The volcanic sequence exposed in the Treia Valley, 40 km north of Rome, promises to be useful in establishing the climate and chronology of the Mediterranean middle Pleistocene. A change in the course of the Tiber River has been a critical factor in the development of this section. The Tiber makes a detour eastward from Civita Castellana around the volcanic area of the Monti Sabatini before reaching Rome and the sea. Stratigraphic relations exposed in the Treia Canyon show that early in the Pleistocene the “Paleotiber” flowed due south from Civita Castellana to Rome, and was diverted by the first products of the Sabatini volcanoes. The former bed of the “Paleotiber” can be identified, and the occurrence of lake beds clarifies the way in which damming took place. The present course of the Tiber probably formed when the lake backed into a tributary valley and overflowed into the headwaters of another tributary.