Fig. 196.—Erratic Blocks in the Alps.
At Pravolta, in the Alps, going towards Monte Santo-Primo, upon a calcareous rock, we find the mass of granite represented in [Fig. 196]. This erratic block exists, with thousands of others, on the slopes of the mountain. It is about fifty feet long, nearly forty feet broad, and five-and-twenty in height; and all its edges and angles are perfect. Some parallel striæ occur along the neighbouring rocks. All this clearly demonstrates that a glacier existed, in former times, in this part of the Alps, where none appear at the present time. It is a glacier, then, which has transported and deposited here this enormous block, weighing nearly 2,000 tons.
In the Jura Mountains, on the hill of Fourvières, a limestone eminence at Lyons, blocks of granite are found, evidently derived from the Alps, and transported there by the Swiss glaciers. The particular mode of transport is represented theoretically in [Fig. 197]. A represents, for example, the summit of the Alps, B the Jura Mountains, or the hill of Fourvières, at Lyons. At the glacial period, the glacier A B C extended from the Alps to the mountain B. The granitic débris, which was detached from the summit of the Alpine mountains, fell on the surface of the glacier. The movement of progression of this glacier transported these blocks as far as the summit B. At a later period the temperature of the globe was raised, and when the ice had melted, the blocks, D E, were quietly deposited on the spots where they are now found, without having sustained the slightest shock or injury in this singular mode of transport.
Fig. 197.—Transported blocks.
Every day traces, more or less recognisable, are found on the Alps of ancient glaciers far distant from their existing limits. Heaps of débris, of all sizes, comprehending blocks with sharp-pointed angles, are found in the Swiss plains and valleys. Blocs perchés (Perched blocks), as in [Pl. XXXI.], are often seen perched upon points of the Alps situated far above existing glaciers, or dispersed over the plain which separates the Alps from the Jura, or even preserving an incredible equilibrium, when their great mass is taken into consideration, at considerable heights on the eastern flank of this chain of mountains. It is by the aid of these indications that the geologist has been able to trace to extremely remote distances signs of the former existence of the ancient glaciers of the Alps, to follow them in their course, and fix their point of origin, and where they terminated. Thus the humble Mount Sion, a gently-swelling hill situated to the north of Geneva, was the point at which three great ancient glaciers had their confluence—the glacier of the Rhône, which filled all the basin of Lake Leman, or Lake of Geneva; that of the Isère, which issued from the Annecy and Bourget Lakes; and that of the Arve, which had its source in the valley of Chamounix, all converged at this point. According to M. G. de Mortillet, who has carefully studied this geological question, the extent and situation of these ancient glaciers of the Alps were as follows:—Upon its northern flank the glacier of the Rhine occupied all the basin of Lake Constance, and extended to the borders of Germany; that of the Linth, which was arrested at the extremity of the Lake of Zurich—this city is built upon its terminal moraine—that of the Reus, which covered the lake of the four cantons with blocks torn from the peaks of Saint-Gothard;—that of the Aar, the last moraines of which crown the hills in the environs of Berne;—those of the Arve and the Isère, which, as we have said, debouched from Lake Annecy and Lake Bourget respectively;—that of the Rhône, the most important of all. It is this glacier which has deposited upon the flanks of the Jura, at the height of 3,400 feet above the level of the sea, the great erratic blocks already described. This mighty glacier of the Rhône had its origin in all the lateral valleys formed by the two parallel chains of the Valais. It filled all the Valais, and extended into the plain, lying between the Alps and the Jura, from Fort de L’Écluse, near the fall of the Rhône, up to the neighbourhood of Aarau.
The fragments of rocks transported by the ice-sea which occupied all the Swiss plain follow, in northerly direction, the course of the valley of the Rhine. On the other hand, the glacier of the Rhône, after reaching the plain of Switzerland, turned off obliquely towards the south, received the glacier of the Arve, then that of the Isère, passed between the Jura and the mountains of the Grande-Chartreuse, spread over La Bresse, then nearly all Dauphiny, and terminated in the neighbourhood of Lyons.