Jökull - 01.12.1975, Page 45

Fig. 5. The ash layers in
the bank near Skammá,
in a matrix of sandy silt.
The lowest part of the
visible deposit is 8 cm of
fine black ash, overlain
by 12 cm of rather thin
(0.5—1 cm) layers of co-
arse white ash separated
by sandy silt.
Mynd 5. Öskulög í barði
ndlægt Skammá. Neðst
sjást 8 cm af fingerðri
svartri ösku, yfir henni
liggja 12 cm af fremur
þunnum (0,5—1 cm) lög-
um af grófum hvitum
vikri með millilögum af
sandblöndnum leir.
lower layer of rather mixed stone composition.
The relief of the area north of the Nordlinga-
fljót is noteworthy, since the low, rolling hil-
locks from Kleppar to a point opposite to
Vopnalág and thence through Thorvaldsháls,
and for some kilometres north of the river have
summits in a height range 410—430 m. It may
be suggested that this is an area in which the
original overlying basalts have been entirely
stripped off and the softer materials below
selectively eroded. These softer materials, which
may be partly tillites, partly sedimentary sand-
stones and conglomerates, may appear to be
deposits when they are, in fact, bedrock re-
sulting from an earlier phase of deposition.
An anomalous deposit was examined just west
of the Skammá stream on the south shore of
Arnarvatn stóra. This stream flows from Rétta-
vatn at 549 m and then through two small
lakes just below this level before falling steeply
about 8 m over the escarpment into Arnarvatn
in a rapid about 25 m in length which is com-
paratively little incised into the escarpment
especially in its lower part. About 100 m west
of the rapid and 7 m above the level of Arnar-
vatn thick layers of ash occur in tlie eroded
bank of soil forming the edge of the escarp-
ment. About 20 cm of this ash could be un-
covered, the upper layers being of coarse white
ash, with a thick layer of fine black ash below,
but the deposits were well back in the bank in
a hollow probably excavated by sheep and it
was not considered advisable to remove the very
large area of turf required to obtain a clear
section (Fig. 5). Such a thickness of ash is to-
tally uncharacteristic of tliis area in which ash
layers were being particularly looked for, but
none found: even some distance to the south-
west, at Álftakrókur, the ash layers found were
a millimetre or two thick.
It can be suggested that this thick deposit of
ash is the result of collection in the drainage
area of Réttavatn and transport by the Skammá
into an Arnarvatn with a level only a little
below that of Réttavatn and deposition on to
a delta at the entrance to the lower lake. In
that case, the rather immature fall on the pre-
sent Skammá is due to the relatively recent fall
in level of Arnarvatn.
ICE MOVEMENT
Ice scratches in the area summarised from
the geological map (1965), Atkins (1971) and
from the author’s observations are shown in
Fig. 1. It is clear that the main direction of ice
movement over the area was from a mainly
east-north-easterly direction, which is maintain-
ed into the Borgarfjördur—Mýrar Lowland, to
the south-west (Kjartansson, 1955, p. 159). The
situation to the north of Langjökull is uncer-
JÖKULL 25. ÁR 43