Geomorphological hazard affecting a geosite in the Northern Apennines (Italy): the historical cliff of Canossa

The ancient site of Canossa (Northern Apennines, Italy) has been studied, owing to its relevance from both the cultural and geomorphological viewpoint. The ruins of the Canossa castle are located on top of a calcarenite cliff overlying clay-matrix polymictic breccias. The calcarenites making up the Canossa cliff are stratified in some 6-7 m thick layers, alternated with thin marly beds. The entire calcarenite cliff is affected by various subvertical discontinuity systems which subdivide it into several large rock strips. The presence of layers showing different levels of competence leads to the formation of ledges and indentations. Numerous niches, corresponding to eroded epigenetic concretions are also typical of the rock slopes. The boundary between the calcarenite rocks and the underlying polymictic breccias has created a contrasting landscape between the steep cliff slopes, the gently sloping cultivated fields to the east and the rugged badlands to the west. Similarly to many other historical sites found all over the Northern Apennines, also the area of Canossa is particularly prone to mass wasting processes, with landslides affecting both the calcarenite cliff and the clayey bedrock. In addition, active gully erosion is in progress. The most active processes are found on the NE slope, where dense jointing and dip-downstream layers favour the onset of rock falls and block spreads. Other movements are located on the SW slope, where tensile stresses have caused the opening of several rock discontinuities. These joints are subject to progressive widening with time which leads eventually to rock block detachment and fall, with risk not only for the preservation of the geosite itself but also for transport infrastructures and residential buildings. This area has a great historical importance since it was the centre of a very powerful fief in the Middle Ages and has been a national monument for two centuries but the degradational problems affecting both the rock slopes and the underlying clayey soils pose serious problems to the preservation of the castle and the safety of visitors. For assessing the geomechanical characteristics of the calcarenites making up the Canossa cliff in relation to its proneness to instability processes, the modified Rock Mass Rating system was used, relating its parameters to the Q-System. These engineering classification systems are based on semi-quantitative measurements and observations concerning the strength of the rock and the density and space orientation of its joints, such as roughness coefficient, spacing, alteration and presence of groundwater. The analytical approach adopted has allowed the degradational processes at present affecting this site to be properly identified in order to propose adequate remedial measures and ensure the future conservation of this important geomorphosite and its safe fruition for tourists.