Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 121

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rites of Ástaðafjall and Mosfellsheiði may once have heen
connected.
A thin cover of dolerite is found on many of the flat-topped
uplifted blocks along the mountain chain farther southwest,
thus on Meitill, Vífilsfell, Bláfjöll, Geitafell, and Langahlíð.
Those on Langahlíð are porphyritic and closely similar to the
dolerites in the neighbouring lowland area east of Hafnar-
fjörður. To explain these flat-topped mountains, some of them
quite large, by the sub-glacial eruption hypothesis is in my
view out of the question. The tectonic uplift explains the
facts quite naturally and the dolerite on Langahlíð and on the
westem lowland fits very well into the scheme.
On the west side of Hengill we can also make some observa-
tions which are in harmony with the assumption of a rela-
tively late uplift. Here, at the mouth of the Engidalur, is a
low mound, 500X700 m2, of sediments through which the
brook Engidalskvísl has cut a gorge. Two types of material
are found. The lower is a tuff consisting mostly of fine pu-
mice. A few homb fragments and an occasional small homh
are found and it is clear that this material is essentially of
primary origin and not far from its origin. On the other hand
the stratification is such as to suggest an eolian sediment
with large dunes, and foreign material and the worn state of
the pumice verifies that this must be so. I assume that the
material is essentially primary but that the surface of the
loose deposit was moved and redistributed by the wind. On
this material we have a different, clearly secondary material.
This is mostly finegrained windblown basalt debris and pu-
mice but also well rounded fluviatile gravel, especially in the
boundary layer. Sandlenses occur. Finally the lower and up-
per layers have been moved; probably they flowed in a soak-
ed state and were variously wound and even locally over-
turned.
The Engidalskvisl which crosses the sediments carries much
rhyolite but no such material was found in the sediments.
Also they are much finer than the material carried by the