Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

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.20

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.21 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 paper22 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?

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.23 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.

Again, considerable stress has been laid by Mr. Mackintosh upon the occurrence of chalk-flints in the drift-deposits of Blackpool, Dawpool, Parkgate, Halkin Mountain, Wrexham, the peninsula of Wirral, Runcorn, Delamere, Crewe, Leylands, Piethorne (near Rochdale), and other places. “All these flints,” Mr. Mackintosh remarks, “belong to the basin of the Irish Sea, and have almost certainly crossed the general course of the northern boulders on their way from Ireland.” Here, unfortunately, the Irish Sea intervenes to conceal the evidence that is needed to enable us to track the exact path followed by the erratics in question. I am not so certain as Mr. Mackintosh that the chalk-flints he refers to came from the north of Ireland. Chalk-flints occur pretty numerously in the drift-deposits in the maritime districts of north-eastern Scotland, which we have every reason to believe have been derived from an area of Cretaceous rocks covering the bottom of the adjacent sea; and for aught one can say to the contrary, patches of chalk-with-flints may occur in like manner in the bed of the Irish Sea. I cannot at present remember whether any boulders of the basalt-rocks, which are associated with the Chalk in the north of Ireland, have been recognised in the drifts of the west of England; but if the chalk-flints really came from Antrim, it is more than probable that they would be accompanied by fragments of the hard igneous rocks which overlie the Cretaceous strata of north Ireland. Chalk and chalk-flints occur in the boulder-clay of the Isle of Man, where they are associated, Mr. Horne tells us, with Criffel granite and fragments of a dark trap-rock.24 Possibly these last are basalt-rocks from Antrim. It seems reasonable, therefore, to believe that erratics of Irish origin have found their way to the Isle of Man; and if this be so, it may be permissible to assume that the chalk-flints of Blackpool, etc. (and perhaps also some of the basalt-rocks), have come from the same quarter. Mr. Horne has no doubt that the Irish erratics were brought to the Isle of Man by land-ice. Referring to the conclusion arrived at by Mr. Close that the Irish mer de glace “was probably not less than 3000 feet in depth,” he remarks: “It is highly probable that this great mass of Irish ice succeeded, after a hard battle (i. e., with the Scottish ice-sheet), in reaching the Manx coast-line. It is not to be supposed that the normal momentum of the respective ice-sheets remained constant. The moving force must have varied with changing conditions. On the other hand, it is quite possible that there may have been an ‘under-tow’ of the ice from the north-east coast of Ireland, which would easily account for Antrim chalk and chalk-flints in the Manx till.” I would go further, and state my conviction that before the united ice-sheets had attained their maximum development, it is almost certain that the ice flowing into the Irish Sea basin by the North Channel would for a long time exceed in mass the coalescent glaciers that descended from the Southern Uplands of Scotland, and would therefore be enabled to extend much further to the east than it could at a later date, when the general mer de glace had reached its climax. It might thus have advanced as far as and even beyond the Isle of Man. This inference is based upon the simple fact that the area drained by the mer de glace of the North Channel was very much greater than the area extending from the watershed of the Southern Uplands of Scotland to the Isle of Man. Erratics from the north of Ireland would thus travel down the bed of the North Channel, and eventually be distributed over a wide area up to and possibly even some distance beyond the Isle of Man. But as the Scottish and Cumbrian ice-flows gradually increased in importance, the mer de glace coming from the North Channel would be forced further and further to the west, until the ice-flow issuing from the high-grounds of Kirkcudbright at last succeeded in reaching the middle of the Irish Sea basin. This gradual modification of the general ice-flow in that basin would of course give rise to a redistribution of the ground-moraine, and the Irish erratics would then travel onwards underneath the Scottish ice, and eventually reach the low-grounds of Lancashire and Cheshire, along with erratics from Criffel and the Cumbrian mountains. It is, therefore, quite unnecessary to suppose that the mer de glace of the North Channel actually crossed the whole breadth of the basin of the Irish Sea to invade Lancashire, Cheshire, and north Wales. Had this been the case, chalk-flints, chalk, and many other kinds of rock derived from the north of Ireland, and even from Arran and Argyll, would have abounded in the drifts of the west of England. Erratics coming from Ireland could not possibly have travelled underneath Irish ice further east than the Isle of Man. There or thereabouts, as I have said, the mer de glace of the North Channel would begin to encounter the ice streaming down from the uplands of Galloway and the mountains of Cumberland: and as the ice from these quarters increased in thickness, it would gradually override what had formerly been the bottom-moraine or till of the North Channel mer de glace. Thus Irish erratics would become commingled with erratics from Criffel, etc., and be carried forward in a southerly and south-easterly direction. The chalk-flints in the drifts of Lancashire, Cheshire, etc., are probably therefore remaniés– the relics of the bottom-moraine of the North Channel mer de glace rearranged and redistributed. And this is why they and other Irish rocks are so comparatively rare in the glacial accumulations of the west of England.

Thus all the instances of intercrossings adduced by Mr. Mackintosh as favouring the iceberg theory, and condemning its rival, I would cite as proving exactly the opposite. So far from presenting any real difficulty to an upholder of the land-ice theory, they, in point of fact, as I have already remarked, lend that view additional support.

It is not my purpose to criticise all the arguments and reasons advanced by Mr. Mackintosh in favour of his special views, but I may be allowed a few remarks on the somewhat extraordinary character of the agents which, according to him, were mainly instrumental in producing the drift-phenomena of western England. Before doing so, however, I may point out that, in ascribing the transport of erratics in that region (and, by implication, the formation of the boulder-clays, etc., with which most of these erratics are associated) to floating-ice and sea-currents, Mr. Mackintosh has failed to furnish us with any “fossil evidence” to show that western England was under water at the time the boulder-clays and erratics were being accumulated. He speaks of cold and warm currents, but where do we find any traces of the marine organisms which must have abounded in those waters? Where are the raised sea-beaches which must have marked the retreat of the sea? Where do we encounter any organic relics that might help us to map out the zones of shallow and deep water? The sea-shells, etc., which occur in the boulder-clays are undeniably remaniés; they are erratics just as much as the rock-fragments with which they are associated. Similar assemblages of organic remains are met with in the till of Caithness, where shallow-water and deep-sea shells, and shells indicative of genial and again of cold conditions, are all confusedly distributed throughout one and the same deposit. The same or analogous facts are encountered in the Blocklehm of some parts of Prussia, marine and freshwater shells occurring commingled in the boulder-clay. Nay, even in the moraine profonde of the ancient Rhone glacier, broken and well-preserved shells of Miocene and Pliocene species appear enclosed in the tumultuous accumulation of clay, sand, and erratics. And precisely similar phenomena confront us in the glacial deposits of the neighbourhood of Lago Lugano. Mr. Mackintosh refers to the so-called “stratification” of the boulder-clay, as if that were a proof of accumulation in water. But a rude kind of bedding, generally marked by differences of colour, and sometimes by lines of stones, was the inevitable result of the sub-glacial formation of the boulder-clay. The “lines of bedding” are due to the shearing of the clay under great pressure, and may be studied in the boulder-clay of Switzerland and Italy, and in the till not only of the Lowlands but of the Highlands of Scotland. Occasionally the “lines” are so close that the clay sometimes presents the appearance of rude and often wavy and irregular lamination – a section of such a boulder-clay reminding one sometimes of that of a gnarled gneiss or crumpled schist. And these appearances may be noted in boulder-clays which occupy positions that preclude the possibility of their being marine – as in certain valleys of the Highlands, such as Strathbraan, and in the neighbourhood of Como, in Italy. This “lamination” is merely indicative of the intense pressure to which the till was subjected during its gradual accumulation under the ice. It is assuredly not the result of aqueous action. Aqueous lamination is due to sifting and winnowing – the coarser or heavier and finer or lighter particles being separated in obedience to their different specific gravity, and arranged in layers of more or less regularity according to circumstances. There is nothing of this kind of arrangement, however, in the so-called stratified boulder-clay. If the clay of an individual lamina be washed and carefully sifted, it will be found to be composed of grains of all shapes, sizes, and weights, down to the finest and most impalpable flour. It is impossible to believe that such a heterogeneous assemblage of grains could have been dropt into water without the particles being separated and sifted in their progress to the bottom. Of course, every one knows that patches and beds of laminated clay and sand of veritable aqueous origin occur now and again in boulder-clay. I suppose there is no boulder-clay without them. I have seen them in the till of Italy and Switzerland, where they show precisely the same features as the similar laminated clays in the till of our own islands. But these included patches and beds point merely to the action of sub-glacial waters, such as we know circulate under the glaciers of the Alps, of Norway, and of Greenland.

Again, I would remark that Mr. Mackintosh has ignored all the evidence which has been brought forward from time to time to demonstrate the sub-glacial origin of boulder-clay, and to prove the utter insufficiency of floating-ice to account for the phenomena. And he adduces no new facts in support of the now discredited iceberg theory, unless it be his statement that flat striated rock-surfaces (such as those near Birkenhead) have been caused by floating-ice – the dome-shaped roches moutonnées being, on the other hand the work of land-ice. As a matter of personal observation, I can assure Mr. Mackintosh that flat striated surfaces are by no means uncommonly associated in one and the same region with roches moutonnées. What are roches moutonnées but the rounded relics of what were formerly rough uneven tors, projecting bosses, and prominent rocks? The general tendency of glacial action is to reduce the asperities of a land-surface; hence projecting points are rounded off, while flat surfaces are simply, as a rule, planed smoother. Mr. Mackintosh might traverse acres of such smoothed rock-surfaces in regions where the strata are comparatively horizontal – for example, in the case of the basaltic plateaux of the Faröes and of Iceland, which have certainly been glaciated by land-ice. Similar flat glaciated surfaces are met with again and again both in the Highlands and Lowlands of Scotland, occupying positions and associated with roches moutonnées and till of such a character as to prove beyond any doubt that they no less certainly are the result of the action of land-ice. But it is needless to discuss the probability or possibility of glaciation of any kind being due to floating-ice. We know that glaciers can and do polish and striate rock-surfaces; no one, however, can say the same of icebergs: and until some one can prove to us that icebergs have performed this feat, or can furnish us with well-considered reasons for believing them to be capable of it, glacialists will continue sceptical.