Giornale di Geologia Applicata 1 (2005) 113 –121, doi: 10.1474/GGA.2005-01.0-11.0011 The Hill of Todi (Umbria, Central Italy) Corrado Cencetti, Pietro Conversini & Paolo Tacconi Department of Civil and Environmental Engineering, University of Perugia Via G. Duranti, 1 - 06125 Perugia (Italy) ABSTRACT. The historical town of Todi, in central Italy, lies on the top of a hill made up of clastic sediments in continental facies, typical of a filling stratigraphic series of a Plio-Pleistocene lacustrine basin (Ancient Lake Tiberino). Its particular geologic-lithologic conformation, with basal clays underlying sands and gravels, is responsible for extensive landslides which have threatened the town’s stability since historical times. The paper describes the geological, geomorphological and hydrogeological characteristics of the Hill of Todi, the physical-mechanical properties of the basal soils (clays, the main cause of the hydrogeological risk detected) and the types of landslides occurring over time on the hill. The landslide areas are subject to intensive monitoring; here, thanks to the administration of the Region of Umbria, the morphological and hydrological systematization works carried out have made it possible to obtain substantial stability in these areas. Key terms: Geomorphology, Landslides, Todi, Umbria. Introduction The town of Todi, in the Province of Perugia (Umbria, central Italy), stands on the top of a hill (Fig. 1), composed of Quaternary continental clastic sediments, attributable to the Umbrian Villafranchiano. Its propensity to instability, favored by lithological, stratigraphic and structural factors, is similar to that of many other historic towns in central Italy located in “Apennine intermountain basins”. These basins are the result of the extensive tectonic stage taking place on the Tyrrhenian side of the Apennine chain in the Plio-Pleistocene, and were gradually filled with clastic sediments in lacustrine and fluvial-lacustrine facies (clays, sands and conglomerates). The basin areas have always been highly populated, also because of important road networks built in the valley, which since ancient times have played an important role in the socioeconomic development of the entire country. The Hill of Todi, situated at the southwestern edge of one of the largest of these basins (the “Ancient Tiberian Lake”, today crossed by the Tiber River – Fig. 2), has suffered many significant landslides over the centuries, distributed almost uniformly over all the slopes, which have in part affected the development of the town. Fig. 1 – Panoramic view of the Hill of Todi.