Part 34 (2/2)
Thus, the action of aqueous infiltrations followed by frost, the chemical decomposition which granite undergoes under the influence of a moist atmosphere, degrade and disintegrate the rocks which const.i.tute the mountains enclosing the glacier. Blocks, sometimes of very considerable dimensions, often fall at the foot of these mountains on to the surface of the glacier. Were it immovable the debris would acc.u.mulate at its base, and would form there a ma.s.s of ruins heaped up without order. But the slow progression, the continuous displacement of the glacier, lead, in the distribution of these blocks, to a certain kind of arrangement: the blocks falling upon its surface partic.i.p.ate in its movement, and advance with it. But other downfalls take place daily, and the new debris following the first, the whole form a line along the outer edge of the glacier. These regular trains of rocks bear the name of ”_moraines_.” When the rocks fall from two mountains, and on each edge of the glacier, and two parallel lines of debris are formed, they are called _lateral moraines_. There are also _median moraines_, which are formed when two glaciers are confluent, in such a manner that the _lateral moraine_, on the right of the one, trends towards the left-hand one of the other. Finally, those moraines are _frontal_, or _terminal_, which repose, not upon the glacier, but at its point of termination in the valleys, and which are due to the acc.u.mulation of blocks fallen from the terminal escarpments of glaciers there arrested by some obstacle. In PLATE x.x.xI. we have represented an actual Swiss glacier, in which are united the physical and geological peculiarities belonging to these enormous ma.s.ses of frozen water: the moraines here are _lateral_, that is to say, formed of a double line of debris.
[Ill.u.s.tration: x.x.xI.--Glaciers of Switzerland.]
Transported slowly on the surface of the glacier, all the blocks from the mountain preserve their original forms unaltered; the sharpness of their edges is never altered by their gentle transport and almost imperceptible motion. Atmospheric agency only can affect or destroy these rocks when formed of hard resisting material. They then remain nearly of the same form and volume they had when they fell on the surface of the glacier; but it is otherwise with blocks and fragments enclosed between the rock and the glacier, whether it be at the bottom or between the glacier and its lateral walls. Some of these, under the powerful and continuous action of this gigantic grinding process, will be reduced to an impalpable mud, others are worn into facets, while others are rounded, presenting a mult.i.tude of scratches crossing each other in all directions. These scratched pebbles are of great importance in studying the extent of ancient glaciers; they testify, on the spot, to the existence of pre-existing glaciers which shaped, ground, and striated the pebbles, which water does not; on the contrary, in the latter, they become polished and rounded, and even natural striations are effaced.
Thus, huge blocks transported to great distances from their true geological beds, that is, _erratic blocks_, to use the proper technical term, rounded (_moutonnees_), polished, and scratched surfaces, _moraines_; finally, pebbles, ground, polished, rounded, or worn into smooth surfaces, are all physical effects of glaciers in motion, and their presence alone affords sufficient proof to the naturalist that a glacier formerly existed in the locality where he finds them. The reader will now comprehend how it is possible to recognise, in our days, the existence of ancient glaciers in different parts of the world. Above all, wherever we may find both _erratic blocks_ and _moraines_, and observe, at the same time, indications of rocks having been polished and striated in the same direction, we may p.r.o.nounce with certainty as to the existence of a glacier during geological times. Let us take some instances.
At Pravolta, in the Alps, going towards _Monte Santo-Primo_, upon a calcareous rock, we find the ma.s.s 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 striae 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 Fourvieres, 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 Fourvieres, at Lyons. At the glacial period, the glacier A B C extended from the Alps to the mountain B. The granitic debris, 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.
[Ill.u.s.tration: Fig. 196.--Erratic Blocks in the Alps.]
[Ill.u.s.tration: 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 debris, of all sizes, comprehending blocks with sharp-pointed angles, are found in the Swiss plains and valleys. _Blocs perches_ (Perched blocks), as in PL. x.x.xI., 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 ma.s.s 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 Rhone, which filled all the basin of Lake Leman, or Lake of Geneva; that of the Isere, 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 _Isere_, which, as we have said, debouched from Lake Annecy and Lake Bourget respectively;--that of the _Rhone_, 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 Rhone 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'ecluse, near the fall of the Rhone, 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 Rhone, after reaching the plain of Switzerland, turned off obliquely towards the south, received the glacier of the Arve, then that of the Isere, pa.s.sed 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.
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-Baltea_ entered the plain of Ivrea, 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 Varese. 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 pa.s.sage 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]
[107] ”Carte des Anciens Glaciers des Alpes,” pp. 8-10. (1860.)
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 alt.i.tude, 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 pa.s.sed near the sources of the Dnieper and the Volga, and thence took a direction towards the sh.o.r.es 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 ma.s.ses of the Alps. Stretching to the west and to the north, they extended to the valley of the Rhone as far as Lyons, then crossing the summit-level of the Jura, they pa.s.sed 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, pa.s.sed near to Udinet, covered Peschiera, Solferino, Como, Varese, and Ivrea, 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 princ.i.p.al valleys of this chain, both on the French and Spanish sides, especially the valleys of the centre, which comprehend those of Luchon, Aude, Bareges, Cauterets, and Ossun. In the Cantabrian chain, an extension of the Pyrenees, the existence of ancient glaciers has also been recognised.
”In the Vosges and the Black Forest they covered all the southern parts of these mountains. In the Vosges, the princ.i.p.al traces are found in the valleys of Saint-Amarin, Giromagny, Munster, the Moselle, &c.
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