Náttúrufræðingurinn - 1978, Qupperneq 53

HEIMILDIR
Eliasson, Sigunjin, 1974: Eldsumbrot í
Jökulsárgljúfrum. Náttúrul'ræðingur-
inn, 44: 52—70, Reykjavík.
Sigurðsson, Oddur et al., 1975: Dettifoss-
virkjun, jarðfræðiskýrsla. Orkustofn-
un, Reykjavík.
Sœmundsson, Kristján, 1973: Straumrák-
aðar klappir kringum Asbyrgi. Nátt-
úrufr. 43: 52—60.
Tómasson, Haukur, 1973: Hamfarahlaup
í Tökulsá á Fiöllum. Náttúrufr. 43:
12-34.
Þórarinsson, Sigurður, 1956: On the Varia-
tions of Svínafellsjökull, Skaftafells-
jökull and Kvíárjökull in Öræfi. Jök-
ull, 6: 1-15.
— 1958: The Öræfajökull Eruption of
1362. Náttúrugripasafn Islands,
Reykjavík.
— 1959: Some Geological Problems in-
volved in the Hydroelectric Develop-
ment of Jökulsá á Fjöllum. Raforku-
málaskrifstofan, Reykjavík.
— 1960: On the Geology and Geophysics
of Iceland by J. Askelsson et al. VIII.
Jökulsárgljúfur and Asbyrgi, 69—74.
— 1964: On the Age of the Terminal
Moraines of Brúarjökull and Hálsa-
jökull. Jökull, 14: 67—75.
— 1971: Aldur ljósu gjóskulaganna úr
Heklu samkvæmt leiðrcttu geislakols-
tímatali. Náttúrufr. 41: 99—105.
S U M M A R Y
Some Remarks on Prehistoric
Floods in the Jökulsá á Fjöllum
Glacial River, Northeast-Iceland
by Siguruin Eliasson,
Skinnastadur, 671 Kópasker, Iceland.
The author traced old flood deposits
in several traverses across the Jökulsá
River Canyons, NE-Iceland, in relation
to C14-dated ashlayers (S. Thorarinsson,
1958, 1971) cf. Fig. 4. The main results
are as follows: Striated lavas of probable
Younger Dryas Period age were found in
the Ásheidi uplands, pointing towards a
more northerly advance of the ice-margin
in that period than argued by K. Sæ-
mundsson (1973). A thin layer of boulders
and gravel in the western part of Ásheidi,
dispersed on pahoehoe lava, is thouglit
to be of glacial and glaciofluvial origin
(Fig. 1). Striations arouncl the dry Ás-
byrgi Canyon, attributed by Sæmundsson
to transport of rock debris by catastrophic
floods in earliest postglacial times, are
identified as genuine glacial striae (Fig.
2). Deposits of three tremendous glacial
floods were found in soil profiles around
the Jökulsá River Canyons, dated re-
spectively at 4600 y B.P., 3000 y B.P. and
2000 y B.P. (C14-age, half-value period
5730 years). High, coarse gravel terraces
in the Forvöd Valley and the Svínadalur
Valley were deposited in the flood of
3000 y B.P. All three floods split in two
main forks a short distance north of the
Hljódaklettar Crater Row, one flowing
down to the Ásbyrgi Canyon, the other
down the more easterly Landsgljúfur
Canyon. The flood of 2000 y B.P. eroded
many of the present wide canyons and
united the two older gorges of the Ás-
byrgi Canyon. On the Eyjan, a high
cliffbound island-remnant of the wall be-
tween the older gorges, is a boidderlayer,
situated on top of the H3-ashlayer (C14-
age 2900 y B.P.), Fig. 7 and 8. The prob-
nble peak flow of this flood was at least
200 thousand m3/discharge/s, and was,
perhaps, the biggest of the three floods.
The Jökulsá River, however, heads in the
Vatnajökull ice-cap. The Ásbyrgi Canyon
is mostly eroded in three successive cata-
strophic floods during the past 5000 years,
but has probably never been the actual
channel of the Jökulsá River.
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