At the head of the Adriatic, between the mountains and the sea, lies that part of the Lombard plain known as the Veneto. The substratum of the plain is a bed of boulders, covered during the lapse of ages by a deposit of rich alluvial soil. The rivers when they debouch from the 'mountains assume an eastern trend in their effort to reach the sea.
The result is that the plain is being gradually extended in an easterly direction, and cities like Ravenna, Adria and Aquileia, which were once seaports, lie now many miles inland.
The encroachment of land on sea has been calculated at the rate of about three miles in a thousand years. A strong current sets round the head of the Adriatic from east to west. This current catches the silt brought down by the rivers and projects it in long banks, or lidi , parallel with the shore. In process of time some of these banks, as in the case of Venice, raised themselves above the level of the water and became the true shore-line, while behind them lay large surfaces of water, called lagoons, formed partly by the fresh water brought down by the rivers, partly by the salt-water tide which found its way in by the channels of the river mouths.
Along the coast-line, roughly speaking between the Apennines at Rimini and the Carnic Alps at Trieste, three main systems of lagoons were thus created, the lagoon of Grado or Marano to the east, the lagoon of Venice in the middle, and the lagoon of Comacchio to the south-west for plan, see Harbour.
All three are dotted with small islands, possibly the remains of some earlier lido. These islands are little else than low mud banks, barely rising above the water-level. On a group of these mud banks about the middle of the lagoon of Venice stands the city of Venice. It would be difficult to imagine a site less adapted for the foundation and growth of a great community.
