Greinar (Vísindafélag Íslendinga) - 01.01.1976, Síða 139

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plateau, reaching a height of 360 m or about 260 m above the lake.
On the west side it ends in a very steep wall of 50 m height, over-
looking in the close neighborhood the surface of the westem dole-
rites which it must be taken to overlie, although a direct contact
is not visible. To the east the formation is thicker than this, but
overlain and largely hidden by the Háhryggur series of (16), and
the presumably underlying dolerite is not exposed.
Our interpretation of the data so far is that the first faulting at
Tórukleif itself was insignificant, whereas subsidence increased
towards the axis of the whole depressed zone. Widespread outpour-
ing of unporphyritic magma then took place, partly in shallow
water and partly on a gradually widening and thickening sheet
of terrigenous sediments, including more or less redistributed and
spread tuffs, in a zone between the westem and eastern dolerites.
The Háhryggur series with its capping lava flows belongs to this
phase and is mainly suberial. The present high position of these
rocks in the Hátindur plateau and the Háhryggur NE-SW striking
long faulted stripes (Fig. 10. C) finds an explanation in a later
vertical displacement by about 200 m (for the horizontal sediments
of the Hátindur plateau), during which also the main Jómkleif
faulting of a very similar magnitude took place. This displacement
is taken to be due to an uplift corresponding to the formation of
the Jórukleif (Svínahlíd) flexure (which is not mentioned in (16)).
For there is no possibility that the fluviatile sediments were formed
at the present high level of over 300 m. No such high land existed
in the area of the present Thingvellir Lake and depression, from
which ranning water could have transported material southwards
— the dolerite surface to the west and east neither is, nor can be
assumed to have been as high as that. More evidence for the uplift
of the Hátindur plateau will soon be given.
The plateau is overlain by numerous erosional remnants of a sub-
aquatic coarsely plagioclase-porphyritic breccia (Hátindur, Jóra-
tindur, Helluhryggir, Fig. 10, C). As seen in the higher parts,
Hátindur is formed of steeply westerly-dipping tuff-breccia layers
in situ; these have been cut by a smooth 5° easterly dipping sum-
mit plain at about 425 m elevation, Fig. 8. (At a similar level we
further find a clear near-horizontal erosional plain, cutting steeply
dipping tuff-breccia layers in situ, forming the western summit of