Náttúrufræðingurinn - 1964, Blaðsíða 47

NÁTTÚ RUFRÆÐINGURINN
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flow indicates an almost complete deglaciation of the country as early as 8000
years ago. Further, the above circumstances show that the isostatic uplift of
the country had by that tirne brought the shoreline down to ils present level
or even lower.
Peat overlying the Thjórsá Lava in the beach at Stokkseyri (19)
1910 ± 250 years (W-909)
Tlie samplc was taken in the tidal zone 150 cm below the high-water mark
from a thick layer of peat. A diatom analysis of the sample by Jón Jónsson
reveals an almost 100 per cent freshwater flora. This fact indicates a consider-
able rise of the sea level during the last 2000 years.
Plant remains in silt deposited in Krókslón (13)
5290 ± 250 years (W-911)
“Krókslón” is a name given to a short-lived lake, formed by a lava-flow dam-
ming up the glacial river Tungná, which in turn nearly filled the lake basin
with silt and then drained it to the bottom by cutting a deep outlet gorge
(Fig. 4). The sample for radiocarbon analysis, consisting of the aquatic nioss
species Hygrohypnum ochreum, was taken from the bottom layer of silt de-
posited on the edge of the damming lava. This lava-flow, called “Stífluhraun”,
is a young — probably the youngest but one — mcmber of the so-called Tungná
Lavas (of which the great Thjórsá Lava seems to be the oldest; see map, Fig. 3).
According to the age determination of the plant remains the age of the “Stíflu-
hraun” may be estimated at about 5400 years.
Carbonized plants underlying the lava Þingvallahraun at the Sog (20)
9130 ± 260 ycars (W-1912)
A stratigraphic section of the gorge of the Sog is sketched in Fig. 6. The
plant remains, mostly or exclusively consisting of the moss species llhacomitri-
um lanuginosum, were obviously carbonized by the heat of the overlying lava-
flow as it flooded the area. This great lava must therefore be of practically
the sante age as the plant remains, or, according to the above determination,
about 9100 years old.
C14 Datings connected with Tephroclironology
by Sigurdur Thorarinsson
Museum of Natural History, Department of Geology, Iteykjavik.
Most of the samples collected by Thorarinsson for C14 dating are connected
witli his attempt to establish a tephrochronological time scale for the post-
glacial deposits in Iceland. The aim has been to date the inost widespread
teplira layers in the country or to support their tephrochronological dating. The
C14 dated layers can be used for datings where the C14 method is not applicable,