Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 54

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face is lower than elsewhere and comes sufficiently close to
the groundwater. Elsewhere, on somewhat higher ground as
far as Hlíðarendi in ölfus, the frost action is general, but of
a different character: The surface is covered with cube-like
blocks in the 30—50 cm range, which have been uplifted just
by interstitial water between the typically thin lava sheets.
The case of Leggjarbrjótur, which we mentioned in Chapter
1 in connection with Þorleifur Einarsson’s attempt to trace the
Younger Dryas rand moraines in Northern Iceland, is almost
too complicated to be discussed here. Kjartansson (7) was much
in doubt of the age of the features he observed along the east
side of Langjökull north of Bláfell. He chose Younger Dryas,
not mentioning Older Dryas, the existence of which he seems
not to have realized. On the other hand, Þorleifur Einarsson’s
wrong picture of the Older Dryas glaciation left no other alter-
native than the Younger Dryas. In our reconstruction, Fig. 2,
both these cold spells may have played a role, but there are
at present no data to separate the effects of these two. There
is, therefore no point in trying at present to distinguish bet-
ween a Younger or Older Dryas age for the shield Leggjar-
brjótur.
At the time of writing, there has not been time to test,
in the above way, the ages of the main shieldvolcanoes in the
northern part of the Eastern volcanic zone, nor have we yet
suggested definitely a genetic connection between the Icelandic
shieldvolcanoes and glacial climate. But if now we do so, we
find an interesting indirect evidence for that connection.
For then the fact becomes significant, that there are no
shieldvolcanoes in the southern part of the Eastern volcanic
zone: This area was ice-covered until the end of the Younger
Dryas.
The present author has shown (8) that in Iceland, an ice-
sheet of more than about 50 m thickness will be able to choke
an eruption under it during the hour of birth, because of the
strength of the ice-sheet against bending upwards and because of
its weight. This argument I used against the strange hypothesis
of the formation of Table mountains in subglacial eruptions.