Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 140

138
V-25" rr-r
Fig. S: The upper part of the Hátindur erosional remnant of plagioclase coarse-
porphyritic tuff-breccia. The layered structure is best seen in the west side, as
indicated. The dip is 25-30°, as is most usual for tuff-breccias of foreset bed origin.
A gently east-dipping summit plain cuts the tuff-breccia layers.
the sub-aquatic Mælifell mountain far ther east; the material of this
part of Mælifell is the same as in Hátindur as far as can be seen.
(The above details of erosion of dipping tuff-breccia layers in situ
are not found in (16)).
Before the 200 m uplift of the Hátindur plateau, the Hátindm-
summit would have been at about 225 m, indicating an erosion-
ally graded adjustment to a quite reasonable level of the lake, if it
was dammed on the east side, either by a glacier or, just as possibly
and more likely, by the newly formed Dráttarhlíd breccia ridge
(which is almost certainly of subglacial origin), so that the out-
flow was not where it had been and is at present, but instead over
the Selflatir pass nearby, at a little less eroded stage of that pass
than at present.
The “Old porphyritic breccia,” at least the one of the Hátindur
plateau, and also in Mælifell, may have been formed in that lake
at this stage, and the initial erosion and leveling of that breccia
would then also have occurred. In Jórutindur, this breccia rests
on a non-glacial sediment, “bedded hyaloclastite” in (16, p. 41),
over a tillite. This suggests a lake stage after a glaciation, not a
subglacial formation of this breccia, as assumed in (16).
About the same water level in the lake as we are speaking of
existed at some unknown time after the Jórukleif uplift, for in this
way is understood the shoreline about 40 m below the edge of the
Jórukleif scarp, Fig. 7. The outflow over Selflatir again existed