Jökull - 01.12.1984, Page 123

mer sea levels. He furthermore presented a
theory for the deglaciation of the Borgarfjördur
region and a scheme for sea level changes.
Briefly, Bárdarson’s observations were the
following (Figs. 2 and 3):
He recognized two clay units in the region,
which sometimes were separated by gravel. Bárd-
arson described the lower clay unit as a “lamin-
ated, distorted clay”, and pointed out that in
some sections the surface of the unit is exten-
sively eroded. He recognized this stratum only in
the area south of the Hafnarfjall-Skardsheidi
massif, and found difficulties in correlating
between the various sections where it is exposed.
On the basis of the fossil fauna composition of
the stratum he inferred that the sea temperature
was similar to the present temperature off the
west coast of Greenland, at least during the
deposition of the lowest part of the stratum. He
concluded that this stratum was deposited during
a period when the glaciers were retreating and
the sea transgressing towards the marine limit at
80—100 m above present sea level. Bárdarson
postulated that this transgression was interrupted
for some time at 50-60 m elevation, but admit-
ted that there was no lithostratigraphical evi-
dence for such an intermission. Molluscs col-
lected from the upper part of the distorted clay
show evidence of a low arctic environment during
its deposition.
Bárdarson explained the lack of distorted clay
in the northern part of the region by stating that
the lowlands there must have been ice covered at
the time. Later, during the period of highest sea
level, the glaciers retreated into the tributary
valleys of Borgarfjördur, where they deposited
terminal moraines before finally melting. Subse-
quent to the retreat of the glaciers there followed
a period of falling sea level, at least down to the
40-50 m level. Bárdarson obviously found it
difficult to explain the presence of gravel in the
middle of a marine sequence, and he stated that
“it is important to answer the question whether
one or two transgressions are registered in the
older marine strata of Borgarfjördur” (Bárdarson
1923, p. 65). He further suggested that the ero-
sion of the distorted clay unit in the Melabakkar
cliffs was the work of bottom currents during a
period of relatively low sea level. In a later work
(Bárdarson 1927) he seemed to doubt that there
had been two transgressions in Late Weichselian
times in the Borgarfjördur region.
Bárdarson (1923) concluded that the sea level
remained at the 40-50 m level for a considerable
period and postulated that the upper clay unit was
deposited in that time. He described this unit as a
“horizontally laminated clay stratum”. He
observed this unit throughout the area north of
Hafnarfjall-Skardsheidi, where the stratum often
constituted the major part of the sequence. Bárd-
arson correlated it with similar clays he observed
in the Melabakkar and Ásbakkar cliffs and in
coastal sections south of Mount Akrafjall. In this
stratum Bárdarson found indications of an
increasing sea temperature during the deposition
of the sediments as the fossil fauna assemblages
change towards the top of the sequence from low
arctic to boreal. Finally, on the basis of sub-
merged bogs near Akranes and Borgarnes, Bárd-
arson suggested a regression of the sea down to a
level of 4—5 m below the present sea level. In the
uppermost part of the Borgarfjördur sequence
Bárdarson mapped various units of sand and
gravel, but did not discuss their depositional
environments.
According to Bárdarson’s results, the Late
Weichselian in the Borgarfjördur region was
characterized by retreating glaciers, except for a
late intermission during which the glaciers were
extended for some period as far as the entrances
to the tributary valleys. Terminal moraines piled
up during this period were later levelled by the
sea and formed into terraces. Bárdarson did not
describe the morainic features in any detail. It is
interesting to note, that although Bárdarson rec-
ognized the nearby presence of glaciers during
the time of deposition of the lower part of the
Borgarfjördur sequence, he did not observe tills
or other glacial deposits on the lowlands. He
postulated the presence of glaciers partly on the
basis of negative evidence, i.e. the lack of certain
sediments, and partly on faunal evidence.
Continued study of the Borgarfjördur sediments
Bárdarson’s study of the Borgarfjördur sedi-
ments remained unchallenged for nearly half a
century, and has been referred to as classical by
Ashwell (1975). Kjartansson (1960, 1968) out-
lined the distribution of sediments in the Borgar-
fjördur region. He recognized outcrops of marine
and alluvial sediments, and considered the sur-
face sediments of Melasveit to be of glacial ori-
gin. Later Sœmundsson and Einarsson (1980)
JÖKULL 34. ÁR 121