I may now mention some of the more remarkable examples of intercrossings of erratics which have been recorded from the glacial accumulations of north Germany, etc. An examination of the glacial striæ, roches moutonnées, and boulder-clays of Saxony leads to the conviction, according to Credner, Penck, Torell, Helland, and others, that the whole of that region has been invaded by the great Scandinavian mer de glace which flowed into Saxony from NNE. to SSW. Erratics from southern Sweden and Gothland occur in the boulder-clay, and the presence of these, taken in connection with the direction of the glaciation, leaves us no alternative but to agree with the conclusions arrived at by the Saxon geologists. But, apparently in direct contradiction of this conclusion, we have evidence to show that boulders of the same kinds of rock occur in Denmark and Holland, pointing to a former ice-flow from north-east to south-west and west. Thus boulders derived from Gothland occur at Gröningen in Holland, while fragments from the island of Öland are met with in Faxö; and erratics from the borders of the Gulf of Finland are encountered at Hamburg. Indeed, when geologists come to examine the erratics in north Germany and Poland generally, they find evidence of apparently two ice-flows—one of which went south-south-west, south, and south-east—spreading out, as it were, in a fan-shape towards the southern limits reached by the great “Northern Drift,”—while the other seems to have followed the course of the Baltic depression, overflowing the low-grounds of northern Prussia, Holland, etc., in a south-west and west direction. Now, it is quite evident that no one mer de glace could have followed these various directions at one and the same time. The explanation of the apparent anomaly, however, is not far to seek. It is reasonable to infer that long before the mer de glace had attained its maximum dimensions, when as yet it was confined to the basin of the Baltic and was only able to overflow the northern regions of Prussia, etc., its course would be determined by the contour of the pavement upon which it advanced. It would, therefore, be compelled to follow the Baltic depression, and for a long time it would carry erratics from Finland, the Baltic islands, and eastern Sweden in a south-west and west-south-west direction. And this would continue to be the direction even after a considerable portion of the low-grounds of Prussia, etc., had been overflowed. But when the ice-sheet was enabled to advance south into Saxony, Poland, and Lithuania, erratics from Finland, the Baltic islands, etc., would necessarily cease to travel towards the west, and hold on a south-south-east, south, and south-south-west course. Again, when the mer de glace was on the decline, a time would return when the ice, as before, would be controlled in its flow by the Baltic depression, and this would give rise to a further distribution of erratics in a prevalent west-by-south direction.[T]
[T] For a fuller discussion of the distribution of erratics on the Continent, I may refer to Appendix, Note B, in Prehistoric Europe, where the reader will find references to the literature of this interesting subject. [Continental geologists now recognise a distinct stage of the Ice Age, during which their “Upper Diluvium” was deposited by a great glacier that occupied the basin of the Baltic. This “Great Baltic Glacier” appears to have been contemporaneous with the local ice-sheets and valley-glaciers of the Highlands and other mountain-tracts of our island. See Article X. 1892.]
No one of late years has been more assiduous in the collection of facts relating to the intercrossing of erratics in the drift-deposits of England than Mr. D. Mackintosh.[U] He has written many instructive and interesting descriptions of the phenomena in question, which he justly thinks are of prime importance from a theoretical point of view. In a recent paper[V] he presents us with the results of a systematic survey of the direction and limits of dispersion of the erratics of the west of England and east of Wales, which he evidently is of opinion afford strong support to the iceberg theory, while at the same time they are directly opposed to the theory of transport by land-ice. I have attentively considered all the arguments advanced by Mr. Mackintosh in favour of his views—the one upon which he apparently lays most stress being that of the intercrossings of erratics observed by him—and I shall now proceed to point out how the phenomena described by him are most satisfactorily explained by the land-ice theory. They seem to me, indeed, to lend additional support to that theory, in the same manner as the intercrossings of boulders observed in Scotland, northern Germany, etc., and sub-alpine regions of France. Mr. Mackintosh calls attention to the fact that erratics of the well-known Criffel granite are found scattered over a large part of the plain of Cumberland, from which they extend south along the coast to near the mouth of the estuary of the Duddon. They reappear on the coast in the neighbourhood of Blackpool and Liverpool, and again at intervals on the coasts of north Wales from Flint to Colwyn Bay, and thence to Penmaenmawr and the neighbourhood of Beaumaris. They are dispersed over the peninsula of Wirral and the Cheshire plain, etc., and they have been followed south-east as far as the neighbourhood of Cardington, near Church Stretton, Burton, Wolverhampton, Stafford, Hare Castle, Macclesfield, and Manchester. This great stream of boulders, therefore, spreads out to south-east, south, and south-west: the erratics, to quote Mr. Mackintosh, “have radiated from an area much smaller than their terminal breadth.” The same is the case, I may remark in passing, with erratics in the boulder-clays of Scotland, Scandinavia, north Germany, etc., as also with those in the drift-deposits of the great Rhone glacier and other ancient glaciers both on the north and south side of the Alps. Now, the course followed by the Criffel erratics is crossed at an acute angle by the path pursued by many boulders of Eskdale granite, and various felspathic rocks derived from the Cumberland mountains. For example, Cumberland erratics of the kinds mentioned occur near St. Asaph and Moel-y-Tryfane and in Anglesey, and they have been followed over a wide district in Cheshire, etc., extending as far south as Church Stretton and Wolverhampton, and as far east as Rochdale. More than this, we find that numerous erratics of felstone, derived from the mountain of Great Arenig, in north Wales, have gone to north-east as far as Halkin Mountain, in Flintshire, Eryrys, near Llanarmon, and Chirk, from which last-named place they have been traced in a south-easterly direction to Birmingham, Bromsgrove, etc. A glance at the map of England will show that this south-easterly drift of erratics crosses at an acute angle the paths followed by the Criffel granite boulders and the erratics derived from Cumberland, so that we have now several intercrossings to account for. How can this be done by the land-ice theory?
[U] This enthusiastic geologist died in 1891.
[V] Quart. Journ. Geol. Soc., vol. xxxv. p. 425
The explanation seems to me obvious, for the phenomena are, after all, less striking than similar appearances which have been observed in Scotland, especially by my colleagues, Messrs. Peach and Horne, in Caithness and the Orkney and Shetland Islands; and they are certainly less intricate than the facts recorded by MM. Falsan and Chantre concerning the intercrossing, interosculation, and direct opposition of erratic paths in Savoy and Dauphiny. We have only to reflect that the great mer de glace—to which, as I believe, all the English phenomena are due—did not come into existence and attain its maximum dimensions in the twinkling of an eye, nor could it afterwards have disappeared in the same sudden manner. On the contrary, a period of local glaciation must have preceded the appearance of the great ice-sheet. At first, and for a long time, permanent snow would be confined to the higher elevations of the land, and glaciers would be limited to mountain-valleys; but as the temperature fell the snow-line would gradually descend, until at last, probably after a prolonged period, it reached what is now the sea-level. Thus the formation of névé and glacier-ice would eventually take place over what are now our low-grounds, and other tracts also, which are now submerged. It is quite impossible that the vast sheets of ice which can be demonstrated to have covered Scotland, a large part of England, Ireland, Scandinavia, and north Germany, and even the limited area of the Faröe Islands, could possibly have been fed by the snow-fields of mountain-heights only. The precipitation and accumulation of snow, and the formation of névé and glacier-ice, must have taken place over enormous regions in what are now the temperate latitudes of Europe.
It is obvious that the direction of ice-flow in the basin of the Irish Sea opposite the south of Scotland and the west of England, while preserving a general southerly trend, would vary at different periods. Before the mer de glace in that basin had attained its climax there must have been a time when the ice, streaming outwards from the high-grounds of Cumberland, was enabled to push its way far westward out into the basin of the Irish Sea. At that time it was still able to hold its own against the pressure exerted by the Scottish ice. But as the general mer de glace increased in thickness, the course of the Cumberland ice would be diverted ever further and further to the south-east, until, eventually, the Scottish ice came to hug the coast of Cumberland, and to overflow Lancashire in its progress towards the south-east. So gorged with ice did the basin of the Irish Sea become, that a portion of the Scottish ice was forced over the plain of Cumberland and up the valley of the Eden, where it coalesced with the ice coming north from the Shap district, and thereafter flowed in an easterly direction to join the great mer de glace of the North Sea basin.
Thus the intercrossings of the Criffel and Cumberland erratics described by Mr. Mackintosh receive a ready explanation by the land-ice theory. Nor do the intercrossings of the Welsh erratics with those derived from Scotland and Cumberland offer any difficulty. The ice coming from the Welsh mountains would naturally be deflected towards south-east by the mer de glace that streamed in that direction, and might quite well have carried its characteristic boulders as far as Birmingham before the general mer de glace had attained its greatest dimensions. But when that period of maximum glaciation arrived, the Welsh boulders would be unable to travel so far towards the east, and the Scottish and Cumberland boulders would then cross the path formerly followed by the felstone erratics from Great Arenig.
Again, it is evident that when the mer de glace was gradually decreasing similar oscillations of the ice-flow would take place, but in reverse order, and thus would give rise to a second series of intercrossings. Moreover, we must remember that the Glacial period was characterised by several great changes of climate. It was not one continuous and prolonged period of cold conditions, but consisted rather of a succession of arctic and genial climates; so that the same countries were overrun at different epochs by successive mers de glace, each of which would rework, denude, and redistribute to a large extent the morainic materials of its predecessor, and thus might well cause even greater complexity in the dispersion of erratics than has yet been recognised anywhere in these islands.
Mr. Mackintosh refers to the occurrence of chalk-flints and Lias fossils associated with northern erratics in the drift-deposits of the west of England, the presence of which, he thinks, is fatal to the theory of transport by land-ice. Thus, he says, chalk-flints, etc., have been met with at Lillieshall (east of Wellington), at Strethill (near Ironbridge), at Seisdon (between Wolverhampton and Bridgenorth), at Wolverhampton, near Stafford, and near Bushbury. Chalk-flints have also been found as far west as Malvern and Hatfield Camp, south of Ledbury. All these erratics have crossed England from the east, according to Mr. Mackintosh and other observers. Not only so, but, as Mr. Mackintosh remarks, those found at Wolverhampton, Birmingham, etc., “must have crossed the course of the northern boulders near its southerly termination.” And since both northern and eastern erratics are found associated in the same drift-deposit, it seems to him “impossible to explain the intercrossing by land-ice or glaciers.” Now, on the contrary, those eastern erratics are scattered over the very districts where I should have expected to find them. The observations of geologists in East Anglia have shown that that region has been invaded by the mer de glace of the North Sea basin.[W] This remarkable glacial invasion is proved not only by the direction followed by stones of local derivation, and by boulders which have come south from Scotland and the northern counties, but by the occurrence in the boulder-clay at Carnelian Bay and Holderness of erratics of certain well-known Norwegian rocks, which have been recognised by Mr. Amund Helland. The occurrence of chalk-flints and fragments of Oolitic rocks in the neighbourhoods mentioned by Mr. Mackintosh thus only affords additional evidence in favour of the land-ice origin of the drift-deposits described by him. The mer de glace that flowed down the east coast of England seems to have encroached more and more upon the land, until eventually it swept over the low-lying Midlands in a south-westerly direction, and coalesced with the mer de glace that streamed inland from the basin of the Irish Sea, and the ice that flowed outwards from the high-grounds of Wales. The united ice-stream would thereafter continue on its south-westerly course down the Severn valley to the Bristol Channel. I have no doubt that Mr. Mackintosh will yet chronicle the occurrence of chalk-flints and other eastern erratics from localities much further to the south than Ledbury.