Náttúrufræðingurinn - 1958, Blaðsíða 33

23
LANDSLAG Á SKAGAFJALLGARÐI
SUMMARY
Development and age of topography in the mountains between Skagafjörður
and Húnavatnssýsla
by Trausti Einarsson
University of lceland, Deparlement of Engineering.
In the area called here Skagafjallgarður two generations of forms are found.
The one is represented by broad valleys and plains graded to a base level about
300 m above present sea level. Lavas of 3 magnetic periods flowed in these valleys.
The second generation is represented by narrower canyons having present sea
level as a base and penetrating into the older forms from the nearest lowland
and cutting through the lavas.
The outer part of Skagi consists entirely of the young lavas (young as com-
jjared with the basalts of which the mountain chain is formed), where they
were studied by I’jeturss. Much later Líndal found that such lavas have a
far wider extension, but his work has remained unpublished. The present
writer has traced the lavas further and studied the sign of their magnetization
with the compass method (3). The lavas of Skagi, of reverse magnetization,
extend to the south between Skagastrandarfjöll and Bjarnarfell. They rest in
the south on a flat floor at the 300 m level, as far as Hallárdalur. A further
patch lies at the same level between Hallárdalur and Norðurárdalur, and a
further one at 380 m in Urðir between Norðurárdalur and Laxárdalur. These
jiarts are the remnants of a valley of the old generation, with the lavas tliat
covered its floor. The valley is continued in the Laxárdalur whose northern
end has been cut down considerably during the second erosion cycle. From
Laxárdalur to Langidalur lead 3 deep passes, Geitaskarð (370 m), Strjúgsskarð
(350 m), practically graded to the 300 m level, and Auðólfsstaðaskarð (about
200 m) somewhat lowered during the second cycle because of capture of
drainage. A fourth graded pass, Litla Vatnsskarð (340 m), goes to the east. A
remnant of young lavas is at about 300 m in the southern end of Laxárdalur.
On the west side of the mountains the picture is similar. At the mouth of
Norðurárdalur there is a flat part, Höskuldsstaðanúpur (337 m) in front of
the slope of Dunufjall. Up to 300 m it consists of older rocks on which rests
a layer of river gravel, covered by a young columnar lava (of normal magne-
tization). Refsborg is probably a similar feature. Sólheimaháls on the SW side
of Langidalur is similar; up to 300 m it consists of older dipping basalts. Here
comes a cover of horizontal river gravel and sand and a young columnar lava
(normal magnetization), a part of which also forms the top of Burfell.
lt is inferred that when the young lavas flowed there was an erosional plain
on the west side of the mountains, at about the present 300 m level, while
broad valleys, graded to the same level, penetrated the mountains and broad
passes liad at many places been cut practically down to the base level. This
older cycle is well recognizable all over the mountain region.