Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 153

149
10 cm thick erosional remnant of hard tillite separates the same
basal sediments and the breccia of Vídihlid. Still further up the
river, the breccia covers the sediments and their erosional slope
down to the level of the present river bed. A short distance up
along the west branch of the river (Thverá) there is another ex-
posure of the basal sediments, now to 160 m, and directly on these
rests the Old feldspar-porphyritic breccia of the Sandfell region.
These facts show that the basal sediments, being probably derived
from Stapafell, were deposited in the lake during a stage at which
the water level was a little higher than that corresponding to the
late-glacial 150 m terrace. The sediments were then covered by a
glacier, and after this glaciation, a similar or deeper river bed than
the present one was cut into the sediments; after this the feldspar-
porphyritic breccia of Vídihlíd was formed.
Unporphyritic dykes and our basal sediment are mentioned in
(16) but the further details and their interpretation is not given.
From Vídihlíd the porphyritic breccia extends without a break
rather far to the southwest, being cut by erosional terraces, first
at the 240-260 m level (similar to that of the Vídihlíd surface),
and finally one giving the surface of the westem part of Súlufell
at 350 m, which corresponds well to the erosional surface of Mæli-
fell (Fig. 10, C). A small patch of the same breccia caps the nar-
row Foldahryggur, a ridge with a valley slope on the west side
and a dislocation wall on the other (overlooking Djáknapollur).
To call this ridge the “Folaldahryggur volcanic line” as done in
(16), is quite unjustified. The same applies to the name “Vídihlíd
volcanic ridge” and many such cases where erosion has created
lengthy forms, whereas no trace of truely volcanic lines are found.
The same porphyritic breccia still forms the main rock in the
uplifted ridge Hrómtmdartindur. Here it is in part covered by an
apparently much younger volcanic tuff formation, and this is cut
by a porphyritic dyke with strong reverse magnetization (position:
375 m NE of highest peak, T 552)x). This supports the high age
we have already foimd.
We have now bridged the entire 8 km wide zone, between the
1)1 am indepted to Dr. Leó Kristjánsson for testing my samples. He considers
the magnetization as TRM, not that of lightning (private communication).