Náttúrufræðingurinn - 1959, Síða 22

182
NÁTTÚRUFRÆÐINGURINN
SUMMARY
The Pleistoccnc volcanism in the north of Skagafjörður and the paleomagnetic dating
of topograpliy in Middle Northern Iceland.
by Trausti Einarsson
University oj Iceland, Department of Engineering.
In a previous paper (3) I showed that in Middle Northern Iceland there are
two generations of topograpliic forms. The one, of Tertiary age, is represented
by broad valleys and plains graded to a base lcvel about 300 m above present sea
level. The other generation is represented by narrower valleys formed at a lover
base level and tliis generation was assumed to represent the Pleistocene. The
amount 'of erosion during the two cyclcs stands in a ratio of about ten to one.
In tlic' present paper it is pointed otit that the fall of base level from the
300 m mark took place in two main steps, there being a long stillstand at 80-100
m above sea level. During this stillstand there was formed a general strandplanc,
at least 2—3 km wide, and all largcr valleys developed a graded floor at this
level. Tlie work done in deepening from the 300 m level to the 100 m level is
something like 5 times that wliich was performed later.
In the outer part of Skagafjörður the strandplane can bc dated with reason-
able certainty. It was first covered witli grey sandstone and conglomerates, poss-
ibly in part of glacial origin, then witli a general layer of brown, probably wind-
blown sandstone, and finally with basaltic volcanic rocks, as seen in Skagi, Málm-
ey, Þórðarhöfði, and Hrolleifshöfði. In Skagi there are 3 lava flows with reverse
magnetic polarity (R. M.) aiul a fourtli with normal polarity (N. M.) (Tjarnar-
fjall). A lowland glaciation seems to have separated two of the reverse lavas. Tn
Málmey there are 3 or 4 rcvcrsc lavas, while in Þórðarhiifði and Hrolleifshöfði
the rocks are of normal polarity.
The last known magnetic transition from reverse to normal polarity is of pre-
Sicilian age (4). This means probably pre-Giinz age; at any rate the transition
lies near the beginning of the „classic" Pleistocene (of Penck and Briickner). We
may therefore assume with confidence that the strandplane was fully developed
in pre-Gunz time. On thc otlier hand its formation and the deepening of thc
valleys froin tlie 300 to the 100 m level may still bc Plcistocene if liereby wc
mean the „extendcd" Pleistocene defined by the 1948 London Conferencc.
Tf the lengtli of thc classic Pleistocene is y2 million years then the wliolc de-
velopment of thc younger valley generation seems to demand 2 or 3 million years,
hardly much less. By this standard the older generation of valleys seems to ex-
tend at least into the Lower Pliocene.
This conclusion leads to a revision of the Tjörnes area (Cf. ref. 5). Recently
it has been pointed out (G) that Neplunea despecta, of Pacific origin, escapcd
through the Bering strait into North Atlantic waters only at the beginning of
the (extended) Pleistocene. This species, according to Bárðarson (7) appears
suddenly in the Cardium groenlandicum zone together with some other northern