Studio sul crioclastismo dell'alta Valle di S. Pellegrino (Dolomiti): indagini sul terreno e sperimentazioni in laboratorio

Abstract

The present paper illustrates a research carried out within the framework of a co-operation programme between the University of Caen (France) and the University of Modena, active in 1985-87 and concerning Geomorphology. The research was made in order io value which factors determined the asymmetrical modelling of the slopes of a Dolomitic valley with an EW orientation (the upper Valle di S. Pellegrino). The study, besides a bibliographical research, involved detailed field observations and laboratory tests; finally field and laboratory data were compared. In particular, the influence of the factors lithology, tectonics and aspect on the evolution of the valley sides was examined. The field observations (detailed gaomorphological survey and observations on fracturation conditions of the rocks of the two slopes) showed, on the one hand, the differential morphology of the valley sides and the asymmetrical distribution of the Quaternary deposits, on the other hand, the constancy of structural conditions (attitude of strata, pattern of fissurations). The bibliographical and field data supplied important indications on the constancy of the factors lithology and tectonics, while, at the same time, emerged the variability of frost shattering, due to the different aspect of the slopes. in the laboratories of the "Centre de Géomorphologie du CNRS" of Caen, determinations of physical and mechanical properties (water content, porosity, capillarity, permeability and microfissuration) of rock samples were carried out; besides experimental frost shattering was done by subjecting the above-mentioned samples to daily freeze-thaw cycles in cold rooms. From these tests data which confirmed the constancy of the factors lithology and tectonics were obtained. Therefore it seems that the asymmetrical modelling of the valley sides has been deeply influenced by the factor aspect: the North-facing slope had been longer protected by glaciers (and by these more intensively shaped) from the effects of freeze-thaw cycles; besides longer lasting snow-coverings occured after the retreat of the glaciers and occur at the present time on the Northfacing slope, where should be ensured a protective action, which is different from the one of the opposite side. On the other hand, it seems that the different exposure when the snow-covering is absent everywhere, originates more remarkable frost shattering phenomena on the South-facing slope, because of a higher frequency of temperature fluctuations around 0°C. A concise bibliographical rewiew concerning frost shattering phenomena and mechanisms is also presented.