Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 21

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aine to correspond to the Older Dryas, but this was, in his view,
the rand position of a „general glaciation" of Iceland (4),
cf. Fig. 1.
Because of that interpretation, Þorleifur Einarsson was un-
able to explain his own findings, on the basis of extensive
palynological work, that the birch occurred earlier in north-
eastern Iceland than in the southern parts of the country. Our
analysis has demonstrated what large areas in the Northeast
were ice-free during the Older Dryas, and would quite certainly
have been ice-free during Alleröd time.
To this should also be added certain particularly favourable
Alleröd conditions, which are a direct result of the severe
frost of Older Dryas time. This is frost lifting. The expansive
pressure of ice, when being formed, is at least 1000 kg/cm2,
corresponding to the weight of a 3800 m thick plate of basalt
of density 2.8 g/cm3. In a country built up of plane-parallel
basalt lavas, the groundwater between a topmost lava of say
1—2 m thickness and the underlying solid layer may freeze in
winter during such severe climate, when the frost reaches such
a depth. And the easiest way of yielding to the ice pressure
is an uplift of the top layer(s), or rather individual blocks
of it. The result of this process is a chaos of blocks, called
urð or grjót in Icelandic, and is an extremely common occur-
rence, due to the Older and Younger Dryas cold periods.
This phenomenon provides a method of testing the age of
late-glacial and postglacial lavas. Thus frost lifting is extrem-
ly clear and impressive in such outskirts of the Skjaldbreið-
ur shieldvolcano lavas, where groundwater is and was near
the surface of the lavas. There is no possibility of mistaking
effects of steam at the time of flowage of the lavas for the
effects of ice lifting, these phenomena are widely different. We
shall relate on such age tests in the second part of Chapter 3.
Due to the frost action on the ice-free surface in north-
eastern Iceland in Older Dryas time, plants and low animals
found ideal shelter in Alleröd time.
It will now be clear that the distribution of glacial centers,