindafélag Íslendinga) - 01.07.1962, Blaðsíða 172

170
These masses, or their main parts, are best understood as
a part of a general tuff-breccia layer which may be consi-
dered to have covered the peneplane on the Young Plateau
Basalts.
We have then remnants of such a general tuff-breccia
layer in the west (the main Súlur tuff-breccia etc.) and
here in the east. In both areas we find that the layer is older
than valley erosion at the 300 m level. (In the eastern area
the work done seems far less than in the west. An explana-
tion of this may be suggested. The submarine slope south of
this area and east of the Westman Isles is very steep and
runs close to the present coast. This slope must be geologically
young, formed by a late subsidence. If we assume that this
occurred towards the end of the 300 m stage the main val-
leys of that stage would have been drowned and only the
relatively slight further valley erosion at the 300 m level
after this event would be evident.)
We have then reasons to believe that in the eastern and
western areas we have the remnants of the same tuff-breccia
layer.
In the intervening area there are similar extensive masses
in the Reykjanes ridge, Grafningsfjöll, Ingólfsfjall, and Laug-
ardalsfjöll. Their main parts would also appear to be rem-
nants of the same layer. But these masses have been disturb-
ed so much relatively recently by tectonic movements that
older erosion forms and old base-levels cannot be recognized
with any certainty. And from our study of the masses them-
selves it would seem that at least those occurring south and
southwest of the Thingvalla lake are largely younger tlian the
main Súlur tuff-breccia. This conclusion rests firstly on the
fact that we have not found any traces of reverse magnetiza-
tion younger than nt of the Ingólfsfjall area. Secondly, the
dolerite phase of n2 in Ingólfsfjall appears to be identical
with the dolerite at Reykjavík which, as we have seen, be-
longs to or is younger than the strandplane stage.