The mountainous alignments that cross Northern Portugal in a longitudinal direction combine a rough relief with very steep slopes and an exceptionally high average yearly rainfall (superior to 2000 mm). These slopes are often regularized by surface formations, which correspond to weathering mantles on granite and quaternary deposits produced by periglacial processes, the last ones more frequent over schist. Mass movements, especially debris flows and landslides, are frequently recorded in this territory, playing a crucial role in slope dynamics. The Marão Hill is a tectonic relief, mostly constituted by metamorphic rocks, with deeply entrenched valleys and accented slopes, where the surface formations have a strong presence. Aiming to show the absolute need of considering these formations in mass movement risk assessment, a field research was developed in Marão Hill, where was completed its exhaustive survey, georeferenciation and representation in a Geographical Information System. Then it was conducted a correlated spatial analysis, with a weighted overlay of the permanent conditioning factors considered more relevant, besides surface formations, within this territorial context, particularly lithology, slope, slope aspect and vegetation cover. The results show a major difference between the two hazard maps produced, by including or neglecting the surface for mations. The risk-prone sites identified matches the areas that combine strong slopes, north quadrant orientation and the presence of surface formations, particularly the ones from the tardiglacial period, characterized by less cohesion and bigger heterometry.
mountains; geomorphology; slope deposits; natural hazards; GIS
