Jökull - 01.12.1976, Blaðsíða 5

delta of Ljósoddi at 39Ó m under Thverfelij
65 m above the lake level at 325 m (Fig. 1).
Westwards from Skotmannsfell the plateau
slopes gently into the shallow Grímsá valley,
but above Oddstadir, near the junction between
Grímsá and Tunguá, the valleyside increases in
height by about 100 m, and behind this lip lies
the conical hill Reydarfell, its eastern side thick-
ly plastered with basal till. The wide area to
the north of this hill, where the open plateau
merges into the upper end of Flókadalur, has
broad areas of rather similar deposits, although
it is not possible to tell from air photographs
whether these have angular or rounded stones.
Probably the whole area was covered at one
time, but streams have removed the material in
the valleys.
THE LUNDARREYKJADALUR VALLEY
As suggested in the Introduction to this paper,
the valley of the now tributary river Tunguá
was the head of the original Grímsá river, un-
til Thverfell blocked up Reydarvatn and form-
ed the source of the modern Grímsá. The mark-
ed contrast in grading between the old Tunguá
valley and the modern upper Grímsá valley is
shown in Fig. 2. Grímsá has probably opened
up an old minor tributary valley on the north
side of the main valley, and the course of the
stream is very youthful, interrupted by many
falls and rapids, while the upper part of the
valley is a shallow trench in the plateau surface.
In contrast, Tunguá now runs in a wide valley,
deeply cut into the plateau, and is graded to
probably three base levels. In contrast to the
Grímsá valley that of the Tunguá contains
several farms and a main road connecting
Reykjavik with the lowland to the west. It is
obviously the ‘main’ valley.
Associated with the well-graded Tunguá
stream are two remarkable features, Englands-
flói and Tungufellsflói (Figs. 1 and 2). Both
appear to be platforms cut in basalt rock, Eng-
landsflói at about 175 m and Tungufellsflói 130
m at its back against the Tungufell mountain,
124 m at the top of a sharp slope down to the
Tunguá. They are apparently old valley floors
of the Tunguá, perhaps dating from the late
Pliocene when the original trenching of the
300—400 m basalt plateau took place (Einarsson,
1958, p. 20). Although the lower Tungufellsflói
is probably the more recent, this does not ac-
count for the major differences in the two plat-
forms. Englandsflói has few major and sharp
breaks of slope whereas Tungufellsflói is en-
tirely bounded by marked and regular slope
changes. The boundary with the steep and
rather straight southern slope of Tungufell, and
the outside edge at 124 m appear to be of very
recent formation. Below the outside edge and
above the Tunguá is a shelf at about 110 m
which runs without a break into a shallow,
north-north-east to south-south-west oriented
channel with highest point 111 m, near a lake
at the summit, below the Tungufell farm. This
channel ends above the Grímsá river at 108 m,
with a narrow step below at 100 m.
The Tungufellsflói platform continues to the
Metres
Fig. 2. Long profiles of Grímsá and Tunguá rivers, and across rock platforms.
Mynd 2. Langsnið af Grimsá og Tunguá.
JÖKULL 26. ÁR 3