Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 37

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the early and late stages of all the Pleistocene main glacia-
tions.
In the area Tunguheiði we see the clear difference between
clear-water erosion and that of glacial rivers coming from
mountain chain glacier tongues such as formed in Older Dryas
time (Chapter 1).
We get again the N-S drainage in Hafralónsá and in a Kverká
in this area. The gorge Dimmugljúfur reveals the work of glacial
runoff from the glaciated mountain chain.
The rivers in the area Þistilfjörður have a clear and con-
sistent SW-NE course. They flow on a flat plain on the strong
Tertiary Basalts, underlying our dolerite group. Thus we have
found a clear indication of the same SW-NE tectonic over-
printing in this area as in southern Iceland, at a time prior to
the formation of our main normal group of dolerite in NE-Ice-
land. As the latter is the first group of zonal volcanism, we
must seek the cause of this ,,sudden“ change of the tectonic
fractures in the same circumstances which gave rise to the
zonal volcanism.
In the lower zone west of Biskupsöxl the N-S fractures are
now mostly marked by small fault steps, whereas the old
shallow valleys are obliterated by sediments. West of the great
glacial river Jökulsá á Fjöllum this faulting continues, also
through the present narrow volcanic zone itself, where also
open fissures abound, together with linear volcanic lines but
also with large and small shield volcanoes.
For the valley volcanoes west of the Thingvellir zone in SW-
Iceland, we assumed the crossing of fractures to be an essen-
tial factor in the formation of shield volcanoes (Part I). And
we have no difficulty to assume the validity of that theory in
the northern part of the Eastern Active zone, now that we
know of the SW-NE fracture system in the somewhat deeper
older basalts underneath. But as the older fracture system is
buried, we cannot prove this crossing of fractures in individual
cases. On the other hand the deeper and older system is evid-
enced by exceptional crater rows of SW-NE direction, which
hitherto have seemed difficult to explain.