Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 53

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Trölladyngja, could possibly also belong to that stage, taking
the estimated northern limit of the “main glacier” in Chapter 1
into account.
A way of testing and distinguishing lavas of Older or Young-
er Dryas age from younger ones, is to look for frostlifting in
the thin margins of the lavas, where groundwater is exist-
ing. Where groundwater is evidently present under the margins
of the field of the Skjaldbreiður lavas, the uplift by frost
action is very clear. A brook flows south along the hill on
the north side of Biskupsbrekka. Within the water course, the
groundwater is also flowing and does not freeze, and here the
lava is flat and smooth and relatively very little affected by
frost lifting. But on the east side of the brook there is a suc-
cession of uplifted 3—4 m high, sometimes elongated hills paral-
lel to the brook, where the groundwater could be stagnant
and freeze in extreme frosts. The frost must have reached
a depth of 3—4 m or more. This strongly suggests Older Dryas
age of Skjaldbreiður (There is no difficulty in distinguishing
between uplift by frost action and uplift of hills due to lava
pressure below a crust during the time of eruption). There is
no frost lifting in the Þjórsárlava at Villingaholtslækur, or
elsewhere in the groundwater-soaked lava whose age is 8000
to 9000 years B.P. This is one of the oldest postglacical lavas
in Iceland, and the observation indicates that lavas with clear
frost action must belong to times of glacial climate. Sofar,
all the shieldvolcanoes tested indicate Older or Younger Dryas
age. Older Dryas is clearly indicated for the Skálafell shield
on Reykjanes, and at least Younger Dryas age for the Lang-
hóll shield.
Of special interest is also the shieldvolcano Heiðin há near
Bláfjöll ridge, Fig. 4. According to our reconstruction of the
Older Dryas Bláfjöll glacier, the orifice was icefree. This agrees
well with the age based on frost action.
Where the lavas come to an area of shallow groundwater
near Hlíðarvatn in Selvogur, we find large hills due to frost
lifting. This is also the case along the road to Þorlákshöfn,
between about Hraun and Hamraendar, where the lava sur-