Jökull - 01.12.1975, Page 44

Fig. 4. View south-east-
wards across Arnarvatn
stóra from Hnúabak,
showing the escarpment
cut across the peninsula
uncler Arnarvatnhaedir.
MyncL 4. Utsýn af Hnúa-
bak til suðausturs yfir
Arnarvatn stóra. — Brún
gengur þvert yfir skagann
undir Arnarvatnshæðum.
the 484 summit is a shelf some 40 m wide fac-
ing north-west. At the bottom of the hill below
the shelf, just above the Refsveina stream, lies
a field of very large angular boulders, some of
them measuring 2 m across the widest dimen-
sion. These boulders are almost certainly de-
rived from a basalt capping on the hills. They
lie in an area of soil erosion and were prob-
ably originally covered with soil.
Similar shelves occur on the hills around
Mordísarvatn (M on Fig. I). One of these on
the north-west-facing side of a liill to the north
of the lake was surveyed and was found to slope
up 5° from the north to the main shelf, about
20 m wide and running absolutely level for
100 m before sloping down at about 40° to-
wards the south and disappearing towards the
lake. Other shelves round the lake have the
same level and general form, although they are
not continuous. The form of the shelves sug-
gests that they are glacial spillways, and the
existence of closed channels cutting through the
hill near Reykjavatn strengthens this assump-
tion.
The rock plateau of Arnarvatnshædir further
north is only thinly covered with drift and falls
steeply to the southern shore of Arnarvatn stóra
and to the flat tops of promontories into the
lake. Further escarpments occur along the east-
ern shore of the lake at the base of the steep
slope from Langijörfi. These escarpments ap-
pear to be erosion features and the flat tops
of the lake promontories may be marginal
channels (Fig. 4). As a general rule the escarp-
ments in the Arnarvatnsheidi, running from
north-east to south-west, with the steepest slopes
facing north-west, all seem to be the result of
water streaming along the fronts of the slopes
42 JÖKULL 25. ÁR
to the south-west, with many lakes occupying
the hollows at the base of the escarpments.
DEPOSITS
Where cleposits are exposed from the cover
of mineral soil, thick peats, or lakes, they appear
to be till-like in form, with some enormous
boulders perched on hill-tops, perhaps as erra-
tics. Boulder fields similar to that described
below the escarpment near Stóralón are found
elsewhere, for instance on the south shore of
Arnarvatn stóra and on the north side of Eld-
borg, near Úlfsvatn, but it is likely that other
such features are covered by soil. The deposits
appear to be thinnest in the north and east,
thickest in the south and west, but identifica-
tion from air photographs is difficult since only
those deposits cut into by wind erosion or
stream action show up, and the distribution
shown on Fig. 1 is probably a major under-
estimate. Where soil erosion has occurred the
surface is littered with stones. An examination
of one such area was made on the north-west
facing side of an escarpment north of the Alfta-
krókur kofi. The surface stones were removed
over an area about half a metre square and a
hole dug to a depth of half a metre. The
material below the surface was silt, only two
small stones being founcl. A party of eight per-
sons examined the shapes of stones in small
areas randomly selected on the surface of the
stone field, and eye estimates gave a mean
value of 41 percent of stones markedly angular,
30 percent markedly rounded, and the re-
mainder intermediate. These results suggest
frost heaving of stones to the surface from a