Náttúrufræðingurinn - 1964, Qupperneq 43

NÁTTÚRUFRÆÐINGU RI N N
37
S U M M A R Y
The Retreat of the Last Glacial Ice-sheet and Some Voleanoes in the
Kjölur Area, Central Iceland
by Gudmundur Kjarlansson.
Museum oj Natural History, Department of Geology, Reykjavik.
The direction of glacial striæ in the Central Highlands of Iceland (cf. Figg.
1 and 2) show clearly that the main ice-divide between the Nortli and the
South of the country ran south of the present topographic water-divide dur-
ing the thinning and retreat of tlie last (Wurrn) ice-sheet.
At tlie time of maximum thickness or at an early stageofthe thinningof the ice-
sheet the ice-divide lay to the north of Mt. Bláfell whose northern shoulder (1160
m. a. s. 1.) became ice-scoured in the direction SW, wliile its summit (1204 m. a. s.
1.) bears no signs of glaciation. This suggests a thickness of about 850 m. for
the ice-sheet when it was at its tliickest on the highland area to the southeast of
Mt. Bláfell. At a later stage the ice-divide moved farther south owing to a more
rapid deglaciation in the North than in the South of the country. The consequ-
ent diversion in the flow of the ice is manifested by a younger set of striæ in a
north ancl northwesterly direction. These striæ are not found on Mt. Bláfell as
the ice-sheet had then beconte too thin to reach above it. But on low ground
the younger set of striæ is found to cross the remainder of the older set at al-
most right angles (Fig. 3).
As thc ice-margin, rctreating soutliwards, passed the col of the Kjölur mountain
pass a lake was dannned up on the south side of thc col. A distinct wave-cut
shore-line on the hills bordering Kjölur on the west side marks the lakc level
at about 630 m. a. s. 1. As this figure is just a probable estimate of the height
of the col at the time of this lake, the latter may be assumed to liave had an
outlet towards the north across the col. Now the col is higher (about 650 m.
a. s. 1.) as in postglacial time it was covered with thick lava from the shield-
volcano Kjalhraun.
Farther south, on the slope of Leggjabrjótur, There is a series of lower shore-
lines representing successive stages in the lowering of the ice-marginal lake level
(Fig. 4) due to the thinning of the ice-sheet south of Kjölur. These lower shore-
lines are less distinct, indicating a variable height ot the damming ice.
A further proof, besides striæ and ice-dammed lakes, for the northward
movement of the last ice-sheet in the Kjölur area is provided by easily recogni-
zable erratic blocks originating in the rhyolite peaks of Kerlitagarfjöll, south
of Kjölur, and found scattered over a sector running far north of tlie present
water-divide.
Two types of volcanoes, shieldvolcanoes and tablemountains, dominate the
landscape of Kjölur. The first category is represented by the postglacial Kjal-
hraun and the ice-scoured, interglacial Baldheidi and Skálpanes. But the most
prominent tablemountains are Hrútfell, Kjalfell, Blágnípa. Skridufell and Blá-