Greinar (Vísindafélag Íslendinga) - 01.01.1976, Síða 149

147
Glaciation, as the conditions at Hvanngil showed. Therefore, as far
as geomagnetic evidence goes, the reverse magnetization observed,
must be at least 0.69 million years.
The unporphyritic topmost tuff-lava group on the eastern part
of Hengill seems best to correspond in age to the Stangarháls brec-
cia. The Hengill Basal Layer must also be of a quite similar age,
whereas the topmost layer on the western part of Hengill, resting
on the tilted and peneplained unporphyritic hasalts, must be taken
to be much younger.
To get the age of the Old dolerites, we have to add to the age
of our geomagnetic fixpoint the time for the formation of the
abrasion bench at Sandfell and Mælifell, including the formation
of the Skolli erosional plain, and the erosion of the Old porphyritic
rocks in general. Then there is the time interval corresponding to
the Hátindur plateau and the Háhryggur series, and also the dura-
tion of the glaciation preceding the Old porphyritic rocks at Jóru-
tindur. With this in mind, we think that the round number one
million years is the best estimate we can give at present for the
age of the Old dolerites. This figure is probably rather too low than
too high, and a comparison with the geomagnetic time scale might
rather suggest an age of 1.7-1.8 My.
Mælifell consists of two main masses, the feldspar-porphyritic
breccia already mentioned, and a little younger olivine-porphyritic
mass of breccia, tuffs, and lavas, forming the northeast part of the
moimtain. Both are of normal magnetization and both have an
eroded surface in the 320-379 m height range, and both are cut by
the abrasion bench.
Sandfell consists of tlie feldspar-porphyritic tuff-breccia, and was
formed separately. Its original slopes had at least in parts a cover
of lava tubes, dipping 25°. Such a remnant reaches from the
summit at 409 m down the southwest slope to the 280 m level,
showing that the separation from MæHfell is original. The exis-
tence of the original surface just here is most likely due to the
sheltered position at the time of the formation of the 280-290 m
abrasion bench.
The Mælifell breccia extends east to the river Thverá, where it
overlies an outlier of the largely erosionally modelled Stapafell of
“andesitic hyaloclastite” (16, p. 75).