Náttúrufræðingurinn - 1958, Síða 30

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NÁTTÚRUFRÆÐINGURINN
ments, globules, veins and dikes o£ a more or less crystalline basalt. These rocks
are of Late Pleistocene age.
The most striking features of the móberg landscape are: (1) its lack or
slight development of surface drainage owing to the permeability of the bed-
rock and resulting in little erosion except by the wind; (2) numerous serrated
mountain ridges running all in the same direction within each móberg area,
and (3) small, steep-sided, isolated plateaux (tablemountains) elongated in
the same direction. Both kinds of mountains consist almost exclusively of mó-
berg, except for the tops of many of the plateaux, which are usually covered by
sheets of lava. Such sheets are more rarely found on the tops of the ridges.
As to the origin of these morphological features there are chiefly two dif-
ferent theories, that of vertical displacement and that of subglacial accumu-
lation. According to the first theory the móberg mountains are strips of the
earth’s crust that have either been lifted above their surroundings (Sonder
1938) or remained when these subsided (Reck 1921—22, Nielsen 1933).
The second theory maintains that the móberg magma was extrucled (or „in-
truded") subglacially into vaults or hollows melted into or through the ice-
sheet by volcanic heat, and that by this process the new material was moulded
between the walls of ice into almost the present shape of the mountains. The
hollows were fillecl with melt-water which caused the magma to solidify into
móberg and pillow-lavas rather than into (normal) flows of lava. The ridges
owe their shape to linear magma vents and the plateaux to more central ones.
The flows of lava covering the flat tops of some of the móberg mountains
are interpreted as having been produced subaerially when the mountains had
grown sufficiently in height to reach above the surface of the melt-water.
This view was set forth as a hypotliesis on the origin of the morphology in
the móberg area of S. W. Iceland by the author (Kjartansson 1943). After-
wards, much the same origin was suggested by W. H. Mathews (1947, 1952)
for several flat-topped mountains („tuyas") in British Columbia. Then this
view — in full agreement with my hypothesis as outlined above — was ex-
pressed by Bemmelen and Rutten (1955) for the mountains of the móberg
area of North Iceland. All these authors seem to have come to their conclu-
sions independently of each other and of my hyothesis which had been published
in Icelandic ancl is not referred to in their papers.
The accumulation theory is applicable also to the móberg area of Middle
S. Iceland, or at least to the northeastern half of it (the Tungnáröræfi)
including Langisjór, as well as to the two other móberg areas of Iceland.
However, the Middle South Area differs from the two others in the foll-
owing respects:
(1) All tlie mountains are of the ridge type, none being tablemountains.
This sliows that volcanic activity was exclusively linear in this area throughout
the time of its formation, which took place during the last glaciation. In fact,
the activity has maintained this character also in postglacial time.
(2) None of the mountains are capped with lava sheets, and normal flows
of lava seem not to occur in the móberg formation of this area (between
Skaftá ancl Tungná). This indicates such a heavy glaciation of this area as