Jökull - 01.12.1965, Blaðsíða 23

IAN Y. ASHWELL
UNIVERSITY OF CALGARY, CANADA:
The Hestháls Glacial Drainage Channels
The inter-valley ridges of tlie Borgarfjarðar-
dalir, a series of approximately parallel valleys
in Western Iceland, are normally regular in
shape, and slope down from the central plateau
to end in a steep slope overlooking the Borg-
arfjörður-Mýrar lowland. These ridges, accord-
ing to Einarsson (1958, p. 20) form part of a
Pliocene erosion surface at 300—400 m above
present sea level.
The ridge, Skorradalsháls, between Lundar-
reykjadalur to the north, 'and Skorradalur to
the south, however, has a number of irregular-
ities, and the serrated skvline of the western
part of this ridge is very noticeable from much
of the district. I was able to examine this part
of the ridge, the so-called Hestháls, in the
summer of 1964, and to confirm a suspicion I
liad gainecl from previous acquaintance with
the area in 1962 that the ridge is broken by a
series of glacial drainage channels. Whilst no
conclusive evidence can be gained from these
channels about the glacial history of Western
Icelancl, there are a number of interesting fea-
tures which add a little to this history.
I
The heights of the channels were measured
by accurate altimeter, which was set from spot
heights of the Geodetisk Institut ‘Survey of Ice-
land’ maps. Checks against these spot heights
were made both before and after any series of
readings, and suitable corrections made to
measured heights frorn a graph of pressure
change during the intervening time. In general,
only the highest points of the channels and the
intervening ridges were measured. It was diffi-
cult to obtain any impression of the detailed
topography from the Geocletisk Institut or the
A.M.S. maps at a scale of 1:50,000, and the
details of the channels, as shown in Fig. 1 have
been obtained from the air photographs of the
area flown in 1956.
II
The highest channels, that is, tliose lying at
the eastern end of the series, are cut in the
normal olivine basalts of the country. These
basalts dip towards the southeast, that is, slight-
ly more to the south than the line of the Skorra-
daisháls ridge, at about 10—15°. The section
of the ridge through which the road runs is
niade of a much weaker material in parts,
either a scoriaceous basalt or a fault breccia.
The end of the ridge, making up most of the
area to the west of the road, including the
knife-edged ridge Hestfjall, is again formed of
solid basalts.
The channels (Fig.l) are arranged in roughly
parallel order across the ridge, and in many
cases tend to follow the strike of the rocks. All
the channels have a strictly ‘humpbacked’ form,
that is, they are higher in the centre of the
long profile than at either encl, but tliere is a
striking contrast between the ends of the chan-
nels. On the northern, Lundarreykjadalur, side
the channels end either high above the vallev
floor, or slope steeply down to that floor. On
the southern, Skorradalur, side the ends of the
channels slope gently down to the valley. Whilst
some of the channels on the Skorradalur side
have tributary channels, on the Lundarreykja-
dalur side several of the channels start from
one point on the valley wall. This is best seen
where the main round debouches into Lundar-
reykjadalur (Figure 2).
The most striking fact about the channels,
however, is the arrangement of the channels
when seen in section. Fig. 4 shows a section
through the highest parts of each channel, and
it will be noticed that the channels descend
in height from the highest on the east to the
centre of the series, then rise again to the Kvígs-
staðasneið at the western end. Many of the
channels have the form of the classic overflow
channel, as described by Peel (1949), and nearer
JÖKULL 129