Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 120

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ridges and steps with the general trend SW-NE (cf. for in-
stance Einarsson 1951 a). Sometimes it can be seen that the
ridges are topped by flatlying layers, lava flows or sediments,
and in spite of the confusing structure one can in a few cases
trace one layer from one step to the next. From this it seems
most natural to explain this topography by a tectonic phase
that cut flatlying strata into a number of elongated blocks
that were variously uplifted. Through this movement the
strata were in some cases much distorted, as for instance the
tuff-breccia layers of Reykjafell near the Ski-Hotel. In indi-
vidual cases supports for various theories, such as subglacial
eruptions and primary sandflows can he found. But the funda-
mental process was the tectonic storm and the various local
observations will usually be seen to find the most natural
explanation on that basis. On the whole, exposures are very
bad in this area, the slopes heing covered with scree. The in-
temal structure is therefore hadly seen and especially diffi-
cult to trace for satisfactory distances. It is thus uncertain
whether the lowest groups of Ingólfsfjall crop out in the area;
we are at any rate mainly concerned with the upper group.
In Kambar the moraine m2 of Ingólfsfjall, resting on nl5 is
at about the same level as in Ingólfsfjall, and this also applies
to Ástaðafjall.
In Ástaðafjall there is a cover of horizontal (porphyritic)
relatively young dolerites on an erosion surface on normal
lavas and breccias which probably belong to nt of Ingólfs-
fjall. These topmost lavas of Ástaðafjall which have been gla-
ciated, rise from about 300 m in the south (also here cut by
faults) to 400 m near Hengill and here they rise in a few
steps to 580 m and form the surface of the south comer of
Hengill. This is the best proof of the uplift of Hengill and
gives also some idea of its age. It seems likely that the uplift
is also younger than the Mosfellsheiði dolerite but a direct
demonstration of this is not at hand. The dolerite remnant on
southem Hengill and a similar but smaller patch on the crest
between Innstidalur and Marardalur suggest that the dole-