The upper Valdarno, a long valley bordered from East by the Pratomagno massif and from West by the hills around Siena.The Casentino, in the province of Arezzo, formed by the upper course of the river until the confluence with the Maestro della Chiana channel. The Val di Chiana, a plain dried in the 18th century, which, until the 18th century, was a marshy area tributary of the Tiber.Casentino is the valley in which the first tract of the river Arno flows till Subbiano.It is one of the four valleys in which the Province of Arezzo is divided. Mount Falterona, from which the Arno starts, represents the northern boundary between the Casentino and Romagna. On the east of the valley are the Alpe di Serra and the Alpe di Catenaia, which separate the Casentino from the highest tract of the Val Tiberina. On the west there is the Pratomagno, which separates the valley from the Valdarno Superiore. Part of the valley is included in the Foreste Casentinesi, Monte Falterona, Campigna National Park