Part 22

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.

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 Robert Ball 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 with intervening 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.

1. Epoch of Earliest Baltic Glacier. Lowest boulder-clay of southern Sweden; lowest boulder-clay of Baltic provinces of Prussia; horizon of the Weybourn Crag.

2. Epoch of Greatest Mer de Glace. Lower boulder-clays of middle and southern Germany, central Russia, British Islands; second boulder-clay of Baltic provinces of Prussia.

3. Epoch of Lesser Mer de Glace. Upper boulder-clay of western and middle Germany, Poland, and west central Russia; upper boulder-clay of Britain; third boulder-clay of Baltic provinces of Prussia.

4. Epoch of Last Great Baltic Glacier. Upper boulder-clay and terminal moraines of Baltic coast-lands; district and valley-moraines of Highlands and Uplands of British Islands.

5. Epoch of Small Local Glaciers. Valley-moraines in mountainous regions of Britain, etc.

The evidence on which these conclusions are based is set forth at some length in a forthcoming re-issue of my Great Ice Age.--Nov. 1, 1892.]

PLATE IV

SKETCH MAP OF NORTHERN EUROPE SHOWING AREAS COVERED BY ICE DURING THE EPOCH OF MAXIMUM GLACIATION, AND BY THE GREAT BALTIC GLACIER AND THE LOCAL ICE-SHEETS OF BRITAIN AT A LATER DATE.