The torrent of Isonzo offers one of the most striking instances of these geological revolutions. It is said to have formerly communicated through subterranean channels with the Istrian Timavo, and that its existence as a separate river does not date very far back. Ancient writers do not enumerate the Isonzo amongst the rivers flowing into the Adriatic. It is first mentioned in a document of the sixth century as a river irrigating some inland valley. On Peutinger’s Table we meet with a station, Ponte Sonti, far to the east of Aquileja, and near the sources of the Timavo. The chronicles are silent with respect to the peripatetics of this river, but a careful examination of the surrounding hills justifies the assumption that the valley of Tolmein, on the Upper Isonzo, was formerly a lake which overflowed towards the north-west through the narrows of Caporetto, and that its pent-up waters found their way through the Natisone into the Adriatic. Subsequently they opened themselves a passage to the south, and another lake was {205} formed at the confluence of Isonzo and Wippach. This lake communicated by subterranean channels with the Timavo, but it has now disappeared, and the Isonzo flows directly into the sea, its bed wandering continuously towards the east. The alluvium carried down by this river has formed the peninsula of Sdobba, and joined several old islands to the mainland.

[Μ]

Fig. 65.—SHINGLE BEDS OF THE TAGLIAMENTO, THE MEDUNA, AND ZELLINE.

From the Austrian Staff Map. Scale 1 : 290,000

The Tagliamento is even a more active geological agent than its neighbour just beyond the frontier. The débris deposited at the mouth of the narrow gorge in which it rises covers many square miles of a once fertile plain. In summer its waters trickle through these accumulations of shingle, but after heavy rain the river is converted into a powerful torrent several miles in width, and all the more formidable as its bed lies higher than many parts of the surrounding country. The Meduna and Zelline, to the west of the Tagliamento, are equally destructive, and an extensive tract at their confluence is covered with shingles. Lower down, in the lagoons, these torrents have thrown up huge embankments of sand on either side of their ancient beds. The alluvium brought down by these torrents to the sea is in every instance deposited to the west, a circumstance accounted for by the direction of the coast current.

The Piave, the most considerable river to the east of the Adige, is likewise a most active geological agent, converting fertile fields into sterile shingle tracts, filling up swamps, and carrying large quantities of matter into the sea. At its {206} mouth the land gains rapidly upon the sea, and Heraclea of the Veneti, now known as Cittanova, which was a seaport once, at the present time lies far inland.

The Piave was formerly supposed to have changed its bed in the same manner as the Isonzo. Below the Capo di Ponte, a wild defile in the Dolomite Alps, the Piave flows towards the south-west, past Belluno, and lower down is joined by the Cordevole. It was, however, supposed that the river originally flowed through the valley of Rai, immediately to the south of the Capo di Ponte, and that the Meschio and Livenzo constituted its lower course. Earthquakes or landslips were supposed to have created a barrier across that valley, and the small lakes still seen there were looked upon as remains of the ancient river bed. But M. de Mortillet has shown that this hypothesis is untenable, for the barrier referred to is merely the moraine of an ancient glacier, and there exist no traces whatever of landslips.

[Μ]

Fig. 66.—THE SUPPOSED OLD BED OF THE PIAVE.