Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 43

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topmost normal group. On the west side of Lambamanna-
sandfell is a deep valley; at its southern end is seen a normal
porphyritic lava at the surface (640 m), overlying a 10—
12 m thick sediment whose floor is not seen. This is a rather
soft sandstone with pronounced cross-bedding, and is most
likely of eolian origin. At the mouth of the gully are seen
two normal lavas (base 610 m) separated by scoriae.
Normal lavas are met with at intervals for 3 km farther
north, and end at a scarp (560 m) where the terrain falls
hy 15—20 m. The base of the normal group thus lies some
40 m lower here than south of Birnuhöfði by a distance of
4 km, while further 3 km east, in öldumóðuflá the limit is
practically the same as near Birnuhöfði. The floor of the
normal group seems at any rate to have been very flat and
the lower part of this group, at any rate, is a typical plateau.
As a whole this group has very probably extended farther
north than now. We have now just an erosion remnant, one
step in the low erosion staircase of the Young Plateau rocks.
Stórisandur represents one step, rising only 100 m on the
next 10 km southwards from Lambamannasandfell. Farther
south, for the next 15 km there is a rise of level of the mostly
fragmental material until it merges at about 1000 m with the
socle of Langjökull. The surface of these masses is often re-
markably even under a final plateau of lavas of a shieldvol-
cano type. The structure of the extensive layer of fragmental
masses, reaching from a height of about 650 m to some 1000 m,
is here mostly very difficrdt to trace because of bad outcrops.
But what one sees are mostly flatlying layers and the surface
is typically flat. This might suggest that this layer could be
understood as an erosional remnant of the thick topmost layer
of an extensive plateau, this layer being only gradually dif-
ferent in structure from the lower Young rocks.
However, in Southern Iceland we shall see that this thick
layer of fragmental rocks is much younger than the Young
Plateau basalts.
As to the origin of the Young rocks we have on the whole