Jökull - 01.12.1975, Blaðsíða 28

whether paleontology and biology might pro-
vide reasonably accurate clocks, each spanning
a certain time interval between very carefully
measured radiometric fixpoints. Doubts on that
point have certainly been expressed.
We shall here only consider geomorphology
as a time-keeper. Geomorphology is concerned
with two main questions: 1) how were the land-
forms modelled?, ancl 2) what is the paleonto-
logical age of the strata involved and, hence
also, what is the absolute age of the forms?
The last question was much pursued until
the advent of radioactivity. But for the Quat-
ernary, Upper Tertiary, and even occasionally
for the Lower Tertiary it seems important to
obtain balanced ideas of time intervals along
this line.
The present author has devoted much work
to the study of the development of Icelandic
topography (his main studies are quoted in
Einarsson, 1971), and acquainted himself with
such work in some other countries, to be able
to arrange morphological stages into a correct
time order and eventually to get a reasonable
estimate of the duration of these stages. This
work led him to assume an age of at least 15—
20 My for the oldest erosional forms in Iceland.
These forms cut the Fljótsdalsheidi area (Ein-
arsson, 1971) and the Eastern Fjords, mention-
ed earlier, the incoherent radiometric ages of
which range from 1.5 My to 12 My.
As an orientation for readers not aquainted
with geomorphological results which are here
of interest for comparison, we shall point out
the following.
For Scotland, Wales, and the middle to north-
ern England, the earliest known Tertiary drain-
age was predominantly to the east, the remn-
ants of upper ends of very smooth valleys of
this stage now being found in high terrain
along the west coast of the island, where now
the drainage is to the west. Just the same fact
is found in western Scandinavia, all along Nor-
way and the adjoining parts of Sweden. And
just such smooth valley forms are the earliest
ones in Iceland, following upon extensive pene-
planation and consequtive uplift of the Terti-
ary basalts.
The facts in Britain and Scandinavia suggest
one or more Tertiary phases of uplift, increas-
ing westwards, and a low precipitation that
26 JÖKULL 25. ÁR
allowed the development of smooth forms. The
earliest smooth valleys in Iceland also suggest
low precipitation.
At later times, westward coastal drainage in
Norway and Britain led to rough forms, most
likely due to much increased precipitation in
coastal ranges. Quite corresponding signs of
much increased precipitation is evidenced by
the second generation of the Icelandic valleys.
For Britain and Scandinavia it has been as-
sumed for a long time that the oldest eastward
drainage was initiated by a tectonic event that
was taken to correspottd to the Middle Miocene
tectonic phase in the Alps. This has not been
provecl paleontologically, nor could it be based
on a solid tectonic theory. But we shall indicate
a different comparison, with the Alps.
An extremely detailed. monumental tectono-
morphological work, with ample paleontological
datings (Winkler-Hermaden, 1957), has led to
the result that smooth Upper Miocene erosional
surfaces are still clearly seen at elevations of up
to about 2800 m in the eastern Alps. The strata
are mainly sediments, but where basalts occur,
down in the Pannonic basin, they remained
largely unaffected by erosion for long parts of
the Plio-Pleistocene.
In British geomornhology, it has long been
an accepted method for reconstructing old pene-
plains on high terrain (e. g. the Exmoore), to
use the peaks of volcanic plugs, standing out
of the much eroded sediments, as essentially fix
points. This method leads convincingly to old
smooth surfaces of denudation.
What might then be a reasonable time for
the development of the smooth erosional sur-
face that was cut into tilted plateau basalts in
Iceland? Add to this the time for the excava-
tion into this surface of a generation of smooth
valleys, graded to a base level now having been
uplifted to about 300 m above sea-level (lst
valley generation). Add further a new erosional
phase deepening the older valleys towards their
head, by unchanged base level, but being char-
acterized by the V-shape cross section (2nd
generation). This new generation was most like-
ly due to increase in precipitation as already
stated, but it is quite certain that glaciers had
no part in this development. It was only after
this development, that glacial erosion set in,
giving the large valleys in the system the U-