Part 30

It is in the Alpine Lands that we encounter the most striking evidence of glacial conditions anterior to the epoch of 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 depression of 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 glace_ of 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 southern seas 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 myalis_ bed) 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 the adjacent 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 the _Leda myalis_ bed) 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 myalis_ and _Astarte 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 where _Leda myalis_ and _Astarte borealis_ flourish. 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 myalis_ does 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, with _Elephas 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 yielded _Elephas meridionalis_ and other associated mammalian forms.

[DC] _Elephas meridionalis_ is 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 that _Elephas meridionalis_ and 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:--

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

{2. Forest-bed of Cromer; Hötting breccia; lignites { of Leffe and Pianico; interglacial beds of Interglacial { central France. { { Earliest recognised interglacial epoch; climate { very genial.

{3. 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.; Glacial { terminal moraines of outer zone. { { The epoch of maximum glaciation; the { British and Scandinavian ice-sheets confluent; { the Alpine glaciers attain their greatest development.

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

{5. 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 Glacial { confluent, but _mer de glace_ does 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.

{6. 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). Interglacial { { 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.

{7. 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 Glacial { 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.

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

{9. 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 Glacial { 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 lesser _mer 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.