Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 114

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Grafningur—LyngdalsheiSi. The breccia mass continues in
the mountains to the north and northwest of Stóri Háls, in
part uplifted. But it is also found at a lower level in the area
farther northeast, starting with Háafell and Úlfljótsvatnsfjall
and extending under Ljngdalsheiði to Laugarvatnsfjall, most-
ly covered by a thin sheet of dolerites. The continuation of
this sheet is found on the uplifted Laugarvatnsfjall in the
north and on the similarly uplifted chain Ingólfsfjall—Grafn-
ingsfjöll in the south. The conditions in the central, lowlying
area shall now be described in some detail (see Map III).
Lyngdalsheiði is bounded by fault-steps on three sides. On
the south side is a fault extending from Gálgagil to the WNW
with a throw of about 100 m. Below the step the dolerite is
mainly seen east of Hæðarendi at a height of about 120 m;
it forms a thin sheet (5—6 m) resting on thin gravel over
the primary breccia. At the top of the step the plane surface
of the breccia is at 230 m and is covered by a 15—20 m thick
remnant of the dolerites. In Gálgagil the breccia has the
common “fore-set bedding” stratification, dipping NW.
On the east side there is a quite similar step with a NE
trend; these two faults meet in Hrólfshólar, a small area
where the breccia has been uplifted to about 325 m, or about
100 m higher than along the faults themselves. To the north-
west of Hrólfshólar is a further step of 40—50 m which is
probably also a fault. The maximum thickness of the dole-
rite sheet at the centre of Lyngdalsheiði is hardly more than
100 m and may be considerably less. Most students have call-
ed Lyngdalsheiði a shield-volcano on account of its broad
outlines (no crater is found). But as the form is at least partly
determined by uplifts I must consider this view as question-
able.
On the western slope of Lyngdalsheiði are the prominent
faults Nautabrekkur and Drift with NE-trend.
In the SW the very flat Lyngdalsheiði ends in the step
Grenjabrekkur, a fault with the WNW-trend and a throw
of 70—80 m. Above the step the dolerite has a height of