Direct), situated on the left bank of the Tiber, 945 ft.
Indeed it forms the right arm of the Tiber, by which navigation is carried on at the present day, and is known as the Fossa Trajana.
In prehistoric times the river ran straight on along the valley of the Chiana and joined the Tiber near Orvieto; and there was a great lake, the north end of which was at Incisa and the south at the lake of Chiusi.
South of this the country between the frontier of Tuscany and the Tiber is in great part of volcanic origin, forming hills with distinct crater-shaped basins, in several instances occupied by small lakes (the Lake of Bolsena, Lake of Vico and Lake of Bracciano).
Beginning from the group called the Alpi della Luna near the sources of the Tiber, which attain 4435 ft., they are continued by the Monte Nerone (5010 ft.), Monte Catria (5590), and Monte Maggio to the Monte Pennino near Nocera (5169 ft.), and thence to the Monte della Sibilla, at the source of the Nar or Nera, which attains 7663 ft.