Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

But leaving these and other points which serve to show the weakness of the cause which Mr. Mackintosh supports with such keen enthusiasm, I may, in conclusion, draw attention to certain very remarkable theoretical views of his which seem to me to be not only self-contradictory, but opposed to well-known natural laws. Briefly stated, his general view is that the erratics of the west of England have been distributed by floating-ice during a period of submergence – the scattering of erratics and the accumulation of the associated glacial deposits having commenced at or about the time when the land began to sink, and continued until the submergence reached some 2000 feet below the present sea-level. In applying this hypothesis to explain the phenomena, Mr. Mackintosh makes rather free use of sea-currents and winds. For example, he holds that a current coming from Criffel carried with it boulder-laden ice which flowed south-west to the Isle of Man, south to north Wales, and south-east in the direction of Blackpool and Manchester, Liverpool and Wolverhampton, Dawpool and Church Stretton. Now, in the first place, it is very strange that there is not a vestige or trace of any such submergence, either in the neighbourhood of Criffel itself or in the region to the north of it. The whole of that region has been striated and rubbed by land-ice coming down from the watershed of the Galloway mountains, to the north of which the striæ, roches moutonnées, and tracks followed by erratics, indicate an ice-flow towards the north-west, north, and north-east. It is, therefore, absolutely certain that at the time the granite erratics are supposed to have sailed away from Criffel on floating-ice, the whole of the Southern Uplands of Scotland were covered with a great ice-field extending from Wigtown to Berwickshire; so that, according to Mr. Mackintosh’s hypothesis, we should be forced to believe that an ocean-current originated in Criffel itself! But waiving this and other insuperable objections which will occur to any geologist who is familiar with the glacial phenomena of the south of Scotland, and confining myself to the evidence supplied by the English drifts, I would remark that Mr. Mackintosh’s hypothesis is not consistent with itself. A current flowing in the direction supposed could not possibly have permitted floating-ice to sail from Cumbria to the Isle of Man, to Moel-y-Tryfane and Colwyn Bay. Mr. Mackintosh admits this himself, but infers that the transport of the Cumbrian erratics may have taken place at a different time. But how could this be, seeing that the Criffel and Cumbrian erratics occur side by side in one and the same deposit? Again, the hypothesis of an ocean-current coming from Criffel is inconsistent with the presence of the Irish chalk-flints in the drifts of the west of England. Did these also come at a different time? And what about the dispersion of erratics from Great Arenig, which have gone north-east and north-north-east, almost exactly in the face of the supposed Criffel current? Here an ocean-current is obviously out of the question; and accordingly we are told that this dispersion of Welsh boulders was probably the result of wind. But why should this wind have propelled the floating-ice so far and no further in an easterly direction? Surely if floating-ice was swept outwards from Great Arenig as far as Eryrys, bergs must have been carried now and again much further to the east. And if they did not sail eastwards, what became of them? Did they all melt away immediately when they came into the ice-laden current that flowed towards the south-east?25 A still greater difficulty remains. The Criffel and Cumbrian erratics suddenly cease when they are followed to the south, great quantities of them being accumulated over a belt of country extending from beyond Wolverhampton to Bridgenorth. What was it that defined the southern limits of these northern boulders? It is clear that it could not have been high-ground, for the Severn valley, not to speak of low-lying regions further to the north-east, must have been submerged according to Mr. Mackintosh’s hypothesis. There was therefore plenty of sea-room for the floating-ice to escape southwards. And yet, notwithstanding this, vast multitudes of bergs and floes, as soon as they arrived at certain points, suddenly melted away and dropt their burdens! In what region under the sun does anything like that happen at the present day? Mr. Mackintosh thinks that the more or less sharply-defined boundary-line reached by the erratics “could only have resulted from close proximity to a persistent current of water (or air?) sufficiently warm to melt the boulder-laden ice.” He does not tell us, however, where this warm current of water or air came from, or in what direction it travelled. He forgets some of his own facts connected with the appearance of erratics of eastern derivation, and which, according to him, point to an ocean-current that flowed across from Lincolnshire into the very sea in which the Criffel granite and Cumbrian boulders were being dropt. The supposed warm ocean-current, then, if such it was rather than air, could hardly have come from the east. Neither is it at all likely that it could have come from the west, sheltered as the region of the Severn valley must have been by the ice-laden mountains of Wales. Again, the south is shut to us; for there are no erratics in the south of England from which to infer a submergence of that district. If it be true that all the northern erratics which are scattered over the low-grounds of England, Denmark, Holland, Germany, Poland, and Russia, owe their origin to boulder-laden ice carried by ocean-currents, no such warm water as Mr. Mackintosh desiderates could possibly have come from the east or south-east. We are left, then, to infer that the supposed warm current26 must have flowed up the Severn valley directly in the face of the Criffel current, underneath which it suddenly plunged at a high temperature, the line of junction between it and the cold water being sharply defined, and retaining its position unchanged for a long period of time! However absurd this conclusion may be, it is forced upon us if we admit the hypothesis at present under review. For we must remember that the floating-ice is supposed to have melted whenever it came into contact with the warm current. The erratics occur up to a certain boundary-line, where they are concentrated in enormous numbers, and south of which they do not appear. Here, then, large and small floes alike must have vanished at once! Certainly a very extraordinary case of dissolution.

If we dismiss the notion of a warm ocean-current for that of a warm wind, we do not improve our position a whit. Where did the warm wind come from? Not, certainly, from the ice-laden seas to the east. Are we to suppose, then, that it flowed in from the south or south-west? If so, we might well ask how it came to pass that in the immediate proximity of such a very warm wind as the hypothesis demands, great snow-fields and glaciers were allowed to exist in Wales? Passing that objection, we have still to ask how this wind succeeded in melting large and small masses of floating-ice with such rapidity that it prevented any of them ever trespassing south of a certain line? It is obvious that it must have been an exceedingly hot wind; and that, just as the hypothetical warm ocean-current must have suddenly dived under the cold water coming from the north, so the hot wind, after passing over the surface of the sea until it reached a certain more or less well-defined line, must have risen all at once and flowed vertically upwards into the cold regions above.

Thus, in seeking to escape from what he doubtless considers the erroneous and extravagant views of “land-glacialists,” Mr. Mackintosh adopts a hypothesis which lands him in self-contradictions and a perfect “sea of troubles” – a kind of chaos, in fact. In attempting to explain the drifts of western England and east Wales he has ignored the conditions that must have obtained in contiguous regions – thus forgetting that “nothing in the world is single,” and that one ought not to infer physical conditions for one limited area without stopping to inquire whether these are in consonance with what is known of adjacent districts, or in harmony with the existing phenomena of nature.

I have so strongly opposed Mr. Mackintosh’s explanation of the sudden termination of the northern erratics in the neighbourhood of Wolverhampton and elsewhere, that perhaps I ought to offer an explanation of my own, that it may, in its turn, undergo examination. I labour under the disadvantage, however, of not having studied the drifts in and around Wolverhampton, etc., and the suggestion which I shall throw out must therefore be taken for what it is worth. It seems to me, then, that the concentration of boulders in the neighbourhood of Wolverhampton, and the limits reached by the northern erratics generally, mark out, in all probability, the line of junction between the mer de glace coming from the basin of the Irish Sea and that flowing across the country from the vast mer de glace that occupied the basin of the German Ocean. Along this line the southerly transport of the northern boulders would cease, and here they would therefore tend to become concentrated. But it is most likely that now and again they would get underneath the ice-flow that set down the Severn valley, and I should anticipate that they will yet be detected, along with erratics of eastern origin, as far south even as the Bristol Channel. If it be objected to this view that erratics from Great Arenig have been met with south of Wolverhampton, at Birmingham and Bromsgrove, I would reply that these erratics were probably carried south either before or after the general mer de glace had attained its climax – at a period when the Welsh ice was able to creep out further to the east than it could when the invasion of the North Sea ice was at its height.

I cannot conclude this paper without expressing my admiration for the long-continued and successful labours of the well-known geologist whose views I have been controverting. Although I have entered my protest against his iceberg hypothesis, and have freely criticised his theoretical opinions, I most willingly admit that the practical results of his unwearied devotion to the study of those interesting phenomena with which he is so familiar have laid all his fellow-workers under a debt of gratitude.

VIII.

Recent Researches in the Glacial Geology of the Continent. 27

THE President of this section must often have some difficulty in selecting a subject for his address. It is no longer possible to give an interesting and instructive summary of the work done by the devotees of our science during even one year. So numerous have the students of geological science become – so fertile are the fields they cultivate – so abundant the harvests they reap, that one in my present position may well despair of being able to take stock of the numerous additions to our knowledge which have accumulated within the last twelve months. Neither is there any burning question which at this time your President need feel called upon to discuss. True, there are controversies that are likely to remain unsettled for years to come – there are still not a few matters upon which we must agree to differ – we do not yet see eye to eye in all things geological. But experience has shown that as years advance truth is gradually evolved, and old controversies die out, and so doubtless it will continue to be. The day when controversies shall cease, however, is yet, I hope, far in the future; for should that dull and unhappy time ever arrive, it is quite certain that mineralogists, petrologists, palæontologists, and geologists shall have died out of the world. Following the example of many of my predecessors, I shall confine my remarks to certain questions in which I have been specially interested; and in doing so I shall endeavour to steer clear, as far as I can, of controversial matters. My purpose, then, is to give an outline of some of the results obtained during the last few years by Continental workers in the domain of glacial geology.

Those who are not geologists will probably smile when they hear one declare that wielders of the hammer are extremely conservative – that they are slow to accept novel views, and very tenacious of opinions which have once found favour in their eyes. Nevertheless, such is the case, and well for us that it is so. However captivating, however imposing, however strongly supported by evidence a new view may appear to be, we do well to criticise, to sift the evidence, and to call for more facts and experiments, if such are possible, until the proofs become so strong as to approach as near a demonstration as geologists can in most cases expect such proofs to go. The history of our science, and indeed of most sciences, affords abundant illustration of what I say. How many long years were the views of sub-aërial erosion, as taught by Hutton and Playfair, canvassed and controverted before they became accepted! And even after their general soundness had been established, how often have we heard nominal disciples of these fathers of physical geology refuse to go so far as to admit that the river-valleys of our islands have been excavated by epigene agents! If, as a rule, it takes some time for a novel view to gain acceptance, it is equally true that views which have long been held are only with difficulty discarded. Between the new and the old there is a constant struggle for existence, and if the latter should happen to survive, it is only in a modified form. I have often thought that a history of the evolution of geological theories would make a very entertaining and instructive work. We should learn from it, amongst other things, that the advance of our science has not always been continuous – now and again, indeed, it has almost seemed as if the movement had been retrograde. Knowledge has not come in like an overwhelming flood – as a broad majestic river – but rather like a gently-flowing tide, now advancing, now retiring, but ever, upon the whole, steadily gaining ground. The history I speak of would also teach us that many of the general views and hypotheses which have been from time to time abandoned as unworkable, are hardly deserving of the reproach and ridicule which we in these latter days may be inclined to cast upon them. As the Scots proverb says: “It is easy to be wise behindhand.” It could be readily shown that not a few discarded notions and opinions have frequently worked for good, and have rather stimulated than checked inquiry. Such reflections should be encouraging to every investigator, whether he be a defender of the old or an advocate of the new. Time tries all, and each worker may claim a share in the final establishment of the truth.

Perhaps there is no department of geological inquiry that has given rise to more controversy than that which I have selected for the subject of this address. Hardly a single step in advance has been taken without vehement opposition. But the din of contending sides is not so loud now – the dust of the conflict has to some extent cleared away, and the positions which have been lost or maintained, as the case may be, can be readily discerned. The glacialist who can look back over the last twenty-five years of wordy conflict has every reason to be jubilant and hopeful. Many of those who formerly opposed him have come over to his side. It is true he has not had everything his own way. Some extreme views have been abandoned in the struggle; that of a great Polar ice-sheet, for example, as conceived of by Agassiz. I am not aware, however, that many serious students of glacial geology ever adopted that view. But it was quite an excusable hypothesis, and has been abundantly suggestive. Had Agassiz lived to see the detailed work of these later days, he would doubtless have modified his notion and come to accept the view of large continental glaciers which has taken its place.

The results obtained by geologists who have been studying the peripheral areas of the drift-covered regions of our Continent, are such as to satisfy us that the drifts of those regions are not iceberg-droppings, as we used to suppose, but true morainic matter and fluvio-glacial detritus. Geologists have not jumped to this conclusion – they have only accepted it after laborious investigation of the evidence. Since Dr. Otto Torell, in 1875, first stated his belief that the Diluvium of north Germany was of glacial origin a great literature on the subject has sprung up, a perusal of which will show that with our German friends glacial geology has passed through much the same succession of controversial phases as with us. At first icebergs are appealed to as explaining everything – next we meet with sundry ingenious attempts at a compromise between floating-ice and a continuous ice-sheet. As observations multiply, however, the element of floating-ice is gradually eliminated, and all the phenomena are explained by means of land-ice and “Schmelz-wasser” alone. It is a remarkable fact that the iceberg hypothesis has always been most strenuously upheld by geologists whose labours have been largely confined to the peripheral areas of drift-covered countries. In the upland and mountainous tracts, on the other hand, that hypothesis has never been able to survive a moderate amount of accurate observation. Even in Switzerland – the land of glaciers – geologists at one time were of opinion that the boulder-clays of the low-grounds had a different origin from those which occur in the mountain-valleys. Thus, it was supposed that at the close of the Pleistocene period the Alps were surrounded by great lakes or by gulfs of some inland sea, into which the glaciers of the high valleys flowed and calved their icebergs – these latter scattering erratics and earthy débris over the drowned areas. Sartorius von Waltershausen28 set forth this view in an elaborate and well-illustrated paper. Unfortunately for his hypothesis no trace of the supposed great lakes or the inland sea has ever been detected: on the contrary, the character of the morainic accumulations, and the symmetrical grouping and radiation of the erratics and perched blocks over the foot-hills and low-grounds, show that these last have been invaded and overflowed by the glaciers themselves. Even the most strenuous upholders of the efficacy of icebergs as originators of some boulder-clays, admit that the boulder-clay or till, of what we may call the inner or central region of a glaciated tract is the product of land-ice. Under this category comes the boulder-clay of Norway, Sweden, and Finland, and of the Alpine Lands of central Europe, not to speak of the hilly parts of our own islands.

When we come to study the drifts of the peripheral areas, it is not difficult to see why these should be considered to have had a different origin. They present certain features which, although not absent from the glacial deposits of the inner region, are not nearly so characteristic of such upland tracts. I refer especially to the frequent interstratification of boulder-clays with well-bedded deposits of clay, sand, and gravel; and to the fact that these boulder-clays are often less compressed than those of the inner region, and have even occasionally a silt-like character. Such appearances do seem at first to be readily explained on the assumption that the deposits have been accumulated in water opposite the margin of a continental glacier or ice-sheet – and this was the view which several able investigators in Germany were for some time inclined to adopt.

But when the phenomena came to be studied in greater detail, and over a wider area, this preliminary hypothesis did not prove satisfactory. It was discovered, for example, that “giants’ kettles”29 were more or less commonly distributed under the glacial deposits, and such “kettles” could only have originated at the bottom of a glacier. Again, it was found that pre-glacial accumulations were plentifully developed in certain places below the drift, and were often involved with the latter in a remarkable way. The “brown-coal formation” in like manner was violently disturbed and displaced, to such a degree that frequently the boulder-clay is found to underlie it. Similar phenomena were encountered in regions where the drift overlies the Chalk – the latter presenting the appearance of having been smashed and shattered – the fragments having often been dragged some distance, so as to form a kind of friction-breccia underlying the drift, while large masses are often included in the clay itself. All the facts pointed to the conclusion that these disturbances were due to tangential thrusting or crushing, and were not the result of vertical displacements, such as are produced by normal faulting, for the disturbances in question die out from above downwards. Evidence of similar thrusting or crushing is seen in the remarkable faults and contortions that so often characterise the clays and sands that occur in the boulder-clay itself. The only agent that could produce the appearances, now briefly referred to, is land-ice, and we must therefore agree with German geologists that glacier-ice has overflowed all the drift-covered regions of the peripheral area. No evidence of marine action in the formation of the stony clays is forthcoming – not a trace of any sea-beach has been detected. And yet, if these clays had been laid down in the sea during the retreat of the ice-sheet from Germany, surely such evidence as I have indicated ought to be met with. To the best of my knowledge the only particular facts which have been appealed to, as proofs of marine action, are the appearance of bedded deposits in the boulder-clays, and the occasional occurrence in the clays themselves of a sea-shell. But other organic remains are also met with now and again in similar positions, such as mammalian bones and freshwater shells. All these, however, have been shown to be derivative in their origin – they are just as much erratics as the stones and boulders with which they are associated. The only phenomena, therefore, that the glacialist has to account for are the bedded deposits which occur so frequently in the boulder-clays of the peripheral regions, and the occasional silty and uncompressed character of the clays themselves.

The intercalated beds are, after all, not hard to explain. If we consider for a moment the geographical distribution of the boulder-clays, and their associated aqueous deposits, we shall find a clue to their origin. Speaking in general terms, the stony clays thicken out as they are followed from the mountainous and high-lying tracts to the low-grounds. Thus they are of inconsiderable thickness in Norway, the higher parts of Sweden, and in Finland, just as we find is the case in Scotland, northern England, Wales, and the hilly parts of Ireland. Traced south from the uplands of Scandinavia and Finland, they gradually thicken out as the low-grounds are approached. Thus in southern Sweden they reach a thickness of 43 metres or thereabout, and of 80 metres in the northern parts of Prussia, while over the wide low-lying regions to the south they attain a much greater thickness – reaching in Holstein, Mecklenburg, Pomerania, and west Prussia, a depth of 120 to 140 metres, and still greater depths in Hanover, Mark Brandenburg, and Saxony. In those regions, however, a considerable portion of the diluvium consists, as we shall see presently, of water-formed beds.

The geographical distribution of the aqueous deposits, which are associated with the stony clays, is somewhat similar. They are very sparingly developed in districts where the boulder-clays are thin. Thus they are either wanting, or only occur sporadically in thin irregular beds, in the high-grounds of northern Europe generally. Further south, however, they gradually acquire more importance, until in the peripheral regions of the drift-covered tracts they come to equal and eventually to surpass the boulder-clays in prominence. These latter, in fact, at last cease to appear, and the whole bulk of the diluvium, along the southern margin of the drift area, appears to consist of aqueous accumulations alone.

The explanations of these facts advanced by German geologists are quite in accordance with the views which have long been held by glacialists elsewhere, and have been tersely summed up by Dr. Jentzsch.30 The northern regions, he says, were the feeding-grounds of the inland-ice. In those regions melting was at a minimum, while the grinding action of the ice was most effective. Here, therefore, erosion reached its maximum – ground-moraine or boulder-clay being unable to accumulate to any thickness. Further south melting greatly increased, while ground-moraine at the same time tended to accumulate – the conjoint action of glacier-ice and sub-glacial water resulting in the complex drifts of the peripheral area. In the disposition and appearance of the aqueous deposits of the diluvium we have evidence of an extensive sub-glacial water-circulation – glacier-mills that gave rise to “giants' kettles” – chains of sub-glacial lakes in which fine clays gathered – streams and rivers that flowed in tunnels under the ice, and whose courses were paved with sand and gravel. Nowhere do German geologists find any evidence of marine action. On the contrary, the dovetailing and interosculation of boulder-clay with aqueous deposits are explained by the relation of the ice to the surface over which it flowed. Throughout the peripheral area it did not rest so continuously upon the ground as was the case in the inner region of maximum erosion. In many places it was tunnelled by rapid streams and rivers, and here and there it arched over sub-glacial lakes, so that accumulation of ground-moraine proceeded side by side with the formation of aqueous sediments. Much of that ground-moraine is of the usual tough and hard-pressed character, but here and there it is somewhat less coherent and even silt-like. Now a study of the ground-moraines of modern glaciers affords us a reasonable explanation of such differences. Dr. Brückner31 has shown that in many places the ground-moraine of the Alpine glaciers is included in the bottom of the ice itself. The ground-moraine, he says, frequently appears as an ice-stratum abundantly impregnated with silt and rock-fragments – it is like a conglomerate or breccia which has ice for its binding material. When this ground-moraine melts out of the ice – no running water being present – it forms a layer of unstratified silt or clay, with stones scattered irregularly through it. Such being the case in modern glaciers, we can hardly doubt that over the peripheral areas occupied by the old northern ice-sheet boulder-clay must frequently have been accumulated in the same way. Nay, when the ground-moraine melted out and dropt here and there into quietly-flowing water it might even acquire in part a bedded character.