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distinct floors of morphologically graded valleys (11). For the pre-
sent purpose it is sufficient to say that we find valleys which were
graded to base levels now found at elevations of 250-300 m, and
50-100 m, and finally such graded to present sea-level. In the pre-
sent area volcanoes were formed in the axial line of such valleys
at these successive morphological stages; hence the time succession
of these eruptions can be seen.
The first point to be made is that there is definitely no relation-
ship between the age of the volcanoes and their distance from the
Thingvellir axis, cf. Fig. 4. We shall now eniunerate the volcanoes
in order of decreasing age.
1, Sú (Fig. 4 and Fig. 1) is the remnant of the large Súlm- (short
for Botnssúlur) volcanic centre. Of the present group of volcanoes,
it lies closest to the Thingvellir axis, and is undoubtedly the oldest
one, for it is quite clearly older than the neighbouring graded
valleys in the plateau basalts. In fact, no erosional unconformity
is found between a thick layer of pillows forming the lower half
Fig. 4: Remnants of Pleistocene volcanoes, formed at different graded erosional
stages of the Basalt area west of the axis of the Median Zone. Projections on a
line at right angle to tlie Zone. A — increasing distance from the axis of the
present Zone. A for IV, M is 20 km. Further explanation in the main text.
of Súlur and the extensive plateau group called N2 (12) in this
area; the main part of the Súlur heap may even belong to the
final stages of the plateau basalts of the area, and paleomagneti-
cally to N2 (2, pp. 41 and 70). The Súlur material, in particular
the higher tuff-breccia and lava members, form an uninterrupted
extensive complex of blocks including Gagnheidi, Ármannsfell, and
Lágafell. In other words, the oldest material in this region after
the formation of the plateau basalts, goes in tectonic steps right
L