The Antiquity of Man Part 31

It may even be a question whether the "limon des plateaux," or upland loam of the Somme valley, before alluded to,* may not be a part of the same formation.

(* Number 4 Figure 7.)

As to the higher and lower level gravels of that valley, which, like that of the Seine, contain no foreign rocks, we have seen that they are each of them covered by deposits of loess or inundation-mud belonging respectively to the periods of the gravels, whereas the upland loam is of much older date, more widely spread, and occupying positions often independent of the present lines of drainage. To restore in imagination the geographical outline of Picardy, to which rivers charged with so much h.o.m.ogeneous loam and running at such heights may once have belonged is now impossible.*

(* See above, Chapter 8. )

In the valley of the Rhine, as I before observed, the body of the loess, instead of having been formed at successively lower and lower levels as in the case of the basin of the Somme, was deposited in a wide and deep pre-existing basin, or strath, bounded by lofty mountain chains such as the Black Forest, Vosges, and Odenwald. In some places the loam acc.u.mulated to such a depth as first to fill the valley and then to spread over the adjoining table-lands, as in the case of the Lower Eifel, where it encircled some of the modern volcanic cones of loose pumice and ashes. In these instances it does not appear to me that the volcanoes were in eruption during the time of the deposition of the loess, as some geologists have supposed. The interstratification of loam and volcanic ejectamenta was probably occasioned by the fluviatile mud having gradually enveloped the cones of loose scoriae after they were completely formed. I am the more inclined to embrace this view after having seen the junction of granite and loess on the steep slopes of some of the mountains bounding the great plain of the Rhine on its right bank in the Bergstra.s.se. Thus between Darmstadt and Heidelberg perpendicular sections are seen of loess 200 feet thick, at various heights above the river, some of them at elevations of 800 feet and upwards. In one of these may be seen, resting on the hill side of Melibocus in the Odenwald, the usual yellow loam free from pebbles at its contact with a steep slope of granite, but divided into horizontal layers for a short distance from the line of junction. In these layers, which abut against the granite, a mixture of mica and of unrounded grains of quartz and felspar occur, evidently derived from the disintegration of the crystalline rock, which must have decomposed in the atmosphere before the mud had reached this height. Entire sh.e.l.ls of Helix, Pupa, and Succinea, of the usual living species, are embedded in the granitic mixture. We may therefore be sure that the valley bounded by steep hills of granite existed before the tranquil acc.u.mulation of this vast body of loess.

During the re-excavation of the basin of the Rhine successive deposits of loess of newer origin were formed at various heights; and it is often difficult to distinguish their relative ages, especially as fossils are often entirely wanting, and the mineral composition of the formation is so uniform.

The loess in Belgium is variable in thickness, usually ranging from 10 to 30 feet. It caps some of the highest hills or table-land around Brussels at the height of 300 feet above the sea. In such places it usually rests on gravel and rarely contains sh.e.l.ls, but when they occur they are of Recent species. I found the Succinea oblonga, before mentioned, and Helix hispida in the Belgian loess at Neerepen, between Tongres and Ha.s.selt, where M. Bosquet had previously obtained remains of an elephant referred to E. primigenius. This pachyderm and Rhinoceros tichorhinus are cited as characterising the loess in various parts of the valley of the Rhine. Several perfect skeletons of the marmot have been disinterred from the loess of Aix-la-Chapelle. But much remains to be done in determining the species of mammalia of this formation and the relative alt.i.tudes above the valley-plain at which they occur.

If we ascend the basin of the Neckar, we find that it is filled with loess of great thickness, far above its junction with the Rhine. At Canstadt near Stuttgart, loess resembling that of the Rhine contains many fossil bones, especially those of Elephas primigenius, together with some of Rhinoceros tichorhinus, the species having been lately determined by Dr. Falconer. At this place the loess is covered by a thick bed of travertine, used as a building stone, the product of a mineral spring. In the travertine are many fossil plants, all Recent except two, an oak and poplar, the leaves of which Professor Heer has not been able to identify with any known species.

Below the loess of Canstadt, in which bones of the mammoth are so abundant, is a bed of gravel evidently an old river channel now many feet above the level of the Neckar, the valley having there been excavated to some depth below its ancient channel so as to lie in the underlying red sandstone of Keuper. Although the loess, when traced from the valley of the Rhine into that of the Neckar, or into any other of its tributaries, often undergoes some slight alteration in its character, yet there is so much ident.i.ty of composition as to suggest the idea that the mud of the main river pa.s.sed far up the tributary valleys, just as that of the Mississippi during floods flows far up the Ohio, carrying its mud with it into the basin of that river. But the uniformity of colour and mineral composition does not extend indefinitely into the higher parts of every basin. In that of the Neckar, for example, near Tubingen, I found the fluviatile loam or brick-earth, enclosing the usual Helices and Succineae, together with the bones of the mammoth, very distinct in colour and composition from ordinary Rhenish loess, and such as no one could confound with Alpine mud. It is mottled with red and green, like the New Red Sandstone or Keuper, from which it has clearly been derived.

Such examples, however, merely show that where a basin is so limited in size that the detritus is derived chiefly or exclusively from one formation, the prevailing rock will impart its colour and composition in a very decided manner to the loam; whereas, in the basin of a great river which has many tributaries, the loam will consist of a mixture of almost every variety of rock, and will therefore exhibit an average result nearly the same in all countries. Thus, the loam which fills to a great depth the wide valley of the Saone, which is bounded on the west side by an escarpment of Inferior Oolite, and by the chain of the Jura on the east, is very like the loess found in the continuation of the same great basin after the junction of the Rhone, by which a large supply of Alpine mud has been added and intermixed.

In the higher parts of the basin of the Danube, loess of the same character as that of the Rhine, and which I believe to be chiefly of Alpine origin, attains a far greater elevation above the sea than any deposits of Rhenish loess; but the loam which, according to M. Stur, fills valleys on the north slope of the Carpathians almost up to the watershed between Galicia and Hungary, may be derived from a distinct source.

OSCILLATIONS OF LEVEL REQUIRED TO EXPLAIN THE ACc.u.mULATION AND DENUDATION OF THE LOESS.

A theory, therefore, which attempts to account for the position of the loess cannot be satisfactory unless it be equally applicable to the basins of the Rhine and Danube. So far as relates to the source of so much h.o.m.ogeneous loam, there are many large tributaries of the Danube which, during the glacial period, may have carried an ample supply of moraine-mud from the Alps to that river; and in regard to grand oscillations in the level of the land, it is obvious that the same movements both downward and upward of the great mountain-chain would be attended with a.n.a.logous effects, whether the great rivers flowed northwards or eastwards. In each case fine loam would be acc.u.mulated during subsidence and removed during the upheaval of the land. Changes, therefore, of level a.n.a.logous to those on which we have been led to speculate when endeavouring to solve the various problems presented by the glacial phenomena, are equally available to account for the nature and geological distribution of the loess. But we must suppose that the amount of depression and re-elevation in the central region was considerably in excess of that experienced in the lower countries, or those nearer the sea, and that the rate of subsidence in the latter was never so considerable as to cause submergence, or the admission of the sea into the interior of the continent by the valleys of the princ.i.p.al rivers.

We have already a.s.sumed that the Alps were loftier than now, when they were the source of those gigantic glaciers which reached the flanks of the Jura. At that time gravel was borne to the greatest distances from the central mountains through the main valleys, which had a somewhat steeper slope than now, and the quant.i.ty of river-ice must at that time have aided in the transportation of pebbles and boulders. To this state of things gradually succeeded another of an opposite character, when the fall of the rivers from the mountains to the sea became less and less, while the Alps were slowly sinking, and the first retreat of the great glaciers was taking place. Suppose the depression to have been at the rate of 5 feet in a century in the mountains and only as many inches in the same time nearer the coast, still, in such areas as the eye could survey at once, comprising a small part only of Switzerland or of the basin of the Rhine, the movement might appear to be uniform and the pre-existing valleys and heights might seem to remain relatively to each other as before.

Such inequality in the rate of rising or sinking, when we contemplate large continental s.p.a.ces, is quite consistent with what we know of the course of nature in our own times as well as at remote geological epochs. Thus in Sweden, as before stated, the rise of land now in progress is nearly uniform as we proceed from north to south for moderate distances; but it greatly diminishes southwards if we compare areas hundreds of miles apart; so that instead of the land rising about 5 feet in a hundred years as at the North Cape, it becomes less than the same number of inches at Stockholm, and farther south the land is stationary, or, if not, seems rather to be descending than ascending.*

(* "Principles of Geology" chapter 30 9th edition page 519 et seq.)

To cite an example of high geological antiquity, M. Hebert has demonstrated that, during the Oolitic and Cretaceous periods, similar inequalities in the vertical movements of the earth's crust took place in Switzerland and France. By his own observations and those of M. Lory he has proved that the area of the Alps was rising and emerging from beneath the ocean towards the close of the Oolitic epoch, and was above water at the commencement of the Cretaceous era; while, on the other hand, the area of the Jura, about 100 miles to the north, was slowly sinking at the close of the Oolitic period, and had become submerged at the commencement of the Cretaceous. Yet these oscillations of level were accomplished without any perceptible derangement in the strata, which remained all the while horizontal, so that the Lower Cretaceous or Neocomian beds were deposited conformably on the Oolitic.*

(* "Bulletin de la Societe Geologique de France" 2 series volume 16 1859 page 596.)

Taking for granted then that the depression was more rapid in the more elevated region, the great rivers would lose century after century some portion of their velocity or carrying power, and would leave behind them on their alluvial plains more and more of the moraine-mud with which they were charged, till at length, in the course of thousands or some tens of thousands of years, a large part of the main valleys would begin to resemble the plains of Egypt where nothing but mud is deposited during the flood season. The thickness of loam containing sh.e.l.ls of land and amphibious mollusca might in this way acc.u.mulate to any extent, so that the waters might overflow some of the heights originally bounding the valley and deposits of "platform mud," as it has been termed in France, might be extensively formed. At length, whenever a re-elevation of the Alps at the time of the second extension of the glaciers took place, there would be renewed denudation and removal of such loess; and if, as some geologists believe, there has been more than one oscillation of level in the Alps since the commencement of the glacial period, the changes would be proportionally more complicated and terraces of gravel covered with loess might be formed at different heights and at different periods.

HIMALAYAN MUD OF THE GANGES COMPARED TO EUROPEAN LOESS.

Some of the revolutions in physical geography above suggested for the continent of Europe during the Pleistocene epoch, may have had their counterparts in India in the Recent Period. The vast plains of Bengal are overspread with Himalayan mud, which as we ascend the Ganges extends inland for 1200 miles from the sea, continuing very h.o.m.ogeneous on the whole, though becoming more sandy as it nears the hills. They who sail down the river during a season of inundation see nothing but a sheet of water in every direction, except here and there where the tops of trees emerge above its level. To what depth the mud extends is not known, but it resembles the loess in being generally devoid of stratification, and of sh.e.l.ls, though containing occasionally land sh.e.l.ls in abundance, as well as calcareous concretions, called kunkur, which may be compared to the nodules of carbonate of lime sometimes observed to form layers in the Rhenish loess. I am told by Colonel Strachey and Dr. Hooker that above Calcutta, in the Hooghly, when the flood subsides, the Gangetic mud may be seen in river cliffs 80 feet high, in which they were unable to detect organic remains, a remark which I found to hold equally in regard to the Recent mud of the Mississippi.

Dr. Wallich, while confirming these observations, informs me that at certain points in Bengal, farther inland, he met with land-sh.e.l.ls in the banks of the great river. Borings have been made at Calcutta, beginning not many feet above the sea-level, to the depth of 300 and 400 feet; and wherever organic remains were found in the strata pierced through they were of a fluviatile or terrestrial character, implying that during a long and gradual subsidence of the country the sediment thrown down by the Ganges and Brahmaputra had acc.u.mulated at a sufficient rate to prevent the sea from invading that region.

At the bottom of the borings, after pa.s.sing through much fine loam, beds of pebbles, sand, and boulders were reached, such as might belong to an ancient river channel; and the bones of a crocodile and the sh.e.l.l of a freshwater tortoise were met with at the depth of 400 feet from the surface. No pebbles are now brought down within a great distance of this point, so that the country must once have had a totally different character and may have had its valleys, hills, and rivers, before all was reduced to one common level by the acc.u.mulation upon it of fine Himalayan mud. If the latter were removed during a gradual re-elevation of the country, many old hydrographical basins might reappear, and portions of the loam might alone remain in terraces on the flanks of hills, or on platforms, attesting the vast extent in ancient times of the muddy envelope. A similar succession of events has, in all likelihood, occurred in Europe during the deposition and denudation of the loess of the Pleistocene period, which, as we have seen in a former chapter, was long enough to allow of the gradual development of almost any amount of such physical changes.

HUMAN REMAINS IN THE LOESS NEAR STRASBURG.

M. Ami Boue, well known by his numerous works on geology and a well-practised observer in every branch of the science, disinterred in the year 1823 with his own hands many bones of a human skeleton from ancient undisturbed loess at Lahr, nearly opposite Strasburg, on the right side of the great valley of the Rhine. No skull was detected, but the tibia, fibula, and several other bones were obtained in a good state of preservation and shown at the time to Cuvier, who p.r.o.nounced them to be human.

HUMAN REMAINS IN LOESS NEAR MAESTRICHT.

The banks of the Meuse at Maestricht, like those of the Rhine at Bonn and Cologne, are slightly elevated above the level of the alluvial plain. On the right bank of the Meuse, opposite Maestricht, the difference of level is so marked that a bridge with many arches has been constructed to keep up, during the flood season, a communication between the higher parts of the alluvial plain and the hills or bluffs which bound it. This plain is composed of modern loess, undistinguishable in mineral character from that of higher antiquity, before alluded to, and entirely without signs of successive deposition and devoid of terrestrial or fluviatile sh.e.l.ls. It is extensively worked for brick-earth to the depth of about 8 feet. The bluffs before alluded to often consist of a terrace of gravel, from 30 to 40 feet in thickness, covered by an older loess, which is continuous as we ascend the valley to Liege. In the suburbs of that city patches of loess are seen at the height of 200 feet above the level of the Meuse. The table-land in that region, composed of Carboniferous and Devonian rocks, is about 450 feet high, and is not overspread with loess.

A terrace of gravel covered with loess has been mentioned as existing on the right bank of the Meuse at Maestricht. Answering to it another is also seen on the left bank below that city, and a promontory of it projecting into the alluvial plain of the Meuse and approaching to within a hundred yards of the river, was cut through during the excavation of a ca.n.a.l running from Maestricht to Hocht, between the years 1815 and 1823. This section occurs at the village of Smeerma.s.s, and is about 60 feet deep, the lower 40 feet consisting of stratified gravel and the upper of 20 feet of loess. The number of molars, tusks, and bones (probably parts of entire skeletons) of elephants obtained during these diggings, was extraordinary. Not a few of them are still preserved in the museums of Maestricht and Leyden, together with some horns of deer, bones of the ox-tribe and other mammalia, and a human lower jaw, with teeth. According to Professor Crahay, who published an account of it at the time, this jaw, which is now preserved at Leyden, was found at the depth of 19 feet from the surface, where the loess joins the underlying gravel, in a stratum of sandy loam resting on gravel and overlaid by some pebbly and sandy beds. The stratum is said to have been intact and undisturbed, but the human jaw was isolated, the nearest tusk of an elephant being six yards removed from it in horizontal distance.

Most of the other mammalian bones were found; like these human remains, in or near the gravel, but some of the tusks and teeth of elephants were met with much nearer the surface. I visited the site of these fossils in 1860 in company with M. van Binkhorst, and we found the description of the ground, published by the late Professor Crahay of Louvain, to be very correct.*

(* M. van Binkhorst has shown me the original ma.n.u.script read to the Maestricht Athenaeum in 1823. The memoir was published in 1836 in the "Bulletin de l'Academie Royale de Belgique" volume 3 page 43.)

The projecting portion of the terrace, which was cut through in making the ca.n.a.l, is called the hill of Caberg, which is flat-topped, 60 feet high, and has a steep slope on both sides towards the alluvial plain.

M. van Binkhorst (who is the author of some valuable works on the palaeontology of the Maestricht Chalk) has recently visited Leyden, and ascertained that the human fossil above mentioned is still entire in the museum of the University. Although we had no opportunity of verifying the authenticity of Professor Crahay's statements, we could see no reason for suspecting the human jaw to belong to a different geological period from that of the extinct elephant. If this were granted, it might have no claims to a higher antiquity than the human remains which Dr.

Schmerling disentombed from the Belgian caverns; but the fact of their occurring in a Pleistocene alluvial deposit in the open plains, would be one of the first examples of such a phenomenon. The top of the hill of Caberg is not so high above the Meuse as is the terrace of St. Acheul with its flint implements above the Somme, but at St. Acheul no human bones have yet been detected.

In the museum at Maestricht are preserved a human frontal and a pelvic bone, stained of a dark peaty colour; the frontal very remarkable for its lowness and the prominence of the superciliary ridges, which resemble those of the Borreby skull, Figure 5. These remains may be the same as those alluded to by Professor Crahay in his memoir, where he says that in a black deposit in the suburbs of Hocht were found leaves, nuts, and freshwater sh.e.l.ls in a very perfect state, and a human skull of a dark colour. They were of an age long posterior to that of the loess containing the bones of elephants and in which the human jaw now at Leyden is said to have been embedded.

CHAPTER 17. -- POST-GLACIAL DISLOCATIONS AND FOLDINGS OF CRETACEOUS AND DRIFT STRATA IN THE ISLAND OF MOEN, IN DENMARK.

Geological Structure of the Island of Moen.

Great Disturbances of the Chalk posterior in Date to the Glacial Drift, with Recent Sh.e.l.ls.

M. Puggaard's Sections of the Cliffs of Moen.

Flexures and Faults common to the Chalk and Glacial Drift.

Different Direction of the Lines of successive Movement, Fracture, and Flexure.

Undisturbed Condition of the Rocks in the adjoining Danish Islands.

Unequal Movements of Upheaval in Finmark.

Earthquake of New Zealand in 1855.

Predominance in all Ages of uniform Continental Movements over those by which the Rocks are locally convulsed.

In the preceding chapters I have endeavoured to show that the study of the successive phases of the glacial period in Europe, and the enduring marks which they have left on many of the solid rocks and on the character of the superficial drift are of great a.s.sistance in enabling us to appreciate the vast lapse of ages which are comprised in the Pleistocene epoch. They enlarge at the same time our conception of the antiquity, not only of the living species of animals and plants but of their present geographical distribution, and throw light on the chronological relations of these species to the earliest date yet ascertained for the existence of the human race. That date, it will be seen, is very remote if compared to the times of history and tradition, yet very modern if contrasted with the length of time during which all the living testacea, and even many of the mammalia, have inhabited the globe.

In order to render my account of the phenomena of the glacial epoch more complete, I shall describe in this chapter some other changes in physical geography and in the internal structure of the earth's crust, which have happened in the Pleistocene period, because they differ in kind from any previously alluded to, and are of a cla.s.s which were thought by the earlier geologists to belong exclusively to epochs anterior to the origin of the existing fauna and flora. Of this nature are those faults and violent local dislocations of the rocks, and those sharp bendings and foldings of the strata, which we so often behold in mountain chains, and sometimes in low countries also, especially where the rock-formations are of ancient date.

POST-GLACIAL DISLOCATIONS AND FOLDINGS OF CRETACEOUS AND DRIFT STRATA IN THE ISLAND OF MOEN, DENMARK.

A striking ill.u.s.tration of such convulsions of Pleistocene date may be seen in the Danish island of Moen, which is situated about 50 miles south of Copenhagen. The island is about 60 miles in circ.u.mference, and consists of white Chalk, several hundred feet thick, overlaid by boulder clay and sand, or glacial drift which is made up of several subdivisions, some unstratified and others stratified, the whole having a mean thickness of 60 feet, but sometimes attaining nearly twice that thickness. In one of the oldest members of the formation fossil marine sh.e.l.ls of existing species have been found.

Throughout the greater part of Moen the strata of the drift are undisturbed and horizontal, as are those of the subjacent Chalk; but on the north-eastern coast they have been throughout a certain area bent, folded, and shifted, together with the beds of the underlying Cretaceous formation. Within this area they have been even more deranged than is the English Chalk-with-flints along the central axis of the Isle of Wight in Hampshire, or of Purbeck in Dorsetshire. The whole displacement of the Chalk is evidently posterior in date to the origin of the drift, since the beds of the latter are horizontal where the fundamental Chalk is horizontal, and inclined, curved, or vertical where the Chalk displays signs of similar derangement. Although I had come to these conclusions respecting the structure of Moen in 1835, after devoting several days in company with Dr. Forchhammer to its examination,*

(* Lyell, "Geological Transactions" 2nd series volume 2 page 243.)

I should have hesitated to cite the spot as exemplifying convulsions on so grand a scale, of such extremely modern date, had not the island been since thoroughly investigated by a most able and reliable authority, the Danish geologist, Professor Puggaard, who has published a series of detailed sections of the cliffs.