Crevice-type caves as initial forms of rock landslide development in the Flysch Carpathians

Crevice-type caves, formed along joints, are very common in the Polish Flysch Carpathians (more than 600 have been recorded). The caves represent accessible parts of the widened cracks, which are basic, initial forms of development of deep-seated rock landslides. Structural features of four selected caves (situated in different parts of the Carpathians) were studied in order to determine relations between joint systems and landslide formation. The studies prove that the main rock scarps often represent effects of earlier propagation of the cracks (=cutting surfaces) due to loading and unloading shearing stress. The cracks are structural forms developed along joints or faults. The cutting surfaces represented by cracks (caves) develop a long time before the main gravitational mass movements, whereas the sliding surfaces (shearing zones) are formed or significantly modified ad hoc during the gravitational transport of rock masses. Occurrence of the deep initial cracks causes modification of shape of the sliding surfaces, which are noncircular. The mountain slopes cut by many cracks are permanently developed. Thus, the periodic increase of activity of external factors (floods, downpours, earthquakes, etc.) leads to common formation of the landslides earlier prepared by crack development. This model of slope evolution enables us to interpret the occurrence of the landslide phases (in the Holocene) during which increasing of frequency of the landslides is observed.