Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 35

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took place under influence of the river — or dammed lake water
in the old Víðidalur river bed, which also supports the inference
of a fracture, along which the valley developed.
Formation of table mountains and other forms of tuffbreccia
heaps in lceland, under the influence of river and/or dammed
water in vaTleys.
We have already shown in Fig. 5 how the table mountain
Skriða stands astride the Mjóidalur valley and indicated that
this is a most significant fact. It is time now to enter upon
the matter.
In Part I of these studies (15) the author has given a de-
tailed description of the southern part of the Median Active
Zone, and included a discussion of some valley volcanoes of tuff-
breccia in the western outskirts of the zone. Hvalfell is the
best example. We pointed out in Part I that as Hvalfell formed
on the axial line of the valley, first river water, and soon also
the water of a dammed lake must from the very beginning of
the eruption have entered into the eruptive mechanism, with
the result of such explosivity and quenching which leads
straight to tuffbreccia formation. The four cases of valley
eruptions mentioned in Part I led all to brecciaheaps, and are
of very different ages. Only in the case of Hvalfell, we still
have the whole volcano. Just as in the case of Surtsey, the vol-
canic funnel got, in the end, sealed off from the water, and
then lavas flowed over the conical heap, and are in part also
left at the foot of the cone. But the lavas are especially left
as a cap surrounded by high walls. The reason is in this case
the erosion by an insignificant valley glacier up to the foot of the
walls. Thus the upper part of the volcano is a true table moun-
tain with the lava cap and central crater hill still to be seen,
and we cannot have any doubt that if this mountain had formed
in a place like Skriða, the erosion by main Pleistocene glaciers,
and their runoff waters, would have eroded also the lower
parts, so as to form steep walls from the lava cap all the way
down to the level of the surroundings, and leave the most typi-
cal table mountain such as Skriða and Hlöðufell.
3