This volume includes nine fully reviewed papers presented in the fifth session of the VI International Conference onf Geomorphology in Zaragoza, Spain. Five of the papers are concerned with rock glaciers, landforms that consist of valley-filling masses of angular rock blocks, the exact origin of which is still subject of debate (Rock glaciers in N-Iceland, Lithology and distribution, shallow active layer temperatures and DC resistivity of rock glaciers, monitoring rock glacier dynamics by geomatic methods).

Other papers deal with snow cover dynamics of Mediterranean mountains, soil toposequences, block fields beneath pleistocene ice fields and advances of numerical dating of Quaternary glaciations in China.

Rate and intensity of rock weathering differ between rock types and are governed by a number of physical and chemical parameters, cumulatively referred to as "rock hardness“.
These parameters are the key determinants of relief and slope formation.
The nine papers compiled in this thematic volume contribute to the quantification of these key physical and chemical parameters, both by evaluating the results of long-term field and laboratory studies in various regions. The authors have quantified the relvant parameters and use them for assessing the impact of these variables on rock weathering under a range of climatic conditions.

Die physikalischen und chemischen Reaktionen von Gesteinen auf die die Verwitterung bestimmenden Parameter sind je nach Gesteinstyp unterschiedlich und bestimmen über die als Gesteinshärte bezeichnete Größe den zeitlichen Ablauf und die Intensität der Verwitterung und die Hangentwicklung.
In 9 Beiträgen aus unterschiedlichen Regionen und Klimaten werden zum Teil im Gelände, zum Teil in langjährigen Labor- und Feldexperimenten die auf die unterschiedlichen Gesteinstypen einwirkenden physikalischen und chemischen Parameter erfasst und die Auswirkungen quantifiziert.

Rock strength, weathering and slope evolution are important and traditional fields of investigation in Geomorphology, Earth and Environmental Sciences all over the world. In addition, these topics are crucial for applied questions, for example the construction of buildings or roads. Against this background, the volume presents nine case studies from various regions with different climatic, geomorphological and petrographic conditions. Unfortunately, the issue has no commentarial introduction or recapitulatory conclusion.

The first contribution of Beylich et al. (pages 1–26) illustrates the results of a 1.5 year weathering experiment under wet/moist conditions using non-calcareous rock types collected in artic and subarctic areas of Scandinavia and Iceland. Electric conductivity, ion composition and pH values of the water along with the resulting amount of debris were determined. Although a relatively low sample number was investigated (n = 24), the results indicate that the importance of chemical weathering is more related to rock types than to environmental conditions, that there is evidence suggesting quantitative dominance of chemical over physical weathering in the Freeze room series, and that the rates of chemical and physical weathering as well as their mutual importance is more complex than traditionally believed.

Matsukura et al. (p. 27–40) present results of ten year measurements of weathering rates of rock tablets on a forest hillslope in the Abukuma Mountains, a humid temperate region in central Japan. The field experiments were performed using the microweight loss technique. Main findings are that limestone has more distinctive weathering characteristics compared to other rocks, that limestone and granodiorite in a saturated environment are highly succeptible have high susceptibility to chemical weathering, and that physical weathering is dominant on the ground and in unsaturated soil layers.

Mignon & Placek (p. 41–55) used mainly geological mapping and tectonic measurements to investigate rock control and geomorphology of a small rocky sandstone scarp in the Middle Sudets Mountains, SW Poland. They conclude that the influence of fracture density of the ground plan of the cliff lines is only partial, although the very wide spacing of the joints appears to account for the existence of continuous cliff lines in otherwise relatively weak rock.

Millar (p. 57–67) reconstructs the Late-Pleistocene slope evolution based on the characteristics of exposed massive bedrock cliffs that often exhibit a warren-like network of joints and crevices as common features in resistant conglomerates and sandstones of the Allegheny Plateau in Western New York, USA. Measurements of rock hardness using a Schmidt hammer and a digital elevation model were collected to test the hypothesis that slow, downslope movement under periglacial conditions was responsible for the presence and distribution of detached blocks. The spatial distribution of hardness values indicates two distinct periods of downslope transfer and suggests that the timing of block movement is associated with slope instability following maximum glacial conditions and does not correlate with the most intense periglacial activity.

Engel (p. 69–80) also used the Schmidt hammer method and combined it with existing 10Be exposure-age dates to measure and to calculate the age assignment of intact granite rock strength in the Krkonoše Mountains, Czech Republic. A non-linear correlation was found, which can be used to calibrate Schmidt hammer R-values and permits wider dating of surfaces in the studied area.

Mottershead et al. (p. 81–102) investigated the bedrock slope evolution in a saltrock terrain of Cardona, province Cataluña, Spain. As saltrock is highly soluble, it shows relatively fast changes. By the use of erosion pins on both solid outcrop and rockfall debris, the authors conclude that the erosion of the salt terrain is very closely related to rainfall events and calculate surface lowering rates of up to 10 m per century. In addition, they find a clear relationship between slope angle and erosion rate.

Grab (p. 103–113) researched near-surface rockwall temperatures in the high Drakensberg basalt, South Africa, using high frequency monitoring during the winter of 2001 and the summer of 2002. He deduces spatio-temporal differences and possible implications for weathering and proposes that the pronounced spatial variation in rock surface weathering expressions are a product of the corresponding weathering processes, which to a large part are controlled by differences in rock thermal characteristics.

Aoki & Matsukura (p. 115–132) present another study from Japan. They describe the effects of rock strength and location heights on growth rates of tafoni-like depressions on sandstone blocks of a masonry bridge pier in the coastal spray zone, located between Aoshima Island and Kyushu Main Island. Depths of depressions were measured in 1971, 1989 and 2001, corresponding to 20, 38 and 50 years after the construction of the bridge, and were compared with rock hardness using an Equotip hardness tester. The data show that the depressions were shallower in the blocks having larger strength.

The last contribution of the volume again deals with rock surface temperatures. Sumner et al. (p. 133–147) measured them on surfaces with different aspects using thermo sensors over a three year period on a granite-gneiss boulder on top of an inselberg in the southern Namib Desert, Namibia. Regarding the implications for thermally-driven weathering, they conclude that rapid changes, as measured over minute-durations, show that thermal shock is probably limited to cooling during precipitation events but that more data are required to elucidate the relative contribution of weathering processes.

In sum, the contributions of this issue show that investigations concerning rock strength, weathering and slope evolution are important for both applied fields and scientific research in geomorphology. Meanwhile, a lot of techniques and methods are available. However, besides individual case studies, more interdisciplinary, comprehensive, long-term and comparative research efforts and the development of integrated models are necessary.

Olaf Bubenzer, Heidelberg

Zeitschrift für Geomorphologie N.F. 53/2 (2009)