Náttúrufræðingurinn - 1973, Blaðsíða 72

60
N ÁT T Ú RU F RÆÐINGURINN
Þórarinsson, Sigurður. 1951. Laxárgljúfur and Laxárhraun. A tephrochrono-
logical study. Geogr. Annaler, 33, 1—89.
— 1959. Some geological problems involved in the hydroelectric development
o£ the Jökulsá á Fjöllum. A report to the State Electricity Authority,
Reykjavík. 1—35. FjölritaS.
— 1960. Der Jökulsá Canyon und Ásbyrgi. Peterm. Geogr. Mitteilungen. 104,
23, 154-162.
— 1971. Aldur ljósu gjóskulaganna úr Heklu samkvæmt leiðréttu geislakols-
tímatali. Náttúrufr., 41, 99—105.
Walker, G. P. L. 1971. Compound and simple lava flows. Bull. volcanologique,
35, 3, 579-590.
S U M M A R Y
Grooving on lava surfaces at Asbyrgi NE-Iceland
by
Kristján Sœmundsson,
National Encrgy Authority, Laugavegur 116, Reykjavik.
The origin of grooves and striations on the lava surfaces around tlie stecp
Ásbyrgi canyon is discussed. A single compound lava originating at the Stóra-
Víti crater 20 km distant, makes up the walls of the canyon which are up to
80 m high. This lava flow was probably erupted early in the Younger Dryas
period. Later in this period a glacier advanced on to the margin of the lava
flow south of Ásbyrgi. Its greatest extension is marked by lateral and terminal
moraines and kame terraces. This glacier halted 4,5 km south of Ásbyrgi. The
grooving and striations around Ásbyrgi are attributed to transport of rock
debris by catastrophic íloods which have been shown to have also carved out
the canyons of Ásbyrgi and Jökulsá. Tlie author thinks it possible that two
major catastrophic íloods have contributed to the erosion of the canyons. The
first one occurring in earliest Postglacial Time carved out wide and short
canyons. The latter occurring less than 2900 years ago extended the canyons
upstream, the extensions being narrower and longer than the canyon eroded
by the first flood. Ásbyrgi itself, however, shows only the beginning of this
development because the water of the second catastrophic llood soon took a
more easterly course to erode a parallel canyon which liolds the river today.
Preservation of soil older than 7100 years on the edges of the Ásbyrgi canyon
is thought to be incompatible with the idea that the canyons were formed in
single event less than 2900 years ago.