Chapter 13

[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-leavedRhododendron ponticumflourished 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 ofElephas 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, 8esér. i., p. 87.

Two glacial epochs have similarly been admitted for thePyrenees;[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, 2esér. xxiv., p. 577. Jeanbernat:Bull. de Soc. d’Hist. nat. de Toulouse, iv., pp. 114, 138. Piette:Bull. Soc. géol. de France, 3esér. 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 to3 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 ofScrobicularia piperataand 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, seePrehistoric 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 manyof 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 theother 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 extensivemer de glaceof 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.

It is in the Alpine Lands that we encounter the most striking evidence of glacial conditions anterior to the epochof maximum glaciation. The famous breccia of Hötting has already been referred to as of interglacial age. From the character of its flora, Ettinghausen considered this accumulation to be of Tertiary age. The assemblage of plants is certainly not comparable to the well-known interglacial flora of Dürnten. According to the researches of Dr. R. von Wettstein,[CY]the Hötting flora has most affinity with that of the Pontic Mountains, the Caucasus, and southern Spain, and implies a considerably warmer climate than is now experienced in the Inn Valley. This remarkable deposit, as Dr. Penck pointed out some ten years ago, is clearly of interglacial age. His conclusions were at once challenged, on the ground that the flora had a Tertiary and not a Pleistocene facies; consequently, it was urged that, as all glacial deposits were of Pleistocene age, this particular breccia could not be interglacial. But in this, as in similar cases, the palæontologist’s contention has not been sustained by the stratigraphical evidence, and Dr. Penck’s observations have been confirmed by several highly-competent geologists, as by MM. Böhm and Du Pasquier. The breccia is seen in several well-exposed sections resting upon the moraine of a local glacier which formerly descended the northern flanks of the Inn Valley, opposite Innsbruck, where the mountain-slopes under existing conditions are free from snow and ice. Nor is this all, for certain erratics appear in the breccia, which could only have been derived from pre-existing glacial accumulations, and their occurrence in this accumulation at a height of 1150 metres shows that before the advent of the Hötting flora the whole Inn Valley must have been filled with ice. The plant-bearing beds are in their turn covered by the ground-moraine of a later and more extensive glaciation. To bring about the glacial conditions that obtained before the formation of the breccia, the snow-line, according to Penck, must have been at least 1000 metres lower than now; while, to induce the succeeding glaciation, the depressionof the snow-line could not have been less than 1200 metres. These observations have been extended to many other parts of the Alps, and the conclusion arrived at by Professor Penck and his colleagues, Professor Brückner and Dr. Böhm, is briefly this—that the maximum glaciation of those regions did not fall in the “first” but in the “second” Alpine glacial epoch.

[CY]Sitzungsberichte d. Kais. Acad. d. Wissensch. in Wien, mathem.-naturw. Classe, Bd. xcvii. Abth. i., 1888.

The glacial phenomena of northern and central Europe are so similar—the climatic oscillations which appear to have taken place had so much in common, and were on so grand a scale—that we cannot doubt they were synchronous. We may feel sure, therefore, that the epoch of maximum glaciation in the Alps was contemporaneous with the similar epoch in the north. And if this be so, then in the oldest ground-moraines of the Alps we have the records of an earlier glacial epoch than that which is represented by the lower boulder-clays of Britain and the corresponding latitudes of the Continent. In other words, the Hötting flora belongs to an older stage of the Glacial period than any of the acknowledged interglacial accumulations of northern Europe. The character of the plants is in keeping with this conclusion. The flora has evidently much less connection with the present flora of the Alps than the interglacial floras of Britain and northern Europe have with those that now occupy their place. The Hötting flora, moreover, implies a considerably warmer climate than now obtains in the Alpine regions, while that of our interglacial beds indicates a temperate insular climate, apparently much like the present.

The high probability that oscillations of climate preceded the advent of the so-called “first”mer de glaceof northern Europe must lead to a re-examination of our Pliocene deposits, with a view to see whether these yield conclusive evidence against such climatic changes having obtained immediately before Pleistocene times. By drawing the line of separation between the Pleistocene and the Pliocene at the base of our glacial series, the two systems in Britain are strongly marked off the one from the other.There is, in short, a distinct “break in the succession.” From the Cromer Forest-bed, with its abundant mammalian fauna and temperate flora, we pass at once to the overlying arctic freshwater bed and the superjacent boulder-clay that marks the epoch of maximum glaciation.[CZ]Amongst the mammalian fauna of the Forest-bed are elephants (Elephas meridionalis,E. antiquus), hippopotamus, rhinoceros, (R. etruscus), horses, bison, boar, and many kinds of deer, together with such carnivores as bears,Machærodus, spotted hyæna, etc. The freshwater and estuarine beds which contain this extensive fauna rest immediately upon marine deposits (Weybourn Crag), the organic remains of which have a decidedly arctic facies. Here, then, we have what at first sight would seem to be another break in the succession. The Forest-bed, one might suppose, indicated an interglacial epoch, separating two cold epochs. But Mr. Clement Reid, who has worked out the geology of the Pliocene with admirable skill,[DA]has another explanation of the phenomena. It has long been known that the organic remains of the marine Pliocene of Britain denote a progressive lowering of temperature. The lower member of the system is crowded with southern forms, which indicate warm-temperate conditions. But when we leave the Older and pass upwards into the Newer Pliocene those southern forms progressively disappear, while at the same time immigrants from the north increase in numbers, until eventually, in the beds immediately underlying the Forest-bed, the fauna presents a thoroughly arctic facies. During the formation of the Older Pliocene with its southern fauna our area was considerably submerged, so that the German Ocean had then a much wider communication with the seas of lower latitudes. At the beginning of Newer Pliocene times, however, the land emerged to some extent, and all connection between the German Ocean and more southernseas was cut off. When at last the “Forest-bed series” began to be accumulated, the southern half of the North Sea basin had become dry land, and was traversed by the Rhine in its course towards the north, the Forest-bed representing the alluvial and estuarine deposits of that river.

[CZ]In some places, however, certain marine deposits (Leda myalisbed) immediately overlie the Forest-bed.

[DA]Mem. of Geol. Survey, “Pliocene Deposits of Britain.”See postea, footnote, p. 317.

Mr. Reid, in referring to the progressive change indicated by the Pliocene marine fauna, is inclined to agree with Professor Prestwich that this was not altogether the result of a general climatic change. He thinks the successive dying out of southern forms and the continuous arrival of boreal species was principally due to the North Sea remaining fully open to the north, while all connection with southern seas was cut off. Under such conditions, he says, “there was a constant supply of arctic species brought by every tide or storm, while at the same time the southern forms had to hold their own without any aid from without; and if one was exterminated it could not be replaced.” Doubtless the isolation of the North Sea must have hastened the extermination of the southern forms, but the change could not have been wholly due to such local causes. Similar, if less strongly-marked, changes characterise the marine Pliocene of the Mediterranean area, while the freshwater alluvia of France, etc., furnish evidence in the same direction.

The Cromer Forest-bed overlies the Weybourn Crag, the marine fauna of which has a distinctly Arctic facies. The two cannot, therefore, be exactly contemporaneous: the marine equivalents of the Forest-bed are not represented. But Mr. Reid points out that several arctic marine shells of the Weybourn Crag occur also in the Forest-bed, while certain southern freshwater and terrestrial shells common in the latter are met with likewise in the former, commingled with the prevailing arctic marine species. He thinks, therefore, that we may fairly conclude that the two faunas occupied adjacent areas. One can hardly accept this conclusion without reserve. It is difficult to believe that a temperate flora and mammalian fauna like those of the Forest-bed clothed and peopled eastern England when theadjacent sea was occupied by arctic molluscs, etc. Surely the occurrence of a few forms, which are common to the Forest-bed and the underlying Crag, does not necessarily prove that the two faunas occupied adjacent districts. Mr. Reid, indeed, admits that some of the marine shells in the Forest-bed series may have been derived from the underlying Crag. Were the marine equivalents of the Forest-bed forthcoming we might well expect them to contain many Crag forms, but the facies of the fauna would most probably resemble that of the existing North Sea fauna. Again, the appearance in the Weybourn Crag of a few southern shells common to the Forest-bed does not seem to prove more than that such shells were contemporaneous somewhere with an arctic marine fauna. But it is quite possible that they might have been carried for a long distance from the south; and, even if they actually existed in the near neighbourhood of an arctic marine fauna, we may easily attach too much importance to their evidence.[DB]I cannot think, therefore, that Mr. Reid’s conclusion is entirely satisfactory. After all, the Cromer Forest-bed rests upon the Weybourn Crag, and the evidence as it stands is explicable in another way. It is quite possible, for example, that the Forest-bed really indicates an epoch of genial or temperate conditions, preceded, as it certainly was eventually succeeded, by colder conditions.

[DB]The inference that the Forest-bed occupies an interglacial position is strengthened by the evidence of certain marine deposits which immediately overlie it. These (known collectively as theLeda myalisbed) occur in irregular patches, which, from the character of their organic remains, cannot all be precisely of the same age. In one place, for example, they are abundantly charged with oysters, having valves united, and with these are associated other species of molluscs that still live in British seas. At another place no oysters occur, but the beds yield two arctic shells,Leda myalisandAstarte borealis, and some other forms which have no special significance. Professor Otto Torell pointed out to Mr. Reid that these separate deposits could not be of the same age, for the oyster is sensitive to cold and does not inhabit the seas whereLeda myalisandAstarte borealisflourish. From a consideration of this and other evidence Mr. Reid concludes that it is possible that the deposits indicate a period of considerable length, during which the depth of water varied and the climate changed. Two additional facts may be noted:Leda myalisdoes not occur in any of the underlying Pliocene beds, while the oyster is not found in the Weybourn and Chillesford Crag, though common lower down in the Pliocene series. These facts seem to me to have a strong bearing on the climatic conditions of the Forest-bed epoch. They show us that the oyster flourished in the North Sea before the period of the Weybourn Crag—that it did not live side by side with the arctic forms of that period—and that it reappeared in our seas when favourable conditions returned. When the climate again became cold an arctic fauna (including a new-comer,Leda myalis) once more occupied the North Sea.

If it be objected that this would include as interglacial what has hitherto been regarded by most as a Pliocene mammalian fauna,[DC]I would reply that the interglacial age of that fauna has already been proved in central France. The interglacial beds of Auvergne, withElephas meridionalis, rest upon and are covered by moraines,[DD]and with these have been correlated the deposits of Saint-Prest. Again, in northern Italy the lignites of Leffe and Pianico, which, as I showed a number of years ago,[DE]occupy an interglacial position, have likewise yieldedElephas meridionalisand other associated mammalian forms.

[DC]Elephas meridionalisis usually regarded as a type-form of the Newer Pliocene, but long ago Dr. Fuchs pointed out that in Hungary this species is of quaternary age:Verhandl. d. k. k. geolog. Reichsanstalt, 1879, pp. 49, 270. It matters little whether we relegate to the top of the Pliocene or to the base of the Pleistocene the beds in which this species occurs. That it is met with upon an interglacial horizon is certain; and if we are to make the Pleistocene co-extensive with the glacial and interglacial series we shall be compelled to include in that system some portion of the Newer Pliocene.

[DD]Julien:Des Phènoménes glaciaires dans le Plateau central, etc., 1869. Boule:Revue d’Anthropologie, 1879.

[DE]Prehistoric Europe, p. 306. Professor Penck writes me that he and the Swiss glacialist, Dr. Du Pasquier, have recently examined these deposits, and are able to confirm my conclusion as to their interglacial position.

There can be no doubt, then—indeed it is generally admitted—that the cold conditions that culminated in our Glacial period began to manifest themselves in Pliocene times. Moreover, as it can be shown thatElephas meridionalisand its congeners lived in central Europe after an epoch of extensive glaciation, it is highly probable that the Forest-bed, which contains the relics of the same mammalian fauna, is equivalent in age to the early interglacial beds of France and the Alpine Lands. We seem,therefore, justified in concluding that the alternation of genial and cold climates that succeeded the disappearance of the greatest of our ice-sheets was preceded by analogous climatic changes in late Pliocene times.

I shall now briefly summarise what seems to have been the glacial succession in Europe:—

Weybourn Crag; ground-moraine of great Baltic glacier underlying lower diluvium; the oldest recognised ground-moraines of central Europe.

These accumulations represent the earliest glacial epoch of which any trace has been discovered. It would appear to have been one of considerable severity, but not so severe as the cold period that followed.

Forest-bed of Cromer; Hötting breccia; lignites of Leffe and Pianico; interglacial beds of central France.

Earliest recognised interglacial epoch; climate very genial.

Lower boulder-clays of Britain; lower diluvium of Scandinavia and north Germany (in part); lower glacial deposits of south Germany and central Russia; ground-moraines and high-level gravel-terraces of Alpine Lands, etc.; terminal moraines of outer zone.

The epoch of maximum glaciation; the British and Scandinavian ice-sheets confluent; the Alpine glaciers attain their greatest development.

Interglacial freshwater alluvia, peat, lignite, etc., with mammalian remains (Britain, Germany, etc., central Russia, Alpine Lands, etc.); and marine deposits (Britain, Baltic coast-lands).

Continental condition of British area; climate at first cold, but eventually temperate. Submergence ensued towards close of the period, with conditions passing from temperate to arctic.

Upper boulder-clay of Britain; lower diluvium of Scandinavia, Germany, etc., in part; upper glacial series in central Russia; ground-moraines and gravel-terraces in Alpine Lands.

Scandinavian and British ice-sheets again confluent, butmer de glacedoes not extend quite so far as that of the preceding cold epoch. Conditions, however, much more severe than those of the next succeeding cold epoch. Alpine glaciers deposit the moraines of the inner zone.

Freshwater alluvia, lignite, peat, etc. (some of the so-called post-glacial alluvia of Britain; interglacial beds of north Germany, etc.; Alpine Lands(?); marine deposits of Britain and Baltic coast-lands).

Britain probably again continental; climate at first temperate and somewhat insular; submergence ensues with cold climatic conditions—Scotland depressed for 100 feet; Baltic provinces of Germany, etc., invaded by the waters of the North Sea.

Ground-moraines, terminal moraines, etc., of the mountain regions of Britain; upper diluvium of Scandinavia, Finland, north Germany, etc.; great terminal moraines of same regions; terminal moraines in the large longitudinal valleys of the Alps (Penck).

Major portion of Scottish Highlands covered by ice-sheet; local ice-sheets in Southern Uplands of Scotland and mountain districts in other parts of Britain; great valley-glaciers sometimes coalesce on low-grounds; icebergs calved at mouths of Highland sea-lochs; terminal moraines dropped upon marine deposits then forming (100-feet beach). Scandinavia shrouded in a great ice-sheet, which broke away in icebergs along the whole west coast of Norway. Epoch of the last great Baltic glacier.

Freshwater alluvia (with arctic plants); “lower buried forest and peat” (Britain and north-west Europe generally). Carse-clays and raised beaches of 45 to 50-feet level in Scotland.

Britain again continental; climate at first cold, subsequently becoming temperate: great forests. Eventual insulation of Britain; climate humid, and probably colder than now.

Local moraines in mountain-valleys of Britain, here and there resting on 45 to 50-feet beach; so-called “post-glacial” moraines in the upper valleys of the Alps.

Probably final appearance of glaciers in our islands. Some of these glaciers attained a considerable size, reaching the sea and shedding icebergs. It may be noted here that the decay of these latest glaciers was again followed by emergence of the land and a recrudescence of forest-growth (“upper buried forest”).

A word of reference may now be made to that remarkable association of evidence of submergence, with proofs of glacial conditions, which has so frequently been noted by geologists. Take, for example, the succession in Scotland, and observe how each glacial epoch was preceded and apparently accompanied by partial submergence of the land:—

1.Epoch of Greatest Mer de Glace(lower boulder-clay); British and Scandinavian ice-sheets coalescent. Followed by wide land-surface = Continental Britain, with genial climate. Submergence of land—to what extent is uncertain, but apparently to 500 feet or so.

2.Epoch of Lesser Mer de Glace(upper boulder-clay); British and Scandinavian ice-sheets coalescent. Followed by wide land-surface = Continental Britain, with genial climate. Submergence of land for 100 feet or thereabout.

3.Epoch of Local Ice-sheets in Mountain Districts;glaciers here and there coalesce on the low-grounds; icebergs calved at mouths of Highland sea-lochs (moraines on 100-feet beach). Followed by wide land-surface = Continental Britain, with genial climate. Submergence of land for 50 feet or thereabout.

4.Epoch of Small Local Glaciers, here and there descending to sea (moraines on 50-feet beach).

These oscillations of the sea-level did not terminate with the emergence of the land after the formation of the 50-feet beach. There is evidence to show that subsequent to the retreat of the small local glaciers (4) and the emergence of the land, our shores extended seawards beyond their present limits, but how far we cannot tell. With this epoch of re-emergence the climate again became more genial, our forests once more attaining a greater vertical and horizontal range. Submergence then followed (the 25 to 30-feet beach), accompanied by colder and more humid conditions, which, while unfavourable to forest-growth, tended greatly to increase the spread of peat-bogs. We have no evidence, however, to show that small local glaciers again appeared. Finally the sea retired, and the present conditions ensued.

It will be seen that the submergence which preceded and probably accompanied the advent of the lessermer de glace(2) was greater than that which heralded the appearance of the local ice-sheets (3), as that in turn exceeded the depression that accompanied the latest local glaciers (4). There would seem, therefore, to be some causal connection between cold climatic conditions and submergence. This is shown by the fact that not only did depression immediately precede and accompany the appearance of ice-sheets and glaciers, but the degree of submergence bore a remarkable relation to the extent of glaciation. Many speculations have been indulged in as to the cause of this curious connection between glaciation and depression; these, however, I will not consider here. None of the explanations hitherto advanced is satisfactory, but the question is one well deserving the attention of physicists, and its solution would be of great service to geology.

A still larger question which the history of these times suggests is the cause of climatic oscillations. I have maintained that the well-known theory advanced by James Croll is the only one that seems to throw any light upon the subject, and the observations which have been made since I discussed the question at length, some fifteen years ago, have added strength to that conviction. As Sir RobertBall has remarked, the astronomical theory is really much stronger than Croll made it out to be. In his recently-published work,The Cause of an Ice Age, Sir Robert says that the theory is so thoroughly well based that there is no longer any ground for doubting its truth. “We have even shown,” he continues, “that the astronomical conditions are so definite that astronomers are entitled to direct that vigorous search be instituted on this globe to discover the traces of those vast climatic changes through which astronomy declares that our earth must have passed.” In concluding this paper, therefore, I may shortly indicate how far the geological evidence seems to answer the requirements of the theory.

Following Croll, we find that the last period of great eccentricity of the earth’s orbit extended over 160,000 years—the eccentricity reaching its highest value in the earlier stages of the cycle. It is obvious that during this long cycle the precession of the equinox must have completed seven revolutions. We might therefore expect to meet with geological evidence of recurrent cold or glacial and genial or interglacial epochs; and not only so, but the records ought to show that the earlier glacial epoch or epochs were colder than those that followed. Now we find that the epoch of maximum glaciation supervened in early Pleistocene times, and that three separate and distinct glacial epochs of diminished severity followed. Of these three, the first would appear to have been almost as severe as that which preceded it, and it certainly much surpassed in severity the cold epochs of the later stages. But the epoch of maximum glaciation, or the first of the Pleistocene series, was not the earliest glacial epoch. It seems to have been preceded by one of somewhat less severity than itself, but which nevertheless, as we gather from the observations of Penck and his collaborators, was about as important as that which came after the epoch of maximum glaciation. Hence it would appear that the correspondence of the geological evidence with the requirements of the astronomical theory is as close as we could expect it to be. Four glacial withintervening genial epochs appear to have fallen within Pleistocene times; while towards the close of the Pliocene, or at the beginning of the Pleistocene period, according as we choose to classify the deposits, an earlier glacial epoch followed by genial interglacial conditions, supervened.

In this outline of a large subject it has not been possible to do more than indicate very briefly the general nature of the evidence upon which the chief conclusions are based. I hope, however, to have an opportunity ere long of dealing with the whole question in detail.

[Note.—Since the original publication of this Essay, renewed investigation and study have led me to conclude that the correlation of the British and Continental glacial series is even more simple than I had supposed. I believe the use of the terms “Lower” and “Upper” in connection with the “Diluvial” deposits of the Continent has hitherto blinded us to the obvious succession of the boulder-clays. In Britain we have, as shown above, a “lower boulder-clay,” an “upper boulder-clay,” and the still younger boulder-clays (ground-moraines), and terminal moraines of our district ice-sheets and valley-glaciers. In the low-grounds of the Continent the succession is precisely similar. Thus the lower boulder-clay that sweeps south into Saxony represents the lower boulder-clay of Britain. In like manner, the upper boulder-clay of western and middle Germany, of Poland, and western and north-western Russia, is the equivalent of our own upper boulder-clay. Lastly, the so-called “upper diluvium” and the great terminal moraines of the Baltic coast-lands are on the horizon of the younger boulder-clays and terminal moraines of the mountainous areas of the British Islands. The so-called “lower diluvium” of the Baltic coast-lands thus represents not thelowerbut theupperdiluvium of western and middle Germany, Poland, etc. German geologists are of opinion that the upper boulder-clays of the Baltic coast-lands and of the valley of the Elbe are the ground-moraines of one and the same ice-sheet, which, on its retreat, piled up the terminal moraines of the Baltic Ridge. I believe the two boulder-clays in question are quite distinct, and that the terminal moraines referred to mark the furthest advance of the last great Baltic glacier. The contemporaneity of the two boulder-clays has been taken for granted simply because they are each underlaid by a lower boulder-clay. But, as we have seen, the upper boulder-clay of the Baltic coast-lands is underlaid not by one only, but by two, and in some places even by three other boulder-clays—phenomena which never present themselves in the regions not invaded by the last great Baltic glacier. Three or four boulder-clays occur in the coast-lands of the Baltic because those regions were overflowed successively by three or four separate ice-sheets. Only two boulder-clays are met with south and east of the Baltic Ridge, because the tracts lying south and south-east of that ridge were traversed by only twomers de glace—namely, by that of the epoch of maximum glaciation and by the less extensive ice-sheet of the next succeeding cold period. In the region between the Elbe and the mountains of middle Germany only one boulder-clay appears, because that region has never been invaded by more than one ice-sheet. The succession thus indicated may be tabulated as follows:—

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