Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 137

135
d) Volcanism and Tectonism on the South Side
of the Thingvellir Lake1)
The dolerites drop in steps towards Thingvellir Lake and De-
pression, most conspicuously on the west side. A distinction be-
tween this subsidence and later uphft must be carefully observed.
This applies in particular to the highest step (170 m), the Jóru-
kleif escarpment at the SW-side of the lake. It is the expression of
a relatively late flexure along the Svínahlíd, where the high step
fades out completely towards the farm Heidabær.
The beginning of faulting at Jórukleif is explained in Fig. 7. The
close intergrowth of the unporphyritic volcanic breccia, 3 Fig. 7,
with the fractured dolerite edge suggests that some subaquatic vol-
1) This area has been described rather recently (16) in its broad outlines. The
paper describes correctly the main types of rocks, their field- and microscopic
appearance, and their distribution in the field and is in that respect of value.
But it’s shortcomings are in the obsei*vation of field details. It mentions only
rather few of the details given in the present summary of my observations; but
tlie details are most essential for the understanding of the geology of this area.
In (16) no erosional valleys are recognized and individual ridges are, therefore,
understood and given names as individual volcanic ridges formed subglacially
during the Last Glaciation. But our details prove the existence of even broad
erosional valleys and broad or narrow positive linear erosional forms, into which
extensive individual volcanic tuff-breccia masses have been cut by erosional
agencies. Flat surfaces, so clearly coimected with erosional base levels (cf. stages
of the Thingvellir lake), are treated as some unspecified foim due to subglacial
volcanism. Vertical layers of dislocation breccia, so clearly a part of vertical dis-
location walls, are in no case noticed. Even the significance of a clear surface of
denudation on the whole Hengill mountain, as well as on the narrow Sleggja
ridge, is not understood, in spite of the fact that the top member of Hengill is
said to rest on a surface of denudation (“auf denudierter Basalt Unterlage”, p. 54-
55). This denudational plain of Hengill-Sleggja can be proved to be an uplifted
part of a wider plain formed by normal erosion at a low level, but in (16) it
is suggested that it may correspond to such non-denudational structural plains
in table mountains as shown in Fig. 2. In (16) all the brecciaceous rocks in d)
are assumed to have been formed subglacially during the Last Glaciation (p. 32),
whereas we have found three different glaciations of the area, ages beyond the
Brunhes epoch, and verjr clear evidence of interglacial sub-aquatic origin of most
of the rocks. A clue to this unsatisfactory analysis in (16) may be that author’s
firm belief (p. 9) that tuff-breccias in Iceland are in general of subglacial origin.
We think it necessary to aid the reader by making a detailed comparison with
(16) in the main text. The maps given in (16) are very useful for this aim, and
generally for reading our discussion of this area.