Greinar (Vísindafélag Íslendinga) - 01.01.1976, Síða 141

139
at the end of the Last Glaciation (17), now at a level of about
150 m. This time an ice dam near the leveled Dráttarhlíd ridge
was necessary, as already pointed out in (17).
Tracing the porphyritic breccia on the Hátindur plateau from
Helluhryggir farther southwest into the lower situated Dyrafjöll,
it is seen to be overlain in the western part by an outlier of a
very extensive, partly sub-aquatic tuff-breccia layer of a peculiar,
and thus easily identifiable fine-porphyritic texture. This layer is
the oldest rock in the present d)-area which can be considered to
be centered on the Hengill and will, therefore, be called here the
Hengill Basal Layer.
This important layer — from a tectonic point of view — has a
surface height of 560-580 m across most of the northem slope of
the Hengill moxmtain (H in Fig. 10, E). The bedding is here most-
ly horizontal.
This is already an uplifted position, as the layer rests on E-
Háhryggur, and contains here a clear water level at 500 m (Fig.
10, E and WL in Ej). But the layer rises further in tectonic steps
in the mountain slope to over 650 m (Fig. 10, E1? E2, E3).
It can also be traced in tectonic steps to lower levels towards the
recent Nesjavellir depression, down to about 220 m (Fig. 10, D).
In (16) is given a useful petrographic description of this layer. It
is the “4-5 hyaloclastites of Hengill (glomeroporphyritic)” in
Abb. 15, and describing Hengill, the following description in given,
p. 55: “. . . Olivinbasalt, der sich wegen seiner glomeroporphyris-
chen Struktur gegeniiber anderen angrenzenden Eruptionspro-
dukten auszeichnet und auf diese Weise schon makroskopisch eine
Unterscheidung ermöglicht. Diese Gesteine sind gesprenkelt mit
kleinen Nestern aus basischem Plagioklas und untergeordnet
Pyroxen und Olivin. Die Nester haben einen Durchmesser von
0,5-1,2 mm, und sie können bis zu 20% des Gesteins ausmachen”.
It is this texture of the Hengill Basal Layer which makes it easy
to trace. In (16) this layer is said to be found in 3-4 steps in the
north slope of Hengill (p. 54) but these are assumed to be strati-
graphic steps and none of the details are studied, such as those
shown in Fig. 10, which show them to be due to faulting.
At 650 m in the eastern part of the north face of Hengill, the
Hengill Basal Layer is covered by a subaerial series of unporphyri-