Náttúrufræðingurinn - 1958, Blaðsíða 34

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NÁTTÚRUFRÆÐINGURINN
Aíter an uplift of the order of 300 m Lhe second cycle began. Within the
mountain area it is represented by canyons which often led to river capture
(Þröngidalur, Gönguskarðsá). Outside tliis area erosion was much greater, pro-
bably because of exposure to the work of main glaciers and glacial rivers. The
broad Skagafjörður valley has become graded to the new level and extensive
lowlands were fornted on the west side of the mountains.
Tlie 3—4 km broad pass Stóra Vatnsskarð is a remnant of the older forms,
and here a large river from the south may have entered the Skagafjörður valley.
Conditions are, however, very much altered trougli the young lavas. We meet
them south of the niiddle line of the pass. The lavas flowed from the south
on both sides of a ridge separating Blöndudalur and northern Svartárdalur.
On the east side the lavas probably flowed into Skagafjörður. Where Svartá
cuts through these lavas in the bend between Hvammur and Skollastaðir they
liave a thickness of 250 m. The olcl Bliindudalur, whose western side was
Svínadalsháls, was nearly filled upp by the lavas that here entered an open
plain (Sólheimaháls). The lavas are left with a thickness of 150 m in Stóra-
dalsháls and there they form 3 alternating magnetic groups, the topmost one
being of reverse polarity. The middle group is thought to correspond to Sól-
heimaháls and possibly to Höskuldsstaðanúpur.
Studies of paleomagnetism during the last few years show an alternation
of periods with respectively normal and reverse polarity of the geomagnetic
field, as recorded by lavas. The last period of reverse polarity occurred at the
base of the I’leistocene (1—7). On this basis at least two of the lava groups in
Stóradalsháls are pre-Pleistocene. It is concluded that the topographic forms
of the older generation were fully developed before the end of tlie Pliocene.
The amount of erosion carried out in the okler cycle is estimatecl in 4 zones
(Fig. 2). The zones are divided into square kilometres, and the lieight of each
taken froni the map (1:100.000). Height of original surface is inferred from
flat mountain tops. The erosion then corresponds to a general lowering by
465 m, which, befor the 300 m uplift, corresponded to 75% of the original
thickness of the plateau. In contrast the average lowering after the uplift is
estimated as 25—50 m or 1 /20 to 1 /10 of the former figure. In a theoretical
consideration of the relative rate of average erosion (Cf. fig. 1) il is concluded
that this ratio also applies approximately to the times involved if climatic
factors were similar. Studies in Tjörnes have led the author to the assumption
that the 300 m uplift, which was practically general for Iceland, occurred just
before the Pleistocene, and the old cycle is thought to have started with more
localized uplifts in various parts of tlie country not long after the Middle-
Pliocene but nevertheless after deposition of the lower part of the Tjörnes
Crag. The two generations of topographic forms are traced in some other
parts of the country. The Borgarfjörður valleys, cut in a 3—400 m plateau,
contain both forms. The old floor of the Skorradalur, about 250 m high,
carries remnants of lavas of both normal and reverse polarity. Both forms are
also found at Hvalfjörður so that the moraines here contained in the basalts
are older than the old valley period. And as the basalts are tilted and were