Part 20

For a long time it has been supposed that the glacial deposits of the central regions of Russia were accumulated during the advance and retreat of one and the same ice-sheet. In 1888, however, Professor Pavlow brought forward evidence to show that the province of Nijnii Novgorod had been twice invaded by a general mer de glace. During the first epoch of glaciation the ice-sheet overflowed the whole province, while only the northern half of the same region was covered by the mer de glace of the second invasion. Again, Professor Armachevsky has pointed out that in the province of Tchernigow two types of glacial deposits appear, so unlike in character and so differently distributed that they can hardly be the products of one and the same ice-sheet. But until recently no interglacial deposits had been detected, and the observations just referred to failed, therefore, to make much impression. The missing link in the material evidence has now happily been supplied by M. Krischtafowitsch. At Troïzkoje, in the neighbourhood of Moscow, occur certain lacustrine formations which have been long known to Russian geologists. These have been variously assigned to Tertiary, lower glacial, post-glacial, and pre-glacial horizons. They are now proved, however, to be of interglacial age, for they rest upon and are covered by glacial accumulations. Amongst their organic remains are oak (Quercus pedunculata), alder (Alnus glutinosa, A. incana), white birch, hazel, Norway maple (Acer platanoides), Scots fir, willow, water-lilies (Nuphar, Nymphæa), mammoth, pike, perch, Anadonta, wing-cases of beetles, etc. The character of the plants shows that the climate of central Russia was milder and more humid than it is to-day.

Bull. de la Soc. Impér. des Naturalistes de Moskau, No. 4, 1890.

It is obvious that the upper and lower glacial deposits of central Russia cannot be the equivalents of the upper and lower diluvium of the Baltic coast-lands. The upper diluvium of those regions is the bottom-moraine of the so-called great Baltic glacier. At the time that glacier invaded north Germany, Finland was likewise covered with an ice-sheet, which flowed towards the south-east, but did not advance quite so far as the northern shores of Lake Ladoga. A double line of terminal moraines, traced from Hango Head on the Gulf of Finland, north-east to beyond Joensuu, puts this beyond doubt. The morainic deposits that overlie the interglacial beds of central Russia cannot, therefore, belong to the epoch of the great Baltic glacier. They are necessarily older. In short, it is obvious that the upper and lower glacial accumulations near Moscow must be on the horizon of the lower diluvium of north Germany. And if this be so, then it is clear that the latter cannot be entirely the product of one and the same mer de glace. When the several boulder-clays described by Schröder and others as occurring in the Baltic provinces of Germany are reinvestigated, they may prove to be the bottom-moraines of as many distinct and separate glacial epochs.

Sederholm, Fennia, i., No. 7; Frosterus, ibid., iii., No. 8; Ramsay, ibid., iv., No. 2.

It may be contended that the glacial and interglacial deposits of central Russia are perhaps only local developments--that their evidence may be accounted for by the oscillations of one single mer de glace. This explanation, as already pointed out, has been applied to the boulder-clays and intercalated aqueous beds of the lower diluvium of north Germany, and the prevalent character of the associated organic remains makes it appear plausible. It is quite inapplicable, however, to the similar accumulations in central Russia. During the formation of the freshwater beds of Troïzkoje, no part of Russia could have been occupied by an ice-sheet; the climate was more genial and less "continental" than the present. Yet that mild interglacial epoch was preceded and succeeded by extremely arctic conditions. It is impossible that such excessive changes could have been confined to central Russia. Germany, and indeed all northern and north-western Europe, must have participated in the climatic revolutions.

So far, then, as the evidence has been considered, we may conclude that three glacial and two interglacial epochs at least have been established for northern Europe. If this be the case, then a similar succession ought to occur in our own islands; and a little consideration of the evidence already adduced will suffice to show that it does. It will be remembered that the lower and upper boulder-clays of the British Islands are the bottom-moraines of two separate and distinct ice-sheets, each of which in its time coalesced on the floor of the North Sea with the inland-ice of Scandinavia. It is obvious, therefore, that our upper boulder-clay cannot be the equivalent of the upper diluvium of the Baltic coast-lands, of Sweden, Denmark, and Schleswig-Holstein. De Geer and others have shown that while the great Baltic glacier was accumulating the upper diluvium of North Germany, etc., the inland-ice of Norway calved its icebergs at the mouths of the great fiords. Thus, during the so-called "second" glacial epoch of Scandinavian and German geologists, the Norwegian inland-ice did not coalesce with any British mer de glace. The true equivalent in this country of the upper diluvium is not our upper boulder-clay, but the great valley-moraines of our mountain-regions. It is our epoch of large valley-glaciers which corresponds to that of the great Baltic ice-flow. Our upper and lower boulder-clays are on the horizon of the lower diluvium of Germany and the glacial deposits of central Russia.

It will now be seen that the evidence in Britain is fully borne out by what is known of the glacial succession in the corresponding latitudes of the Continent. I had inferred that our epoch of large valley-glaciers formed a distinct stage by itself, and was probably separated from that of the preceding ice-sheet by a prolonged interval of interglacial conditions. One link in the chain of evidence, however, was wanting: I could not point to the occurrence of interglacial deposits underneath the great valley-moraines. But these, as we have seen, form a well-marked horizon on the Continent, and we cannot doubt that a similar interglacial stage obtained in these islands. We may feel confident, in fact, that genial climatic conditions supervened on the dissolution of the last great mer de glace in Britain, and that the subsequent development of extensive snow-fields and glaciers in our mountain-regions was contemporaneous with the appearance of the last great Baltic glacier.

We need not be surprised that interglacial beds should be well developed underneath the bottom-moraine of that great glacier, while they have not yet been recognised below the corresponding morainic accumulations of our Highlands and Uplands. The conditions in the low-grounds of the Baltic coast-lands favoured their preservation, for the ice in those regions formed a broad mer de glace, under the peripheral areas of which sub-glacial erosion was necessarily at a minimum and the accumulation at a maximum. In our Scottish mountain-valleys, however, the very opposite was the case. The conditions obtaining there were not at all comparable to those that characterised the low-grounds of northern Germany, etc., but were quite analogous to those of Norway, where, as in our own mountain-regions, interglacial beds are similarly wanting. It is quite possible, however, that patches of such deposits may yet be met with underneath our younger moraines, and they ought certainly to be looked for. But whether they occur or not in our mountain-valleys, it is certain that some of the older alluvia of our Lowlands must belong to this horizon. Hitherto all alluvial beds that overlie our upper boulder-clay have been classified as post-glacial; but since we have ascertained that our latest mer de glace was succeeded by genial interglacial conditions, we may be sure that records of that temperate epoch will yet be recognised in such Lowland tracts as were never reached by the glaciers of the succeeding cold epoch. Hence, I believe that some of our so-called "post-glacial" alluvia will eventually be assigned to an interglacial horizon. Amongst these may be cited the old peat and freshwater beds that rest upon the upper boulder-clay at Hailes Quarry, near Edinburgh. To the same horizon, in all probability, belong the clays, with Megaceros, etc., which occur so frequently underneath the peat-bogs of Ireland. An interesting account of these was given some years ago by Mr. Williams, who, as a collector of Megaceros remains, had the best opportunity of ascertaining the nature of the deposits in which these occur. He gives a section of Ballybetagh Bog, nine miles south-east of Dublin, which is as follows:--

1. Boulder-clay. 2. Fine tenacious clay, without stones. 3. Yellowish clay, largely composed of vegetable matter. 4. Brownish clay, with remains of Megaceros. 5. Greyish clay. 6. Peat.

Geol. Mag., 1881, p. 354.

The beds overlying the boulder-clay are evidently of lacustrine origin. The fine clay (No. 2), according to Mr. Williams, is simply reconstructed boulder-clay. After the disappearance of the mer de glace the land would for some time be practically destitute of any vegetable covering, and rain would thus be enabled to wash down the finer ingredients of the boulder-clay that covered the adjacent slopes, and sweep them into the lake. The clay formed in this way is described as attaining a considerable thickness near the centre of the old lake, but it thins off towards the sides. The succeeding bed (No. 3) consists so largely of vegetable débris that it can hardly be called a clay. Mr. Williams describes it as a "bed of pure vegetable remains that has been ages under pressure." He notes that there is a total absence in this bed of any tenacious clay like that of the underlying stratum, and infers, therefore, that the rainfall during the growth of the lacustrine vegetation was not so great as when the subjacent clay was being accumulated. The remains of Megaceros occur resting on the surface of the plant-bed and at various levels in the overlying brownish clay, which attains a thickness of three to four feet. The latter is a true lacustrine sediment, containing a considerable proportion of vegetable matter, interstratified with seams of clay and fine quartz-sand. According to Mr. Williams, it was accumulated under genial or temperate climatic conditions like the present. Between this bed and the overlying greyish clay (from 30 inches to 3 feet thick) there is always in all the bog deposits examined by Mr. Williams a strongly-marked line of separation. The greyish clay consists exclusively of mineral matter, and has evidently been derived from the disintegration of the adjacent granitic hills. Mr. Williams is of opinion that this clay is of aqueo-glacial formation. This he infers from its nature and texture, and from its abundance. "Why," he asks, "did not this mineral matter come down in like quantity all the time of the deposit of the brown clay which underlies it? Simply because, during the genial conditions which then existed, the hills were everywhere covered with vegetation; when the rain fell it soaked into the soil, and the clay being bound together by the roots of the grasses, was not washed down, just as at the present time, when there is hardly any degradation of these hills taking place." He mentions, further, that in the grey clay he obtained the antler of a reindeer, and that in one case the antlers of a Megaceros, found embedded in the upper surface of the brown clay, immediately under the grey clay, were scored like a striated boulder, while the under side showed no markings. Mr. Williams also emphasises the fact that the antlers of Megaceros frequently occur in a broken state--those near the surface of the brown clay being most broken, while those at greater depths are much less so. He shows that this could not be the result of tumultuous river-action--the elevation of the valley precluding the possibility of its receiving a river capable of producing such effects. Moreover, the remains show no trace of having been water-worn, the edges of the teeth of the great deer being as sharp as if the animal had died but yesterday. Mr. Williams thinks that the broken state of the antlers is due to the "pressure of great masses of ice on the surface of the clay in which they were embedded, the wide expanse of the palms of the antlers exposing them to pressure and liability to breakage; and even, in many instances, when there was 12 or 14 inches in circumference of solid bone almost as hard and sound as ivory, it was snapped across." It is remarkable that in this one small bog nearly one hundred heads of Megaceros have been dug up.

Mr. Williams' observations show us that the Megaceros-beds are certainly older than the peat-bogs with their buried timber. When he first informed me of the result of his researches (1880), I did not believe the Megaceros-beds could be older than the latest cold phase of the Ice Age. I thought that they were later in date than our last general mer de glace, and I think so still, for they obviously rest upon its ground-moraine. But since I now recognise that our upper boulder-clay is not the product of the last glacial epoch, it seems to me highly probable that the Megaceros-beds are of interglacial age--that, in short, they occupy the horizon of the interglacial deposits of north Germany, etc. The appearances described by Mr. Williams in connection with the "grey clay" seem strongly suggestive of ice-action. Ballybetagh Bog occurs at an elevation of 800 feet above the sea, in the neighbourhood of the Three Rock Mountain (1479 feet), and during the epoch of great valley-glaciers the climatic conditions of that region must have been severe. But without having visited the locality in question I should hesitate to say that the phenomena necessarily point to local glaciation. Probably frost, lake-ice, and thick accumulations of snow and névé might suffice to account for the various facts cited by Mr. Williams.

I have called special attention to these Irish lacustrine beds, because it is highly probable that the post-glacial age of similar alluvia occurring in many other places in these islands has hitherto been assumed and not proved. Now that we know, however, that a long interglacial stage succeeded the disappearance of the last general mer de glace, we may feel sure that the older alluvia of our Lowland districts cannot belong exclusively to post-glacial times. The local ice-sheets and great glaciers of our "third" glacial epoch were confined to our mountain-regions; and in the Lowlands, therefore, which were not invaded, we ought to have the lacustrine and fluviatile accumulations of the preceding interglacial stage. A fresh interest now attaches to our older alluvia, which must be carefully re-examined in the new light thus thrown upon them.

Turning next to the Alpine Lands of central Europe, we find that geologists there have for many years recognised two glacial epochs. Hence, like their confrères in northern Europe, they speak of "first" and "second" glacial epochs. Within recent years, however, Professor Penck has shown that the Alps have experienced at least three separate periods of glaciation. He describes three distinct ground-moraines, with associated river-terraces and interglacial deposits in the valleys of the Bavarian Alps, and his observations have been confirmed by Professor Brückner and Dr. Böhm. The same glacialists, I understand, have nearly completed an elaborate survey of the eastern Alps, of which they intend shortly to publish an extended account. The results obtained by them are very interesting, and fully bear out the conclusions already arrived at from their exploration of the Bavarian Alps. A similar succession of glacial epochs has quite recently been determined by Dr. Du Pasquier in north Switzerland. Nor is this kind of evidence confined to the north side of the Alps. On the shores of Lake Garda, between Salò and Brescia, three ground-moraines, separated by interglacial accumulations, are seen in section. The interglacial deposits consist chiefly of loams--the result of sub-aërial weathering--and attain a considerable thickness. From this Penck infers that the time which has elapsed since the latest glaciation is less than that required for the accumulation of either of the two interglacial series--a conclusion which, he says, is borne out by similar observations in other parts of the Alpine region.

Morlot: Bulletin de la Soc. Vaud. d. Sciences nat., 1854, 1858, 1860. Deicke: Bericht. d. St. Gall. naturf. ges., 1858. Heer: Urwelt der Schweiz. Mühlberg: Festschrift d. aarg. naturf. Ges. z. Feier ihrer 500 Sitz., 1869. Rothpletz: Denkschr. d. schweizer. Ges. f. d. ges. Naturwissensch., Bd. xxviii., 1881. Wettstein: Geologie v. Zurich u. Umgebung, 1885. Baltzer: Mitteil. d. naturf. Ges. Bern, 1887. Renevier: Bull. de la Soc. helvèt. d. Sciences nat., 1887.

Penck: Die Vergletscherung d. deutschen Alpen, 1882. Brückner: "Die Vergletscherung des Salzachgebietes," Geogr. Abhandl. Wien, Bd. i. Böhm: Jahrb. der k. k. geol. Reichsanst., 1884, 1885. See also O. Fraas, Neues Jahrb. f. Min. Geol. u. Palæont., 1880, Bd. i. p. 218; E. Fugger and C. Kastner, Verhandl. d. k. k. geol. Reichsanst., 1883, p. 136.

Mittheil. des deutsch. u. oesterreich. Alpenvereins, 1890, No. 20 u. 23.

Beiträge z. geolog. Karte der Schweiz, 31 Lief., 1891; Archiv. d. Sciences phys. et nat., 1891, p. 44.

"Die grosse Eiszeit," Himmel u. Erde.

Although the occurrence of such sub-äerial products intercalated between separate morainic accumulations is evidence of climatic changes, still it does not tell us how far the glaciers retreated during an interglacial stage. Fortunately, however, lignite beds and other deposits charged with plant remains are met with occupying a similar position, and from these we gather that during interglacial times the glaciers sometimes retired to the very heads of the mountain-valleys, and must have been smaller than their present representatives. Of such interglacial plant-beds, which have been met with in some twenty localities, the most interesting, perhaps, is the breccia of Hötting, in the neighbourhood of Innsbruck. This breccia rests upon old morainic accumulations, and is again overlaid by the later moraines of the great Inn glacier. From the fact that the breccia yielded a number of supposed extinct species of plants, palæontologists were inclined to assign it to the Pliocene. Professor Penck, however, prefers to include it in the Pleistocene system, along with all the glacial and interglacial deposits of the Alpine Lands. According to Dr. von Wettstein, the flora in question is not Alpine but Pontic. At the time of the formation of the breccia the large-leaved Rhododendron ponticum flourished in the Inn Valley at a height of 1200 metres above the sea; the whole character of the flora, in short, indicates a warmer climate than is now experienced in the neighbourhood of Innsbruck. It is obvious, therefore, that in interglacial times the glaciers must have shrunk back, as Professor Penck remarks, to the highest ridges of the mountains.

Penck: Die Vergletscherung der deutschen Alpen, p. 228. Verhandl. d. k. k. geol. Reichsanst., 1887, No. 5; Himmel und Erde, 1891. Böhm: Jahrb. d. k. k. geol. Reichsanst., 1884, p. 147. Blaas: Ferdinandeums Zeitschr., iv. Folge; Bericht. d. naturwissensch. Vereins, 1889, p. 97.

We may now glance at the glacial succession which has been established for central France. More than twenty years ago Dr. Julien brought forward evidence to show that the region of the Puy de Dôme had witnessed two glacial epochs. During the first of these epochs a large glacier flowed from Mont Dore. After its retreat a prolonged interglacial epoch followed, during which the old morainic deposits and the rocks they rest upon were much eroded. In the valleys and hollows thus excavated freshwater beds occur which have yielded relics of an abundant flora, together with the remains of Elephas meridionalis, Rhinoceros leptorhinus, etc. After the deposition of these freshwater alluvia, glaciers again descended the valleys and covered the interglacial beds with their moraines. Similar results have been obtained by M. Rames from a study of the glacial phenomenon of Cantal, which he shows belong to two separate epochs. The interval between the formation of the two series of glacial accumulations must have been prolonged, for the valleys during that interval were in some places eroded to a depth of 900 feet. M. Rames further recognises that the second glacial epoch was distinguished by two advances of valley-glaciers, separated by a marked episode of fusion. Dr. Julien has likewise noted the evidence for two episodes of fusion during the first extension of the glaciers of the Puy de Dôme.

Des Phénomènes glaciaires dans le Plateau central de la France, &c.; Paris, 1869.

Bull. Soc. géol. de France, 1884; see also M. Boule, Bull. de la Soc. philomath. de Paris, 8^e sér. i., p. 87.

Two glacial epochs have similarly been admitted for the Pyrenees; but Dr. Penck some years ago brought forward evidence to show that these mountains, like the Alps, have experienced three separate and distinct periods of glaciation.

Garrigou: Bull. Soc. géol. de France, 2^e sér. xxiv., p. 577. Jeanbernat: Bull. de Soc. d'Hist. nat. de Toulouse, iv., pp. 114, 138. Piette: Bull. Soc. géol. de France, 3^e sér. ii., pp. 503, 507.

Mitteilungen d. Vereins f. Erdkunde zu Leipzig, 1883.

We may now return to Scotland, and consider briefly the changes that followed upon the disappearance of the local ice-sheets and large valley-glaciers of our mountain-regions. The evidence is fortunately clear and complete. In the valley of the Tay, for example, at and below Perth, we encounter the following succession of deposits:--