Jökull - 01.01.2010, Page 190

M. Da̧bski and P. Angiel
acceptable for assessing ages of glacier landforms
developed over the last century (Da̧bski, 2007),
especially since measured lichen diameters were not
larger than 55 mm (Bradwell and Armstrong, 2007).
In order to obtain further information about the
minimum age of the moraines, annual rings of Salix
arctica at the southeastern slope of Skálabjörg were
counted (Table 2). Each lateral moraine was searched
for the thickest specimen. Cross-sections were done
at the thickest part of the main trunk of each tree.
Table 2. Number of tree rings of Salix arctica growing
on the southeastern slope of Skálabjörg. – Fjöldi trjá-
hringja á víðirunnum í suðausturhlíðum Skálabjarga.
site alt. m a.s.l. no. of tree rings beg. growth (AD)
697 33 1975
681 23 1985
670 21 1987
655 24 1984
636 19 1989
610 12 1996
Detailed mapping of periglacial landforms was
performed on the top surface of the southern part
of Skálabjörg (Figure 6). Excavations were done
in selected places to check ground structure, depth
of frost sorting and to collect samples for granu-
lometric analyses. Measurements of orientation of
stones (their largest cross-sectional planes) was per-
formed in chosen sorted stripes and solifluction lobes.
Rounding of superficial stones was assessed in several
places in order to obtain information about origin of
superficial material (Reichelt, 1961).
GLACIAL MODIFICATION OF
SKÁLABJÖRG
The most prominent glacial depositional landform in
the southern part of Skálabjörg is a moraine ridge
(0,5–2 m high) clearly visible in several places, at an
altitude between 690 and 742 m a.s.l. on the eastern
side of the nunatak and c. 700 m a.s.l. on the south-
western side (Figures 2C, 2I and 6). It is currently 85
to 98 m above the glacier margin. Lichenometric dat-
ing (Table 1) shows that initial exposure of this ridge
occurred not earlier than AD 1896 and not later than
AD 1944 for the eastern part of the ridge. For the
southwestern part of the ridge, the initial exposure
occurred between AD 1915 and 1954. The earliest
date (AD 1896) corresponds well with the beginn-
ing of the glacier marginal retreat in AD 1894 (Evans
and Twigg 2002). The latest dates (AD 1944 and AD
1954) cannot be accepted because they contradict car-
tographic documentation (a map shows the situation
in AD 1937–1938) and a photograph from AD 1951
(published in Jökull 40). At the turn of 1940s and
1950s the glacier margin must have been lower.
However, we can infer that the surface of the
glacier at Esjufjöll could be at its maximum position
inherited from the Little Ice Age (LIA) for a longer
time than the front margin of the glacier, maintaining
maximum position during the first decades of the 20th
century. This is in accordance with Gudmundsson
(1997) who, after analysing numerous works, reached
the conclusion that the LIA maxima in southern Ice-
land ended in the 1930s. The glacier margin has been
receding since AD 1932 (Evans and Twigg, 2002).
Our data show a maximum age of AD 1896 for the
highest lateral moraine ridge at eastern Skálabjörg and
AD 1915 for southwestern Skálabjörg, whereas the
minimum age is AD 1930 for both places. Therefore,
the rate of glacier surface lowering since the end of
LIA has been between 0,8 and 1,3 m/yr on average
(Table 3).
Measuring of annual tree rings of Salix arctica
proved to be of a very limited use in determining the
glacier lowering rate (Table 2). It revealed that the
oldest specimen found on the highest moraine started
to grow in AD 1975, suggesting at least 45-year-long
colonisation lag time (Figure 7). However, age of the
specimen found just 9 m above the glacier surface
appeared to be 12 years old. Therefore, the colon-
isation lag time of Salix arctica may have significantly
shortened since the beginning of the 20th century due
to climate warming.
Bedrock surfaces, exposed during 20th century
deglaciation, in places covered by a veneer of glacial
sediments, are inclined 17◦ to 32◦ on the eastern
slopes and 15◦–25◦ on the southwestern slopes of the
nunatak (Figure 7). In both cases the inclination stea-
190 JÖKULL No. 60