Jökull - 01.11.1998, Blaðsíða 22

formed in lodgement tills (Figure 3). The moraines
are up to 2 km in length and up to 20 m high and in
most cases made of large clasts lacking finer matrix.
No ice was detected in the moraines, although in
some cases its existence is not excluded. These
moraine ridges lie close together. Their interval are in
most cases few meters.
Three types of moraines can be detected in front
of the Brækur outlet glacier. First, the glacier ad-
vanced down a highly inclined surface of a fan which
resulted in a formation of two push moraines of the
same age. The moraines have an U-shaped form, seen
from above, are 25 m high and consist of sand sup-
ported cobbles. Supraglacially transported boulders
generally over 2 m in diameter are found on the sur-
face of the moraines. They are unstable and therefore
not favourable for lichenometry. The lowest part of
the east moraine is at 550 m a.s.l. and the west
moraine is 570 m a.s.l. Closer to the glacier, along the
eastern snout, there are two ice-cored lateral mo-
raines, at least 5 m high. Similar moraines as in the
Stallurinn proglacial zone are situated in front of the
western Brækur outlet glacier. These moraines are 5-
10 m high and 40-350 m long built by similar materi-
al as the moraines in the Stallurinn proglacial area.
A different type of glacial landscape can be found
in front of the Klofajökull outlet glacier. The glacier
has constructed an extensive landform in the pro-
glacier area. The northern part of the formation is 80
m high and roughly 1 km wide in N-S direction and
2.8 km in an E-W directions. The southem part, near
the glacier snout, is 120 m high. The formation is
tongue or lobate shaped covering about 2 km2 in front
of the outlet (Figure 4). The total volume of the mate-
rial, including superimposed ice remnants, is about
0.57 km1. It is poorly sorted, ranging from silt to boul-
ders, up to 6 m in diameter. Ephemeral sediment load-
ed lakes are located on top of the formation near the
ice-debris boundary. Three distinctive types of sur-
face relief can be observed on the landform; transver-
sal ridges, longitudinal ridges and the kettle and kame
topography which is the most dominant surface relief
(Figure 4). Vertically, the moraine is composed of two
distinct units (Figure 5) a matrix supported pebbles
unit with boulders incorporated underlying a sand
supported pebbles layer with large boulders embod-
ied. Large boulders are resting on top of the upper
unit, varying in size from 2-4 m in diameter.
Moraines of different morphology also occur in
front of the Ögmundarjökull glacier. A 35-40 m high
end-moraine forms the maximum LIA limit. These
moraines have an alluvial fan as a substrate. Similar
end-moraines as in Stallurinn and west-Brækur pro-
glacier areas are located north of the glacier margin.
Their height is not known but they are between 100-
500 m long.
Dating evidence and glacier fluctuation
The Klofajökull outlet glacier reflects the greatest
recent extension of the ice cap (Figure 6). Neither
tephrochronology nor lichenometry could be used to
date this advance but geomorphological evidence in-
dicate that the initial formation occurred before the
Norse settlement of Iceland (ca. 900 AD). The early
historical Hallmundarhraun lava (Jóhannesson, 1989)
apparently flowed around the outermost extension of
the Klofajökull deposition, showing that the landform
predates the lava. The implication is that the feature
post-dates the last Termination and predates the Norse
settlement. The precise date of the Klofajökull forma-
tion can not be inferred, however, it is likely to reflect
repeated advances of the Klofajökull glacier during
the Holocene. According to lichenometry, the glacier
re-advanced during the LIA and reached the outer-
most limit around 1885+9 (Table I). The glacier then
began a continuous retreat that lasted until about
1934+8. No lichens grow on younger substrates due
Table I. Thallus diameters of Rhizocarpon geographicum
and calculated dates for the Brækur, Ögmundarjökull and
Klofajökull outlet glaciers. In each case, the largest inscribed
circle of the thalli was measured.
- Aldursgreiningar með fléttum á jökulgörðum.
Site x5* (i mm) Dates
Brækur outlet . 21.8 , 1881+9
Brækur outlet . 22.2 , 1879+9
Brækur outlet . 22.0 1880+9
Klofajökull 1 . 21.2 . 1885+9
Klofajökull 2 . 14.6 1912+3
Klofajökull 3 ... 9.4 1934+8
Ögmundarjökull outlet. .. 10.0 1923+2
* Mean of the five largest lichens on each site.
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JOKULL, No. 46, 1998