Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 28

26
are able to conclude that they did in reality form during a
severe spell of climate at the end of the first Pleistocene glacia-
tion of Iceland.
We shall now proceed to gradually older rock groups in the
country.
Older than the Lyngdalsheiði are the blocks, uplifted to form
a plateau north of the Laugarvatn area, i.e. north of Gríms-
nes and Lyngdalsheiði. The plateau is crowned by horizontal
basalt lavas, forming a sheet that must originally have con-
sisted of at least 10-20 lavas. This sheet rests on the plain
surface of a 500-600 m thick tuffbreccia, which from top to
bottom is formed of 20—30° dipping foreset beds, particularly
easy to see in the east side of Efstadalsfjall (16). A 400 m
deep mature valley, Mjóidalur, in which the river Skillandsá
flows to the south, has been eroded into the plateau in the west
side of Miðdalsfjall (which is crowned by such erosional lava-
remnants as Gullkista (681 m above sea-level), and Miðdals-
skersli (768 m). The Mjóidalur is shown in the air photograph
Fig. 5, with the typical Table Mountain Skriða resting centrally
across the prolongation of the valley course. The meaning of
this, and the important general inference, will be discussed
later in this chapter. Here, we just point out our discussion of
the tuffbreccia valley-volcanoes in (15).
The structure of the breccias in the plateau under considera-
tion demands a water depth of up to 500 m or 600 m, for the
progression of the foreset beds to be possible. In Efstadals-
fjall and the separate Bjarnarfell near Geysir, it is obvious
that the eruptions started by a water depth indicated very
clearly by signs of a water level (worn gravel) at a present
height of 285 m and 300 m respectively (16, Fig. 27 and 26).
Without any long break the eruptions continued by a depth
corresponding to the higher signs of the water surface as al-
ready mentioned, i.e. the level where the extensive lava sheet
rests on the flat surface at the top of the foreset beds. The two
different levels seem to indicate subsidence during the formation
of these rocks rather than a rise of water level per se. The in-
ference of a final uplift of the plateau by at least 500—600 m