Jökull - 01.12.1989, Blaðsíða 23

The Ages of the Fossvogur Layers
and the Alftanes End-Moraine, SW-Iceland
ÁRNI HJARTARSON
National Energy Authority,
Grensásvegi 9,108 Reykjavík, Iceland.
ABSTRACT
The Reykjavík area is well known in Iceland’ s
geology for its fossiliferous sediments. Two locali-
ties, Elliðavogur and Fossvogur, are classic loca-
tions for shell-bearing sedimentary layers of late
Quaternary age. The Elliðavogur layers underlie the
Reykjavík olivine tholeiite basalt group, whereas the
Eossvogur layers were deposited on top of this lava
unit. The shell-bearing Fossvogur Layers have been
thought to be an interglacial formation, most likely
from the end of the Saale Glacial, or the beginning
°f the Eemian Interglacial. Their age was previously
estimated at about 120,000 years BP. However, new
radiocarbon age determinations have given the
unexpected age of 11,000 years BP (conventional
radiocarbon years). The Fossvogur layers, the
former type locality for the Eemian Interglacial in
Iceland, now seem to have formed during late
Weichselian. This conclusion calls for a revision of
the late Pleistocene and early Holocene time-scale
°f the Reykjavík area. The till in the uppermost sec-
tion of the Fossvogur layers must now be considered
to have formed during Younger Dryas or in Prebo-
real times. This indicates a much more extensive
glaciation of the Reykjavík area at the very end of
the Weichselian glaciation than previously assumed.
Sedimentary layers in Kópavogur are considered
to be of the same age as the Fossvogur layers. They
are covered with a series of terminal moraines
which must be a little younger than the uppermost
till of the Fossvogur layers. The outermost of those
moraines seem to tie up with the so-called Alftanes
ridge, which has hitherto been considered the type
locality for the Older Dryas stadium in Iceland. This
must now be considered to be of Younger Dryas or
Preboreal age.
INTRODU CTION
The Fossvogur sedimentary layers are a series of
till and marine sediments (Fig. 1). They can be
traced for about 1.7 km from the head of the Foss-
vogur bay to the Reykjavík Airport. They cover the
Reykjavík olivine tholeiite basalt group at the Air-
port and the area between the University and the
Old Cemetery on Suðurgata. The layers are also
visible on the coast of the Skerjafjörður bay (Einars-
son, 1968; Geirsdóttir, 1982).
Many naturalists have investigated the Fossvogur
layers. Since 1840 between 30-40 written accounts
deal with the sediments. Péturss (1904) was the first
to recognize that the layers were formed during
Pleistocene:
"Till on top of the shell-bearing layers is now, according to
the situation on each site, proof beyond doubt that these
layers are not younger than the Ice Age; that they are not
older than the Ice age has often been shown, but they must,
however, have formed after the glacier moved across the area;
which must have been ice-free when the Mya truncata lived
in the silt that is now a hard consolidated rock surrounding
their shells. The glacier readvanced and déposited the till on
top of the shell-bearing layers, whereas in most places it
eroded the layers away" (Transl. O.B. Smárason).
The Icelandic geologist Dr. Helgi Péturss (1872-
1949), was one of the first to realize that the Ice Age
was not just one long lasting and ice-cold winter,
JÖKULL, No. 39, 1989 21