Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 36

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The origin of the Eiríksstaðahneflar is then easily under-
stood and, in short, the tuffbreccia N-S ridges of Hólsfjöll,
and individual high mountains in between are most easily refer-
able to eruptions in the valley system on the surface of what
we have termed (N3n). Which of the two separate exposures
of the reverse polarity over (N3n), which were mentioned above,
would correspond to the Hneflar, is quite uncertain. The over-
whelming majority of the Hólsfjöll tuffbreccia masses are of
normal polarity, and we seem with some confidence to be able
to denominate them by (Nlu). But with the addition of Eiríks-
staðahneflar we now know of two polarities during the valley
stage, whereas we used the pre-valley rocks of the Bruna-
hvammsháls to term the main normally polarized extensive
dolerite layer by (N3n). In reality, we now have no idea how
often the polarity may have changed during the valley stage,
which we are considering. Eiríksstaðahneflar is the only known
indication of reverse polarity during the valley stage. Was it
before or after the normally polarized breccias produced in the
valleys? We might know that but not where to place every
“in-valley-formed” normal or reverse tuffbreccia heap. Nor can
we know whether or not this volcanism registered every polarity
epoch or event during a long span of time. We must use essen-
tially the rmin extensive lava groups, and presently wait for
a connection of our main normal group in Hvammsheiði, with
Tjörnes. We now continue the discussion of our valleys.
Ánavatn — Sænautavatn — Grunnavatnsdalur — Tunguá —
Suðurdalur — Sauðá — Vatnaflói — Hófslækur — Geitasandur —
an unbroken N-S line of small rivers, lakes and, finally, a dry
depression.
Gestreiðarstaðakvísl, Kollseyrudalur, and the upper part of
Hofsá are not as definitely suggestive, as their course might
have been determined by the eastern limit of the tuffbreccia
mountain chain. On the other hand, it now becomes clear, why
the Fossárdalur and the Hofsárdalur deepen here so strikingly
and take a NE-course, away from the mountain chain. Here
we have clearly the evidence of glaeial water from the glacial
mountain chain (cf. Chapter 1); and the erosion may stem from