The backbone of the Italian peninsula is the Apennine mountain system, a continuation of the Alpine system that extends into northern Italy. Some of the ancient Roman roads over its principal passes are still in use, and several railway lines now cross the mountains. The total length of the Apennines is about 870 miles (1,400 kilometers); their width ranges from 25 to 125 miles (40 to 200 kilometers). In the northwest the Apennines form the background of the Italian Riviera. Then the range swings southeastward across the peninsula toward the Adriatic Sea, leaving only a narrow coastal plain. These central Apennines are the widest part of the chain and contain the highest mountains. The loftiest group is the Gran Sasso, which rises to 9,560 feet (2,914 meters) in Mount Corno. In the toe of the Italian boot the Apennines extend to the western coast. Near Naples is a volcanic region (see Vesuvius). The southern division, particularly the toe of Italy, has many earthquakes. The Apennines form the watershed for peninsular Italy. In the north they furnish tributaries to the Po River. On the Adriatic side the streams are mountain torrents. To the west are larger rivers‹the Arno, Tiber, and Volturno. The shorter streams flow only during the winter rainy season. Then their swift descent powers numerous hydroelectric plants. The snow-white Carrara marble quarried in the northern Apennines was used by such famous sculptors as Michelangelo and Antonio Canova. Crops produced on the slopes include olives, citrus fruits, grapes, sugar beets, and potatoes. (Encyclopaedia Britannica Online.)