Jökull - 01.01.2013, Side 65

Surface deterioration of glacially abraded basaltic boulders by Fláajökull
Figure 8. Weathering rind thickness and Schmidt hammer R-values; dates of moraine deposition at test sites;
diagram includes means, 95% confidence intervals for the mean, medians (white dots), minima and maxima;
horizontal dashed line (upper limit of 95% confidence interval for moraine VII+) serves as a reference line. –
Mælingar á lögun og ummyndun steinda á jökulgörðunum.
Figure 9. Correlation between standardized meadian
values of weathering rind thickness and R-values on
moraines of Fláajökull; values obtained for test sites
are marked with numbered dots; solid line marks the re-
gression line (R2=0.55) for the whole population, while
the dashed line (R2=0.85) refers to the population ex-
cluding test site IV.– Mælingar á þykkt veðrunarkápu
(vinstri) bornar saman við endurvarpsgildi Schmidt
hammer mælinga (hægri). Þykkt veðrunarkápu steina
á jökulgörðunum eykst með aldri á meðan endurvarps-
gildið minnkar.
abrasion, and a very high precision of the electronic
profilometer. Hardly visible surface discontinuities
of microscopic abrasion and percussion marks may
exist, making micro-roughness of rock surface sig-
nificantly diversified. However, overall increase of
Rz, Rzmax and Ra from the youngest to the oldest
moraines is visible (Figure 7, Table 2, Appendix 1).
Comparable results were obtained by Da̧bski
(2012a) for limestone foreland of the Biferten glacier
in the Glarus Alps, Switzerland, where the LIA mor-
aines have been subjected to weathering since mid-
19th century. Glacially-abraded softer limestone has
undergone greater increase in micro-roughness in
comparison with Icelandic basalt, which can be at-
tributed mostly to differences in rock resistance to
glacial abrasion and weathering. This is manifested
by smoother rock surfaces near the snout of the Bifer-
ten glacier, and rougher surfaces on the oldest moraine
(Table 2).
Relative stabilization of weathering rind thick-
ness, R-values and micro-roughness parameters (Ra,
Rz, Rzmax) on older moraines (Figures 7, 8, Ap-
pendix 1) brings to mind results obtained by Matthews
and Owen (2008) working in the young (post-LIA
maximum) marginal zone of Storbreen glacier in Nor-
way. They showed that surface strength of pyroxene-
granulite gneiss boulders measured with a Schmidt
hammer gradually decreases as one moves away from
the glacier snout, but only on a distance from the
youngest moraine to the moraine c. 45 years old, if
JÖKULL No. 63, 2013 65