Jökull - 01.12.1970, Blaðsíða 69

Fig. 3. Exposure on the eastern bank of Thjórsá north of Gíslholtsfjall, showing unconformity
between Hreppar Series (base) and an interglacial lava flow (top).
Mynd 3. Holt á a,usturbakka Þjórsár norðan við Gislholtsfjall. Mislœgi milli Hreppamyndunar og
ungu hraunanna.
completely cover the underlying solid rock. The
western border of the young lavas forms steep
escarpments. The escarpment north of Efri-
hamrar and Sumarlidabaer near the main road
is very prominent since it runs perpendicular
to the strike of the Hreppar Series. The west-
ern border of the young lavas north of Ás is
even more conspicuous forming a 20 m high
scarp. West of the lakes Gíslholtsvötn the
young lavas reach the eastern bank of Thjórsá.
From there northwards they are well exposed
up to Árnes. The young lavas form an almost
continuous sheet in the south and become in-
creasingly eroded towards the north where they
disintegrate into a number of hills. North of
Thjórsá a broad, dissected range of hills ex-
tends towards the southwest between the rivers
Hvítá and Thjórsá. The young lavas overlap
the southern part of these hills and have been
preserved as erosion remnants on Galtafell,
Hlídarfjall, Skardsfjall and some others. On
Hlídarfjall the young lavas reach their highest
point of elevation at 373 m above sea level.
The number and thickness of the young
lavas vary from one exposure to another. In
Galtafell (Fig. 4) 5 flows with a maximum
thickness of about 80 m were seen. In Hlídar-
fjall the thickness is similar but the number of
flows is less. In Skardsfjall there are 2—3 flows
about 70 m thick. South of Thjórsá the number
of flows is also at least 3 and their thickness
is about 100 m as seen in Gíslholtsfjall and
Sandskardaheidi.
The thickness of individual flows varies a
great deal. The thinnest flow, only about 5 m
thick, was seen in Galtafell and near Akbraut
but others exceed 50 m in thickness.
SEDIMENTS BELOW THE INTER-
GLACIAL LAVA FLOWS AND
THE PRE-LAVA MORPHOLOGY
In various places a thin sedimentary sub-
stratum below the young lavas was observed.
It consists usually of conglomerates or sand-
stones that are easily recognized as fluviatile in
origin.
In Hreppar a greatest thickness of 15 m was
observed on the northeastern side of Galtafell
wliere crossbedded sandstone and conglomerate
outcrop over a distance of some 400 m. The
crossbedding within the sandstone dips towards
NE and N at this locality. On the northern
side of Hólahnúkar 5 m of a pebbly sandstone
outcrops. Above Núpstún farm a gray moraine-
like sendiment underlies the young lavas. This
can be followed over a distance of about 300 m.
Usually the lavas rest directly upon the flu-
viatile sediment or moraine but in a few cases
a 1—2 cm thick layer of black ash was found
at the base of the lavas.
At Raudilaekur in the Holt area a sandstone
with a thickness of 1 m is exposed below a
young lava. The total thickness of this sedi-
mentary layer, however, is unknown. Similarly
at Árbaejarfoss a 25 m deep drillhole pene-
trated a 20 meter lava flow and ended in 5 m
of sandstone
The sedimentary layer is thinner, in the west-
ern part of the area covered by tlie lavas. A
JÖKULL 20. ÁR 67