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depression. In the eastem part, the Stapafell andesite is the oldest
exposed rock. It produced horizontally stratified sandstone and silt-
stone in a lake whose level was somewhat ahove 160 m. Dykes
indicate a following phase of unporphyritic hasaltic volcanics,
which were carried away before the following development: After
glaciation and deglaciation of the area, the lake level was raised
to ahout 200 m and possibly adjusted to an outflow across an early
Selflatir pass, although an outflow to the south across an area that
much later was uplifted, might also be considered.
The rise of the lake was due either to an ice-dam in the east, or
to a subglacially formed breccia dam, the Dráttarhlíd ridge. The
latter is more likely when we consider all the leveling which took
place later at such a high lake level.
At this stage of the lake, a porphyritic suh-aquatic tuff-breccia
covered nearly the entire zone. The eruptions were probably on
several fissures, and three different centers can he distinguished in
the Sandfell-Mælifell remnant. But no trace of volcanic superstruc-
tures are left from this phase, and there are hardly any dykes to
indicate the volcanic fissures. The hreccia spread during the erup-
tions in the form of forset beds, while later erosion near the lake
level (by surf?) destroyed the possibly formed subaerial lava caps.
Surtsey (2.5 x 1.5 km2) and Skarðsmýrarfjall (2x2 km2) are ex-
amples of the spreading capacity of such masses during the emp-
tion. Three or four eruptive fissures in our 8 km wide zone might
have sufficed to cover it more or less during a geologically very
short volcanic phase, e.g. such ones as the postglacial time has
been divided into in Iceland.
A time of abrasion plains and benches, cut into the porphyritic
rocks at various lake levels, followed. At the same time faulting
and some uplift of the Háhryggur sub-zone took place. It may
have been this uplift which caused the thinning out of the Old
porphyritic breccia where it crosses Köldulaugargil from the east
(Fig. 10, D, K).
Normal valley erosion in the Háhryggur sub-zone took place
(Fig. 10, Ej), possibly during a drop of the lake level.
A second glaciation followed, and after this the lake level again
stood as high as the surface of Háhryggur (probably about 200 m
level at that time), and the Hengill Basal Layer was formed,