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If we turn from the lakes themselves to the Lombard plain at large, which is an immensely older and larger basin, we see traces of the same action on a vastly greater scale. A glance at the map will show the intelligent and ever courteous reader that the ‘wandering Po’–I drop into poetry after Goldsmith–flows much nearer the foot of the Apennines than of the Alps in the course of its divagations, and seems purposely to bend away from the greater range of mountains. Why is this, since everything in nature must needs have a reason? Well, it is because, when the mud first began to accumulate in the old Lombard bay of the Adriatic, there was no Po at all, whether wandering or otherwise: the big river has slowly grown up in time by the union of the lateral torrents that pour down from either side, as the growth of the mud flat brought them gradually together. Careful study of a good map will show how this has happened, especially if it has the plains and mountains distinctively tinted after the excellent German fashion. The Ticino, the Adda, the Mincio, if you look at them close, reveal themselves as tributaries of the Po, which once flowed separately into the Lombard bay; the Adige, the Piave, the Tagliamento farther along the coast, reveal themselves equally as tributaries of the future Po, when once the great river shall have filled up with its mud the space between Trieste and Venice, though for the moment they empty themselves and their store of detritus into the open Adriatic.

Fix your eyes for a moment on Venetia proper, and you will see how this has all happened and is still happening. Each mountain torrent that leaps from the Tyrolese Alps bring down in its lap a rich mass of mud, which has gradually spread over a strip of sea some forty or fifty miles wide, from the base of the mountains to the modern coast-line of the province. Near the sea–or, in other words, at the temporary outlet–it forms banks and lagoons, of which those about Venice are the best known to tourists, though the least characteristic. For miles and miles between Venice and Trieste the shifting north shore of the Adriatic consists of nothing but such accumulating mud banks. Year after year they push farther seaward, and year after year fresh islets and shoals grow out into the waves beyond the temporary deltas. In time, therefore, the gathering mud banks of these Alpine torrents must join the greater mud bank that runs rapidly seaward at the delta of the Po. As soon as they do so the rivers must rush together, and what was once an independent stream, emptying itself into the Adriatic, must become a tributary of the Po, helping to swell the waters of that great united river. The Adige has now just reached this state: its delta is continuous with the delta of the Po, and their branches interosculate. The Mincio and the Adda reached it ages since: the Piave and the Livenia will not reach it for ages. In Roman days Hatria was still on the sea: it is now some fifteen miles inland.

From all this you can gather why the existing Po flows far from the Alps and nearer the base of the Apennines. The Alpine streams in far distant days brought down relatively large floods of glacial mud; formed relatively large deltas in the old Lombard bay; filled up with relative rapidity their larger half of the basin. The Apennines, less lofty, and free from glaciers, sent down shorter and smaller torrents, laden with far less mud, and capable therefore of doing but little alluvial work for the filling in of the future Lombardy. So the river was pushed southward by the Alpine deposits of the northern streams, leaving the great plains of Cisalpine Gaul spread away to the north of it.