Jökull - 01.01.2013, Blaðsíða 57

Surface deterioration of glacially abraded basaltic boulders by Fláajökull
scales, ranging from 1 mm to 40 m, is lower along
profiles parallel to the direction of glacial flow when
compared to that measured perpendicular to ice flow.
They also proved that postglacial weathering homog-
enizes bedrock roughness. None of the above stud-
ies employed electronic micro-roughness profilome-
ters such as Handysurf which works with a vertical
resolution of 0.01 µm, allowing the very first stages
of weathering to be detected. Initial results of the use
of Handysurf E 35-B on Fláajökull moraines and in a
limestone foreland of Biferten glacier in Switzerland
are presented in a recent papers by Da̧bski (2012a,b),
showing that the electronic profilometer can be used
to detect initial stages of weathering micro-relief de-
velopment on basaltic and limestone boulders. This
issue is discussed further below.
Weathering rinds have been used to date glacial
landforms at least several thousand years old (Carroll,
1974; Porter, 1975; Chinn, 1981; Nicholson, 2009).
There are very few studies on weathering rinds devel-
oped over much younger time-scales. A rare example
of such a study is the work of Etienne (2002) who
noticed gradual development of weathering rinds due
to iron oxidation, action of micro-organisms and me-
chanical fracturing on basaltic boulders deposited by
Sólheimajökull (S Iceland) since the LIA maximum.
On the oldest moraines, deposited at the turn of the
19th and 20th centuries, according to Etienne (2002),
the rind thickness is 1.3 mm and this value gradu-
ally decreases in the direction of the glacier snout.
He also noticed that gradual development of weath-
ering rind is accompanied by an increase in micro-
roughness, but did not measure this. According to
Etienne’s (2002) findings, the rinds start to undergo
flaking (exfoliation) after c. 150 years of weathering,
limiting further growth of the rinds.
Schmidt hammer techniques in glacial geomor-
phology proved to be useful in distinguishing between
landforms created during the LIA and those devel-
oped over longer timescales (Matthew and Shakesby,
1984; McCarroll, 1989, 1991; McCarroll and Nesje,
1993, 1996; Aa and Sjåstad, 2000; Kotarba et al.,
2002; Winkler, 2005; Shakesby et al., 2006; Owen
et al., 2007; Nicholson, 2009). According to Da̧bski
(2009), rebound values (R-values) can also indicate
different weathering duration of limestone surfaces
abraded by Biferten glacier in Swiss Alps after LIA
maximum. Evans et al. (1999) proved that R-values
reasonably correlate with mean sizes of Rhizocarpon
thalli on basaltic boulders at Heinabergsjökull, which
is a neighboring glacier to Fláajökull.
According to Matthews and Owen (2008), en-
dolithic lichens growing on recently deposited gneiss
boulders by Storbreen glacier in Norway cause rapid
decrease in R-values, but this trend is not visible on
lichen-free surfaces.
Assessments of weathering degree of glacial land-
forms by micro-roughness, weathering rind thickness
and Schmidt hammer R-values indicate that these
methods are robust. However, limited number of stud-
ies performed within glacier forelands created after
the LIA maximum (i.e. short time-scales) call for fur-
ther research.
MARGINAL ZONE OF FLÁAJÖKULL
The marginal zone of the Fláajökull glacier lies at 50–
80 m a.s.l. (Figure 1). The eastern lobe of the glacier
has formed a clear series of arcuate moraines. The
western lobe is separated from the eastern one by a
mega roche moutonnée. Remnants of moraines de-
posited by the western lobe of Fláajökull connect with
the marginal zones of Heinabergsjökull and Skálafell-
sjökull. According to Evans et al., (1999), the glaciers
formed, together with Fláajökull, a single piedmont
lobe in AD 1860–1870. Figure 1 shows the situa-
tion after the opening of a new subglacial tunnel in
the spring of 2001, causing inundation of large parts
of the marginal zone.
Previous lichenometrical dating of the moraines
(Da̧bski 2002, 2007) was carried out along the Hólms-
árgarður profile, located along the Hólmsá river and
used by the Icelandic Glaciological Society for regu-
lar measuring of glacier front fluctuations since 1934
(Sigurðsson, 1998). Lichenometric dates obtained
by Da̧bski (2007), who used the size-frequency ap-
proach elaborated by Bradwell (2004) for SE Iceland,
correlate relatively well with glaciological measure-
ments (Sigurðsson, 1998), an American map based
on aerial photographs taken in 1946 (US Army Map
Service, 1949), historical data published by Ahlmann
JÖKULL No. 63, 2013 57