Rit (Vísindafélag Íslendinga) - 01.07.1962, Blaðsíða 77

75
topograpliy existed. Most probably during an early erosional
stage of the tuff-breccia layer the lava series was formed, and
finally much later, when the steep northern slope of Súlur
existed the columnar basalt near Krókatjarnir was erupted.
All the evidence seems to indicate that the extensive strati-
fied tuff-breccia is older than the Plateau topography.
The peak “950” is the only one that does not carry a cover
of lavas. The structure of the peak 916, as seen from the SE,
is shown in Fig. 35. The peak consists of a series of thin hori-
zontal lavas resting on the plane erosional surface of steeply
dipping breccia layers. A swarm of dykes cuts the peak. The
other peaks have a quite similar structure. It is possible that
these lavas were formed in the central region of a very large
volcano, whose distal parts would have been wiped out. The
numerous dykes tend to confirm this. But it does not seem
possible, at least on the basis of my material, to reconstruct
one or more craters.
The steep east wall of the Súlur massif is nearly certainly
the result of a displacement. At this fault the Plateau Basalts
drop by more than 300 m and are not seen on the east side.
This wall is a part of a marked border line between the Pla-
teau Basalts and younger rocks. On the NW-side of the line
the Plateau Basalts reach heights of 900 m and more, on the
SE-side they are not seen and have dropped below a height
of 100—200 m and more. While from the west the Plateau
Basalts are dissected by deep and broad valleys the SE-border
is a smooth, rather steep slope where hardly any valleys have
been developed. The rivers run in narrow gorges. This is a re-
latively late faultline at which an eastem highland sunk after
the main features of the topography of the Plateau were fully
developed. It may be stated at once that the fault is older
than Middle Pleistocene, for no changes have here taken place
(only farther east) since the Dolerite lavas flowed from Mos-
fellsheiði through the pass at Stíflisdalur into the Kjós valley.
GagnheiÖi. In the Súlur area the conditions are, however,
quite complicated. The Gagnheiði is not simply a subsided