According to the geological observations of M. Morlot, the following successive phases in the development of ice-action in the Alps are plainly recognisable:--

First. There was a period when the ice was in its greatest excess, when the glacier of the Rhone not only reached the Jura, but climbed to the height of 2015 feet above the Lake of Neufchatel, and 3450 feet above the sea, at which time the Alpine ice actually entered the French territory at some points, penetrating by certain gorges, as through the defile of the Fort de l'Ecluse, among others.

Second. To this succeeded a prolonged retreat of the great glaciers, when they evacuated not only the Jura and the low country between that chain and the Alps, but retired some way back into the Alpine valleys.

M. Morlot supposes their diminution in volume to have accompanied a general subsidence of the country to the extent of at least 1000 feet.

The geological formations of the second period consist of stratified ma.s.ses of sand and gravel, called the "ancient alluvium" by MM. Necker and Favre, corresponding to the "older or lower diluvium" of some writers. Their origin is evidently due to the action of rivers, swollen by the melting of ice, by which the materials of parts of the old moraines were rearranged and stratified and left usually at considerable heights above the level of the present valley plains.

Third. The glaciers again advanced and became of gigantic dimensions, though they fell far short of those of the first period. That of the Rhone, for example, did not again reach the Jura, though it filled the Lake of Geneva and formed enormous moraines on its borders and in many parts of the valley between the Alps and Jura.

Fourth. A second retreat of the glaciers took place when they gradually shrank nearly into their present limits, accompanied by another acc.u.mulation of stratified gravels which form in many places a series of terraces above the level of the alluvial plains of the existing rivers.

In the gorge of the Dranse, near Thonon, M. Morlot discovered no less than three of these glacial formations in direct superposition, namely, at the bottom of the section, a ma.s.s of compact till or boulder-clay (Number 1) 12 feet thick, including striated boulders of Alpine limestone, and covered by regularly stratified ancient alluvium (Number 2) 150 feet thick, made up of rounded pebbles in horizontal beds.

This ma.s.s is in its turn overlaid by a second formation (Number 3) of unstratified boulder clay, with erratic blocks and striated pebbles, which const.i.tuted the left lateral moraine of the great glacier of the Rhone when it advanced for the second time to the Lake of Geneva. At a short distance from the above section terraces (Number 4) composed of stratified alluvium are seen at the heights of 20, 50, 100, and 150 feet above the Lake of Geneva, which by their position can be shown to be posterior in date to the upper boulder-clay and therefore belong to the fourth period, or that of the last retreat of the great glaciers. In the deposits of this fourth period the remains of the mammoth have been discovered, as at Morges, for example, on the Lake of Geneva. The conical delta of the Tiniere, mentioned in Chapter 2 as containing at different depths monuments of the Roman as well as of the antecedent bronze and stone ages, is the work of alluvial deposition going on when the terrace of 50 feet was in progress. This modern delta is supposed by M. Morlot to have required 10,000 years for its acc.u.mulation. At the height of 150 feet above the lake, following up the course of the same torrent, we come to a more ancient delta, about ten times as large, which is therefore supposed to be the monument of about ten times as many centuries, or 100,000 years, all referable to the fourth period mentioned in the preceding page, or that which followed the last retreat of the great glaciers.*

(* Morlot, Terrain quaternaire du Ba.s.sin de Leman "Bulletin de la Societe Vaudoise des Sciences Naturelles" Number 44.)

If the lower flattened cone of Tiniere be referred in great part to the age of the oldest lake-dwellings, the higher one might perhaps correspond with the Pleistocene period of St. Acheul, or the era when Man and the Elephas primigenius flourished together; but no human remains or works of art have as yet been found in deposits of this age or in any alluvium containing the bones of extinct mammalia in Switzerland.

Upon the whole, it is impossible not to be struck with an apparent correspondence in the succession of events of the glacial period of Switzerland and that of the British Isles before described. The time of the first Alpine glaciers of colossal dimensions, when that chain perhaps was several thousand feet higher than now, may have agreed with the first continental period when Scotland was invested with a universal crust of ice. The retreat of the first Alpine glaciers, caused partly by a lowering of that chain, may have been synchronous with the period of great submergence and floating ice in England. The second advance of the glaciers may have coincided in date with the re-elevation of the Alps, as well as of the Scotch and Welsh mountains; and lastly, the final retreat of the Swiss and Italian glaciers may have taken place when Man and the extinct mammalia were colonising the north-west of Europe and beginning to inhabit areas which had formed the bed of the glacial sea during the era of chief submergence.

But it must be confessed that in the present state of our knowledge these attempts to compare the chronological relations of the periods of upheaval and subsidence of areas so widely separated as are the mountains of Scandinavia, the British Isles, and the Alps, or the times of the advance and retreat of glaciers in those several regions and the greater or less intensity of cold, must be looked upon as very conjectural.

We may presume with more confidence that when the Alps were highest and the Alpine glaciers most developed, filling all the great lakes of northern Italy and loading the plains of Piedmont and Lombardy with ice, the waters of the Mediterranean were chilled and of a lower average temperature than now. Such a period of refrigeration is required by the conchologist to account for the prevalence of northern sh.e.l.ls in the Sicilian seas about the close of the Pliocene or commencement of the Pleistocene period. For such sh.e.l.ls as Cyprina islandica, Panopoea norvegica (= P. bivonae, Philippi), Leda pygmaea, Munst, and some others, enumerated among the fossils of the latest Tertiary formations of Sicily by Philippi and Edward Forbes, point unequivocally to a former more severe climate. Dr. Hooker also in his late journey to Syria (in the autumn of 1860) found the moraines of extinct glaciers, on which the whole of the ancient cedars of Lebanon grow, to descend 4000 feet below the summit of that chain. The temperature of Syria is now so much milder that there is no longer perpetual snow even on the summit of Lebanon, the height of which was ascertained to be 10,200 feet above the Mediterranean.*

(* Hooker, "Natural History Review" Number 5 January 1862 page 11.)

Such monuments of a cold climate in lat.i.tudes so far south as Syria and the north of Sicily, between 33 and 38 degrees north, may be confidently referred to an early part of the glacial period, or to times long anterior to those of Man and the extinct mammalia of Abbeville and Amiens.

CHAPTER 16. -- HUMAN REMAINS IN THE LOESS, AND THEIR PROBABLE AGE.

Nature, Origin, and Age of the Loess of the Rhine and Danube.

Impalpable Mud produced by the Grinding Action of Glaciers.

Dispersion of this Mud at the Period of the Retreat of the great Alpine Glaciers.

Continuity of the Loess from Switzerland to the Low Countries.

Characteristic Organic Remains not Lacustrine.

Alpine Gravel in the Valley of the Rhine covered by Loess.

Geographical Distribution of the Loess and its Height above the Sea.

Fossil Mammalia.

Loess of the Danube.

Oscillations in the Level of the Alps and lower Country required to explain the Formation and Denudation of the Loess.

More rapid Movement of the Inland Country.

The same Depression and Upheaval might account for the Advance and Retreat of the Alpine Glaciers.

Himalayan Mud of the Plains of the Ganges compared to European Loess.

Human Remains in Loess near Maestricht, and their probable Antiquity.

NATURE AND ORIGIN OF THE LOESS.

Intimately connected with the subjects treated of in the last chapter, is the nature, origin, and age of certain loamy deposits, commonly called loess, which form a marked feature in the superficial deposits of the basins of the Rhine, Danube, and some other large rivers draining the Alps, and which extend down the Rhine into the Low Countries, and were once perhaps continuous with others of like composition in the north of France. [35]

It has been reported of late years that human remains have been detected at several points in the loess of the Meuse around and below Maestricht.

I have visited the localities referred to; but, before giving an account of them, it will be desirable to explain what is meant by the loess, a step the more necessary as a French geologist for whose knowledge and judgment I have great respect, tells me he has come to the conclusion that "the loess" is "a myth," having no real existence in a geological sense or as holding a definite place in the chronological series.

No doubt it is true that in every country, and at all geological periods, rivers have been depositing fine loam on their inundated plains in the manner explained above in Chapter 3, where the Nile mud was spoken of. This mud of the plains of Egypt, according to Professor Bischoff's chemical a.n.a.lysis agrees closely in composition with the loess of the Rhine.*

(* "Chemical and Physical Geology" volume 1 page 132.)

I have also shown when speaking of the fossil man of Natchez, how identical in mineral character and in the genera of its terrestrial and amphibious sh.e.l.ls is the ancient fluviatile loam of the Mississippi with the loess of the Rhine. But granting that loam presenting the same aspect has originated at different times and in distinct hydrographical basins, it is nevertheless true that during the glacial period the Alps were a great centre of dispersion, not only of erratics, as we have seen in the last chapter, and of gravel which was carried farther than the erratics, but also of very fine mud which was transported to still greater distances and in greater volume down the princ.i.p.al river-courses between the mountains and the sea.

MUD PRODUCED BY GLACIERS.

They who have visited Switzerland are aware that every torrent which issues from an icy cavern at the extremity of a glacier is densely charged with an impalpable powder, produced by the grinding action to which the subjacent floor of rock and the stones and sand frozen into the ice are exposed in the manner before described. We may therefore readily conceive that a much greater volume of fine sediment was swept along by rivers swollen by melting ice at the time of the retreat of the gigantic glaciers of the olden time. The fact that a large proportion of this mud, instead of being carried to the ocean where it might have formed a delta on the coast or have been dispersed far and wide by the tides and currents, has acc.u.mulated in inland valleys, will be found to be an additional proof of the former occurrence of those grand oscillations in the level of the Alps and parts of the adjoining continent which were required to explain the alternate advance and retreat of the glaciers, and the superposition of more than one boulder clay and stratified alluvium.

The position of the loess between Basle and Bonn is such as to imply that the great valley of the Rhine had already acquired its present shape, and in some places, perhaps more than its actual depth and width, previously to the time when it was gradually filled up to a great extent with fine loam. The greater part of this loam has been since removed, so that a fringe only of the deposit is now left on the flanks of the boundary hills, or occasionally some outliers in the middle of the great plain of the Rhine where it expands in width.

These outliers are sometimes on such a scale as to admit of minor hills and valleys, having been shaped out of them by the action of rain and small streamlets, as near Freiburg in the Breisgau and other districts.

FOSSIL Sh.e.l.lS OF THE LOESS.

[Ill.u.s.tration: Figures 44, 45, and 46]

(FIGURE 44. Succinea oblonga.)

(FIGURE 45. Pupa muscorum.)

(FIGURE 46. Helix hispida, Lin.; H. plebeia, Drap.)

The loess is generally devoid of fossils, although in many places they are abundant, consisting of land-sh.e.l.ls, all of living species, and comprising no small part of the entire molluscous fauna now inhabiting the same region. The three sh.e.l.ls most frequently met with are those represented in the annexed figures (44, 45 and 46). The slug, called Succinea, is not strictly aquatic, but lives in damp places, and may be seen in full activity far from rivers, in meadows where the gra.s.s is wet with rain or dew; but sh.e.l.ls of the genera Limnaea, Planorbis, Paludina, Cyclas, and others, requiring to be constantly in the water, are extremely exceptional in the loess, occurring only at the bottom of the deposit where it begins to alternate with ancient river-gravel on which it usually reposes.

This underlying gravel consists in the valley of the Rhine for the most part of pebbles and boulders of Alpine origin, showing that there was a time when the rivers had power to convey coa.r.s.e materials for hundreds of miles northwards from Switzerland towards the sea; whereas at a later period an entire change was brought about in the physical geography of the same district, so that the same river deposited nothing but fine mud, which acc.u.mulated to a thickness of 800 feet or more above the original alluvial plain.

But although most of the fundamental gravel was derived from the Alps, there has been observed in the neighbourhood of the princ.i.p.al mountain chains bordering the great valley, such as the Black Forest, Vosges, and Odenwald, an admixture of detritus characteristic of those several chains. We cannot doubt therefore that as some of these mountains, especially the Vosges, had during the glacial period their own glaciers, a part of the fine mud of their moraines must have been mingled with loess of Alpine origin; although the princ.i.p.al ma.s.s of the latter must have come from Switzerland, and can in fact be traced continuously from Basle to Belgium.

GEOGRAPHICAL DISTRIBUTION OF THE LOESS.

It was stated in the last chapter that at the time of the greatest extension of the Swiss glaciers the Lake of Constance and all the other great lakes were filled with ice, so that gravel and mud could pa.s.s freely from the upper Alpine valley of the Rhine to the lower region between Basle and the sea, the great lake intercepting no part of the moraines whether fine or coa.r.s.e. On the other hand the Aar with its great tributaries the Limmat and the Reuss does not join the Rhine till after it issues from the Lake of Constance; and by their channels a large part of the Alpine gravel and mud could always have pa.s.sed without obstruction into the lower country, even after the ice of the great lake had melted.

It will give the reader some idea of the manner in which the Rhenish loess occurs, if he is told that some of the earlier scientific observers imagined it to have been formed in a vast lake which occupied the valley of the Rhine from Basle to Mayence, sending up arms or branches into what are now the valleys of the Main, Neckar, and other large rivers. They placed the barrier of this imaginary lake in the narrow and picturesque gorge of the Rhine between Bingen and Coblenz: and when it was objected that the lateral valley of the Lahn, communicating with that gorge, had also been filled with loess, they were compelled to transfer the great dam farther down and to place it below Bonn. Strictly speaking it must be placed much farther north, or in the 51st parallel of lat.i.tude, where the limits of the loess have been traced out by MM. Omalius D'Halloy, Dumont, and others, running east and west by Cologne, Juliers, Louvain, Oudenarde, and Courtrai in Belgium to Ca.s.sel, near Dunkirk in France. This boundary line may not indicate the original seaward extent of the formation, as it may have stretched still farther north and its present abrupt termination may only show how far it was cut back at some former period by the denuding action of the sea.

Even if the imbedded fossil sh.e.l.ls of the loess had been lacustrine, instead of being, as we have seen, terrestrial and amphibious, the vast height and width of the required barrier would have been fatal to the theory of a lake: for the loess is met with in great force at an elevation of no less than 1600 feet above the sea, covering the Kaiserstuhl, a volcanic mountain which stands in the middle of the great valley of the Rhine, near Freiburg in Breisgau. The extent to which the valley has there been the receptacle of fine mud afterwards removed is most remarkable.

The loess of Belgium was called "Hesbayan mud" in the geological map of the late M. Dumont, who, I am told, recognised it as being in great part composed of Alpine mud. M. d'Archiac, when speaking of the loess, observes that it envelopes Hainault, Brabant, and Limburg like a mantle everywhere uniform and h.o.m.ogeneous in character, filling up the lower depressions of the Ardennes and pa.s.sing thence into the north of France, though not crossing into England. In France, he adds, it is found on high plateaus 600 feet above some of the rivers, such as the Marne; but as we go southwards and eastwards of the basin of the Seine, it diminishes in quant.i.ty, and finally thins out in those directions.*

(* D'Archiac, "Histoire des Progres" volume 2 pages 169, 170.)