Jökull - 01.12.1966, Blaðsíða 55

Fig. 3. The tephra layers H3 and H4 in a
loessial soil, covered by till. Photo taken near
profile 1, cf. Figs. 1 and 2. Length of foot
rule 1 m.
Ljósu Heklulögin H3 og H4 i fokblöndnum
moldarjarðvegi undir jökulurð Hagafellsjökuls
eystri, skammt vestan brúarinnar á Fari.
Photo S. Thorarinsson.
12 cm long and 4—5 cm thick lens of soil con-
taining in its middle the tephra layer H 1693
which is rather coarse-grained in this area (dia-
m. of grains T 0.6 crn).
As the rate of thickening of the soil in the
nearby soil patch (profile 2 on Fig. 2) between
1693 and 1766 did not exceed 0.7 mm/year and
the thickness of the soil covering H 1693 when
overrun by the ice was at least 2—3 cm we
have liere a proof that Hagafellsjökull evstri
did not advance over this area before 1730
or so. And as it is likely that some soil was
removed, it is possible that the glacier advanc-
ed over the area considerably later. It may tlius
be regarded as definitely proved that the
glacier Hagafellsjökull eystri reached its max.
postglacial extension after 1730. Its maximum
extension it may have reached either during
the 1750/60 “Hochstand” or during the “Hoch-
stand” about the middle of the 19th century.
In the Land Register (Jardabók) of Árnes-
sýsla, written by Á. Magnússon and P. Vídalín,
there is a passage where it is said that the river
Tungufljót has destroyed some pastures of the
Vatnsleysa farm (op. cit. p. 302). It seems likely
that this was because of floods in the river
caused by jökulhlaup from Hagavatn. This
would mean that in 1708 the glacier was either
in its 1929- or 1939-position and that is in
very good agreement with the tephrochrono-
logical results obtained by Green and the pre-
sent writer.
Summing up the tephrochronological results
and considering the most likely cause of the
floods in Tungufljót mentioned in the Lancl
Register of 1708 we reach the following con-
clusions regarding the oscillations of Hagafells-
jökull eystri:
Between 850 and 900 A.D.: Glacier smaller
than or maximally as large as in 1929.
About 1670: The glacier reaches its 1939
position.
1708: Extension of the glacier either the
same as in 1929 or 1939. The glacier is stagnant
or receding.
It may be regarded as absolutely certain tliat
the glacier did not reach the area E of Fagra-
dalsfjall before 1730 or so and its maximum
extension in postglacial times was not reached
until later, either about the middle of the 18th
century or — and just as likely — not until
the “Hochstands” of the 19th century.
The profile measured in 1965 is a striking
example of how small the erosional power of
the advancing glacier has been, as maximally
a few tens of cm of loessial soil have been re-
moved in the place where the profile was mea-
sured.
ÁGRIP
í botnsseti vesturhluta Hagavatns, eins og
það var fyrir 1929, eru setlög með árshvörfum.
í þessu seti er einnig að finna öskulög frá
JÖKULL 209