Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 122

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brook. The facts tend to show that the high and steep west-
ern slope of Hengill cannot have existed when the sediments
were layed down. They represent the flat surface of this
whole area which existed here before the tectonic uplift.
Kleifarvatn area. In this area it is possible to work out the
structure in considerable detail and therefore to reconstruct
the history of some of the younger tuff-breccia masses.
Coming from the east the table mountains come to a sud-
den end at Kleifarvatn. The west end is a large block whose
north side is Langahlíð, southwest corner is Geitahlíð (386 m),
and the west side will be called here Vatnshlíð (Vatnshlíð is
the name of the slope along the east side of Kleifarvatn).
This block is bordered by rather steep slopes; their lower part
is formed by rather coarse breccia at the few places where
this can be seen, but on top there is as usually in the tables
a cover of dolerite lavas (Langahlíð, Geitahlíð). In Geitahlíð
the dolerites have a maximum thickness of 100 m or more;
the surface rises gently towards the centre where it culmi-
nates in a 400—500 m wide and 50—60 m deep crater in
which the surrounding dolerite lavas originated. The surface
of Geitahlíð is severely broken up by frost action into a
mantle of blocks; in the north side, at 320 m, one finds rem-
nants of a hard brown weathering layer or solifluction ma-
terial. There are no traces of a glacier overriding the surface
nor of a local ice cap. But near the summit we have more
worn gravel and sand caused by travel down the gentle slope.
On the north side of the crater considerahle erosion is visible,
probably caused by a local snow patch during temperature
minima, but here is also a sheet of hardened coarse tuff with
small bombs and scoriae, the result of some more explosive
activity in the crater toward the end of activity. The crater
is very little disturbed which is of great interest considering
its relatively great age. The reason for this good preservation
may be in part sheltering by snow but the main reason is
probably the leakage of the precipitation into deeper levels,
reducing the frost action, This result shoujd be kept in mind