Part 29

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.[CI] 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.[CJ] 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.[CK] A similar succession of glacial epochs has quite recently been determined by Dr. Du Pasquier in north Switzerland.[CL] 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.[CM]

[CI] 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.

[CJ] 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.

[CK] _Mittheil. des deutsch. u. oesterreich. Alpenvereins_, 1890, No. 20 u. 23.

[CL] _Beiträge z. geolog. Karte der Schweiz_, 31 Lief., 1891; _Archiv. d. Sciences phys. et nat._, 1891, p. 44.

[CM] "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.[CN] 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.

[CN] 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.[CO] 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.[CP] 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.

[CO] _Des Phénomènes glaciaires dans le Plateau central de la France_, &c.; Paris, 1869.

[CP] 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;[CQ] 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.[CR]

[CQ] 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.

[CR] _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:--

6. Recent alluvia. 5. Carse-deposits, 45 feet above sea-level. 4. Peat and forest bed. 3. Old alluvia. 2. Clays, etc., of 100-feet beach. 1. Boulder-clay.

The old alluvia (3) are obviously of fluviatile origin, and show us that after the deposition of the clays, etc., of the 100-feet beach the sea retreated, and allowed the Tay and its tributaries to plough their way down through the marine and estuarine deposits of the "third" glacial epoch. These deposits would appear to have extended at first as a broad and approximately level plain over all the lower reaches of the valleys. Through this plain the Tay and the Earn cut their way to a depth of more than 100 feet, and gradually removed all the material over a course which can hardly be less than 2 miles in breadth below the Bridge of Earn, and considerably exceeds that in the Carse of Gowrie. No organic remains occur in the "old alluvia," but the deposits consist principally of gravel and sand, and show not a trace of ice-action. Immediately overlying them comes the well-known peat-bed (4). This is a mass of vegetable matter, varying in thickness from a few inches up to 3 or 4 feet. In some places it seems to be made up chiefly of reed-like plants and sedges and occasional mosses, commingled with which are abundant fragments of birch, alder, willow, hazel, and pine. In other places it contains trunks and stools of oak and hazel, with hazel-nuts--the trees being rooted in the subjacent deposits. It is generally highly compressed and readily splits into laminæ, upon the surface of which many small reeds, and now and again wing-cases of beetles, may be detected. A large proportion of the woody débris--twigs, branches, and trunks--appears to have been drifted. A "dug-out" canoe of pine was found, along with trunks of the same tree, in the peat at Perth. The Carse-deposits (5), consisting principally of clay and silt, rest upon the peat-bed. The occurrence in these deposits of _Scrobicularia piperata_ and oyster-shells leaves us in no doubt as to their marine origin. They vary in thickness from 10 up to fully 40 feet.[CS]

[CS] For a particular account of the Tay-valley Succession, see _Prehistoric Europe_, p. 385.

A similar succession of deposits is met with in the valley of the Forth,[CT] and we cannot doubt that these tell precisely the same tale. I have elsewhere[CU] adduced evidence to show that the peat-bed, with drifted vegetable débris, which underlies the Carse accumulations of the Forth and Tay is on the same horizon as the "lower buried forest" of our oldest peat-bogs, and the similar bogs that occur in Norway, Sweden, Denmark, Schleswig-Holstein, Holland, etc. Underneath the "lower buried forest" of those regions occur now and again freshwater clays, charged with the relics of an arctic-alpine flora; and quite recently similar plant remains have been detected in old alluvia at Corstorphine, near Edinburgh. When the beds below our older peat-bogs are more carefully examined, traces of that old arctic flora will doubtless be met with in many other parts of these islands. It was this flora that clothed north-western Europe during the decay of the last district ice-sheets of Britain and the disappearance of the great Baltic glacier.

[CT] _Proc. Roy. Soc. Edin._ 1883-84, p. 745; _Mem. Geol. Survey, Scotland_, Explanation of Sheet 31.

[CU] _Prehistoric Europe_, chaps. xvi., xvii.

The dissolution of the large valley-glaciers of this country was accompanied by a general retreat of the sea--all the evidence leading to the conviction that our islands eventually became united to the Continent. The climatic conditions, as evidenced by the flora of the "lower buried forest," were decidedly temperate--probably even more genial than they are now, for the forests attained at that time a much greater horizontal and vertical range. This epoch of mild climate and continental connection was eventually succeeded by one of submergence, accompanied by colder conditions. Britain was again insulated--the sea-level in Scotland reaching a height of 45-50 feet above present high-water. To this epoch pertain the Carse-clays of the Forth and Tay. A few erratics occur in these deposits, probably betokening the action of floating ice, but the beds more closely resemble the modern alluvial silts of our estuaries than the tenacious clays of the 100-feet terrace. When the Carse-clays are followed inland, however, they pass into coarse river-gravel and shingle, forming a well-marked high-level alluvial terrace of much the same character as the yet higher-level fluviatile terrace which is associated in like manner with the marine deposits of the 100-feet beach.

Of contemporaneous age with the Carse-clays, with which indeed they are continuous, are the raised beaches at 45-50 feet. These beaches occur at many places along the Scottish coasts, but they are seldom seen at the heads of our sea-lochs. When the sea stood at this level, glaciers of considerable size occupied many of our mountain-valleys. In the west they came down in places to the sea-coast, and dropped their terminal moraines upon the beach-deposits accumulating there. Thus, in Arran[CV] and in Sutherland,[CW] these moraines are seen reposing on the raised beaches of that epoch. And I think it is probable that the absence of such beaches at the heads of many of the sea-lochs of the Highland area is to be explained by the presence there of large glaciers, which prevented their formation.

[CV] _British Association Reports_ (1854): Trans. of Sections, p. 78.

[CW] L. Hinxman: Paper read before Edin. Geol. Soc., April 1892.

Thus, there is clear evidence to show that after the genial epoch represented by the "lower buried forest," a recrudescence of glacial conditions supervened in Scotland. Many of the small moraines that occur at the heads of our mountain-valleys, both in the Highlands and Southern Uplands, belong in all probability to this epoch. They are characterised by their very fresh and well-preserved appearance.[CX] It is not at all likely that these later climatic changes could have been confined to Scotland. Other regions must have been similarly affected. But the evidence will probably be harder to read than it is with us. Had it not been for the existence of our "lower buried forest," with the overlying Carse-deposits, we could hardly have been able to distinguish so readily between the moraines of our "third" glacial epoch and those of the later epoch to which I now refer. The latter, we might have supposed, simply marked a stage in the final retreat of the antecedent great valley-glaciers.

[CX] _Prehistoric Europe_ (chaps. xvi., xvii.) gives a fuller statement of the evidence.

I have elsewhere traced the history of the succeeding stages of the Pleistocene period, and adduced evidence of similar, but less strongly-marked, climatic changes having followed upon those just referred to, and my conclusions have been supported by the independent researches of Professor Blytt in Norway. But these later changes need not be considered here. It is sufficient for my general purpose to confine attention to the well-proved conclusion that after the decay of the last district ice-sheets and great glaciers of our "third" glacial epoch genial conditions obtained, and that these were followed by cold and humid conditions, during the prevalence of which glaciers reappeared in many mountain-valleys.

We have thus, as it seems to me, clear evidence in Europe of four glacial epochs, separated the one from the other by protracted intervals of genial temperate conditions. So far, one's conclusions are based on data which cannot be gainsaid, but there are certain considerations which lead to the suspicion that the whole of the complex tale has not yet been unravelled, and that the climatic changes were even more numerous than those that I have indicated. Let it be noted that glacial conditions attained their maximum during the earliest of our recognised glacial epochs. With each recurring cold period the ice-sheets and glaciers successively diminished in importance. That is one of the outstanding facts with which we have to deal. Whatever may have been the cause or causes of glacial and interglacial conditions, it is obvious that those causes, after attaining a maximum influence, gradually became less effective in their operation. Such having been the case, one can hardly help suspecting that our epoch of greatest glaciation may have been preceded by an alternation of cold and genial stages analogous to those that followed it. If three cold epochs of progressively diminished severity succeeded the epoch of maximum glaciation, the latter may have been preceded by one or more epochs of progressively increased severity. That something of the kind may have taken place is suggested by the occurrence of the old moraine of that great Baltic glacier that preceded the appearance of the most extensive _mer de glace_ of northern Europe. The old moraine in question, it will be remembered, underlies the lower diluvium. Unfortunately, the very conditions that attended the glaciation of Europe render it improbable that any conspicuous traces of glacial epochs that may have occurred prior to the period of maximum glaciation could have been preserved within the regions covered by the great inland-ice. Their absence, therefore, cannot be held as proving that the lower boulder-clays of Britain and northern Europe are the representatives of the earliest glacial epoch. The lowest boulder-clay, I believe, has yet to be discovered.