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are certainly not an immediate volcanic layer from an explosion.
On Sleggja these same tuffs are intercalated by lava flows, as
stated before.
Hence, the topmost Hengill tuff series is formed of material,
sometimes transported (by wind) on dry land on which lavas
flowed, most often in shallow sheets of slowly flowing water, such
as may be found today in Iceland in the thawing season, and oc-
casionally there was deposition in a little deeper water, probably
of material blown by wind; today such fine material is transported
by storms from the barren interior of the country. All the factors
of transport and material are quite familiar in present Iceland; but
of course these conditions were not present on the top of moun-
tains such as Hengill or Sleggja.
Concerning Hengill, we finally remark that we have traced the
Hengill Basal Layer right across the north slope, Fig. 10, E, for
more than 2 km, without finding any gap in it. This stretch crosses
the continuation of the Recent active axial zone. This lack of a gap
demonstrates the non-existence here of such a process as “spread-
ing” during an interval covering many himdred thousand years.
The deposition of tillite on the fault slope of the Hengill Basal
Layer where it disappears at the SW-end of the Nesjavellir de-
pressions, and the plain of denudation cutting the series resting
on top of the Basal Layer in Hengill itself, shows that such a time
span must be taken into account. More data on erosional intervals
are given below. But, finally, a case of reverse magnetization sup-
ports a high age of these rocks, as shown below. By the figure for
“spreading” in the Median Active Zone in Iceland, circulating in
the literature i.e. 2 cm/yr, a gap of 2 km should have been formed
in the Hengill Basal Layer during the last 100,000 years, i.e. the
whole length we have traced in it.
We now return to the lower country north of Hengill and shall
point out the main features with reference to Figs. 10-12, and the
published maps.
The unporphyritic Háhryggur series, with its capping lavas, can
be traced in lower and lower tectonic strips east to a gully 800 m
NE of the farm Nesjavellir, where it is overlain by a stríp of rather
young plagioclase-porphyritic sub-aquatic volcanic breccia, Fig. 10,
C and D. In the field this breccia is very similar to the one on the