Upon the southern flank of the Alps, the ancient glaciers, according to M. de Mortillet’s map, occupied all the great valleys from that of the Dora, on the west, to that of the Tagliamento, on the east. “The glacier of the Dora” says de Mortillet, whose text we greatly abridge, “debouched into the valley of the Po, close to Turin. That of the Dora-Baltéa entered the plain of Ivréa, where it has left a magnificent semicircle of hills, which formed its terminal moraine. That of the Toce discharged itself into Lake Maggiore, against the glacier of the Tessin, and then threw itself into the valley of Lake Orta, at the southern extremity of which its terminal moraines were situated. That of the Tessin filled the basin of Lake Maggiore, and established itself between Lugano and Varèse. That of the Adda filled the basin of Lake Como, and established itself between Mendrizio and Lecco, thus describing a vast semicircle. That of the Oglio terminated a little beyond Lake Iseo. That of the Adige, finding no passage through the narrow valley of Roveredo, where the valley became very narrow, took another course, and filled the immense valley of the Lake of Garda. At Novi it has left a magnificent moraine, of which Dante speaks in his ‘Inferno.’ That of the Brenta extended over the plain of that commune. The Drave and the Tagliamento had also their glaciers. Finally, glaciers occupied all the valleys of the Austrian and Bavarian Alps.”[107]
Similar traces of the existence of ancient glaciers occur in many other European countries. In the Pyrenees, in Corsica, the Vosges, the Jura, &c., extensive ranges of country have been covered, in geological times, by these vast plains of ice. The glacier of the Moselle was the most considerable of the glaciers of the Vosges, receiving numerous affluents; its lowest frontal moraine, which is situated below Remiremont, could not be less than a mile and a quarter in length.
But the phenomenon of the glacial extension which we have examined in the Alps was not confined to Central Europe. The same traces of their ancient existence are observed in all the north of Europe, in Russia, Iceland, Norway, Prussia, the British Islands, part of Germany, in the north, and even in some parts of the south, of Spain. In England, erratic blocks of granite are found which were derived from the mountains of Norway. It is evident that these blocks were borne by a glacier which extended from the north pole to England. In this manner they crossed the Baltic and the North Seas. In Prussia similar traces are observable.
Thus, during the Quaternary epoch, glaciers which are now limited to the Polar regions, or to mountainous countries of considerable altitude, extended very far beyond their present known limits; and, taken in connection with the deluge of the north, and the vast amount of organic life which they destroyed, they form, perhaps, the most striking and mysterious of all geological phenomena.
M. Edouard Collomb, to whom we owe much of our knowledge of ancient glaciers, furnishes the following note explanatory of a map of Ancient Glaciers which he has prepared:—
“The area occupied by the ancient Quaternary glaciers may be divided into two orographical regions:—1. The region of the north, from lat. 52° or 55° up to the North Pole. 2. The region of Central Europe and part of the south.
“The region of the north which has been covered by the ancient glaciers comprehends all the Scandinavian peninsula, Sweden, Norway, and a part of Western Russia, extending from the Niemen on the north in a curve which passed near the sources of the Dnieper and the Volga, and thence took a direction towards the shores of the glacial ocean. This region comprehends Iceland, Scotland, Ireland, the isles dependent on them, and, finally, a great part of England.
“This region is bounded, on all its sides, by a wide zone from 2° to 5° in breadth, over which is recognised the existence of erratic blocks of the north: it includes the middle region of Russia in Europe, Poland, a part of Prussia, and Denmark; losing itself in Holland on the Zuider Zee, it cut into the northern part of England, and we find a shred of it in France, upon the borders of the Cotentin.
“The ancient glaciers of Central Europe consisted, first, of the grand masses of the Alps. Stretching to the west and to the north, they extended to the valley of the Rhône as far as Lyons, then crossing the summit-level of the Jura, they passed near Basle, covering Lake Constance, and stretching beyond into Bavaria and Austria. Upon the southern slopes of the Alps, they turned round the summit of the Adriatic, passed near to Udinet, covered Peschiera, Solferino, Como, Varèse, and Ivréa, extended to near Turin, and terminated in the valley of the Stura, near the Col de Tenda.
“In the Pyrenees, the ancient glaciers have occupied all the principal valleys of this chain, both on the French and Spanish sides, especially the valleys of the centre, which comprehend those of Luchon, Aude, Baréges, Cauterets, and Ossun. In the Cantabrian chain, an extension of the Pyrenees, the existence of ancient glaciers has also been recognised.