darwin and the iceberg theory

Darwin and the Iceberg TheoryAuthor(s): William MillsSource: Notes and Records of the Royal Society of London, Vol. 38, No. 1 (Aug., 1983), pp.109-127Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/531346 .

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o09

DARWIN AND THE ICEBERG THEORY

By WILLIAM MILLS

The Library, University of Aston, Birmingham

O N 26 April I858, Charles Darwin wrote toJ. D. Hooker:

I have just had the innermost cockles of my heart rejoiced by a letter from Lyell. I said to him (or he to me) that I believed from the character of the flora of the Azores, that icebergs must have been stranded there; and that I expected erratic boulders would be detected embedded between the

upheaved lava-beds; and I got Lyell to write to Hartung to ask, and now H. [artung] says my question explains what had astounded him, viz. large boulders (and some polished) of mica-schist, quartz, sandstone, &c., some embedded, and some 40 and 50 feet above the level of the sea, so that he had inferred that they had not been brought as ballast. Is this not beautiful? (I).

Darwin's delight is understandable. Not only, on the basis of his

knowledge of the flora of the Azores, had he been able to predict Hartung's discovery - a prediction which provided yet further support for his theory of the origin of species - but the discovery seemed also to offer a conclusive test case in favour of another of his pet theories; that which forms the subject of this

paper (2).

EARLY HISTORY OF THE THEORY

The Iceberg, or Drift, theory was not originated by Darwin, nor was it by Lyell, from whom Darwin first learnt of it. As early as 1742 we find the Swedish mineralogist Daniel Tilas suggesting that erratic boulders had been

transported on floating ice (3) and the idea was re-stated at intervals

throughout the later eighteenth and early nineteenth centuries, as for example by Ferber (1784), Wrede (I804), Gruithuisen (I809) Hall (1812), Goethe

(1821) and Dobson (1826) (4). Wrede's work, in particular, deserves a brief further consideration since, for Darwin, it was he who was the originator of the theory in the form of which he approved in essentials, though not in detail

(5). Wrede was a professor of mathematics and natural philosophy at Berlin and in 1804 he published 'Geognostical researches relative to the countries on the south of the Baltic'. In this, he supposed that the Earth's centre of gravity had undergone a slow change along an axis from north to south which caused



IIO

a gradual reduction in the sea level of the northern, and a gradual rise in the sea level of the southern, hemisphere. Thus he explained the apparent fall in the level of the Baltic Sea first noted by Celsius in the early eighteenth century. Of particular interest for this paper is Wrede's explanation of the erratic boulders which littered the North European Plain. These, he argued, had been transported from the north on drift ice at a time when the sea level was higher (6). Lyell's journey to Sweden in i834 had convinced him that it was the land which was rising rather than the sea falling (7), and Darwin, from his observations in South America, confirmed Lyell's view, but both accepted Wrede's thesis that erratics had been distributed at a time when the land was submerged.

With its origin in the mid-eighteenth century the iceberg theory may be claimed as the first glacial theory, if by that we mean the first attempt to explain the morphology of the Earth by invoking the agency of ice. Initially it provided merely a means for explaining the occurrence of erratics, objects whose location far from the rocks from which they were derived had long puzzled geologists. These had been explained as due to the deluge of Noah, to a sudden 'debacle' - or great wave - resulting from a submarine seismic uplift, or else as being the final residue of former granitic mountains which had otherwise decomposed (8). De Luc had even proposed that erratics had been ejected from underground by 'elastic fluids', materials whose nature remained otherwise ill-defined (9). By the mid-I830's there remained three rival schools of thought seeking to explain not just erratics but also other phenomena now

recognized as associated with them: striated pebbles and pavements, polished rock surfaces and morainic ridges. The 'Diluvial' school attributed all such

phenomena to the action of water alone, though not necessarily in the form of the Noachian flood. This school soon lost ground. The debate then became focussed on the role of ice, between the 'land ice' or glacier, school, whose most eloquent advocate was Agassiz, and the 'sea ice' or iceberg, school. Despite the visit of Agassiz to Britain in 1840 and his success in there enlisting such influential support as that of Buckland and - temporarily - of Lyell (io), until I860 it was to the iceberg school that the majority of British geologists inclined ( i). I suggest that a study of 'Darwin and the iceberg theory' illuminates not merely a neglected, though not inconsiderable, portion of Darwin's work but also is essential in order better to understand the delayed reception of the glacier theory, a delay which was further extended by the very plausibility of Darwin's contributions to the debate on the behalf of

icebergs.



III

DARWIN'S INTRODUCTION TO THE THEORY

During my second year at Edinburgh I attended [Jameson]'s lectures on geology and zoology, but they were incredibly dull. The sole effect they produced on me was the determination never as long as I lived to read a book on geology. Yet I feel sure that I was prepared for a philosophical treatment of the subject; for an old Mr Cotton in Shropshire, who knew a good deal about rocks, had pointed out to me two or three years previously a well-known large erratic boulder in the town of Shrewsbury, called the 'bell-stone'; he told me that there was no rock of the same kind nearer than Cumberland or Scotland, and he solemnly assured me that the world would come to an end before any one would be able to explain how this stone came where it now lay. This produced a deep impression on me, and I meditated over this wonderful stone. So that I felt the keenest delight when I first read of the action of icebergs in transporting boulders, and I gloried in the progress of Geology. (I2) Given the prominence this famous passage from the 'Autobiography'

attaches to the iceberg theory, it is perhaps surprising that - to my knowledge - his work in this area has never been accorded detailed treatment. It is clear that from an early age Darwin was fascinated by erratics, but when did he 'first read of the action of icebergs'? In the preface to his account of the Voyage of the Beagle, he writes of 'the theory of the transportation of erratic blocks' - by which he means specifically the iceberg theory - as an idea 'which I have accidentally met with during the past year' (13). This, however, cannot be altogether accurate since the first volume of the first edition of Lyell's Principles of Geology, which Darwin possessed and eagerly perused from the outset of his

voyage, contained the following paragraph:

Floating icebergs - Marine currents are sometimes instrumental in the

transportation of rock and soil, but floating large masses of ice to great distances from the shore. When glaciers in northern latitudes descend the valleys burdened with alluvial debris, and arrive at the shore, they are

frequently detached, and float off. Scoresby counted five hundred icebergs in latitude 69° and 70° north, rising above the surface from the height of one to two hundred feet, and measuring from a few hundred yards to a mile in circumference. Many of these contained strata of earth and stones, or were loaded with beds of rock of great thickness, of which the weight was conjectured to be from fifty thousand to one hundred thousand tons. As the mass of ice below the level of the water is between seven and eight times greater than that above, these masses may sometimes take the ground



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in considerable numbers, in particular parts of the bottom of the deep, or on the shores of some isles, as may offer perplexing problems to future geologists. Some ice islands have been known to drift from Baffin's Bay to the Azores, ... and from the South Pole to the immediate neighbourhood of the Cape of Good Hope. (14)

Here we have outlined most of the important components of the iceberg theory: large amounts of rock debris carried over great distances and deposited far from their parent material so as to 'offer perplexing problems to future

geologists'. The second volume of Lyell's Principles reached Darwin at Montevideo in November 1832 and contained provoking suggestions concerning the role of icebergs, not as geomorphological agents, but as means of diffusing species which Darwin was later to exploit in On the origin of species (I5). Lyell's third volume, which reached Darwin at Valparaiso in 1834, included three pages on the origin of erratics but, interestingly enough, did not

explain the famous Jura erratics as deposited by icebergs during a period of marine submergence - Lyell's later position - but by a compromise version of the debacle theory. Lyell here envisaged the damming of Alpine valleys containing glaciers by a rock slide, the building up of a lake, the detachment of

icebergs from the glaciers, the collapse of the dam, and the sweeping of the debris-strewn icebergs by floodwater onto the Jura where they melted

depositing boulders, gravel and sand (16). In the Voyage of the Beagle, Darwin refers specifically to Lyell's anniversary

address as President to the Geological Society in 1836 and also to the 5th edition of the Principles (17). These incorporated Lyell's findings on his visit to Sweden in 1834 which had been crucial to the development of his views of

icebergs. There Lyell confirmed Celsius's observation that the level of the Baltic Sea appeared to be falling, but - in contrast to both Celsius and Wrede - he concluded that this was due to the rising of the land rather than to the

falling of the sea. Knowing nothing of isostacy, he attributed this rise to

gradual forces of seismic uplift, just such indeed as he had long been seeking in confirmation of his strictly uniformitarian interpretation of Huttonianism. If the Scandinavian land mass was rising, in the past much more of it must have been submerged. Lyell was also intrigued by the views of Swedish fishermen who reported the contemporary movement of large boulders across the sea

transported by ice floes (I8). Putting these two observations together, he concluded that all Swedish erratics, even those now far from the sea, were the result of the same processes and had been distributed to their present locations by icebergs whilst the land was submerged.



I 3 DARWIN'S CONTRIBUTIONS TO THE THEORY

Darwin's interest in icebergs shortly after his return home is indicated by the number of his publications devoted to this topic. If we include the account of his voyage, which contained important addenda largely devoted to the iceberg theory (I9), five of his earliest publications - that is, those up to 1842 - are concerned with this subject (20).

On the voyage, Darwin first records the presence of erratics in his account of the exploration up the Santa Cruz valley in April and May, I834. He was later to observe many more on the island of Chiloe off central Chile. A comparison of the manuscript diary with its published counterpart bears out his statement that it was only later that he formed views as to their origin, or - at least - that it was only later that he was to express them (21). Darwin was able to confirm Humboldt's observation that no erratics were to be seen in the vast intertropical plains of the eastern side of South America and found none either on the western side of the Cordillera down as far south as central Chile (22). This last point represented for him an important confirmation of the iceberg theory. The non-occurrence of erratics in zones beyond the known range to which icebergs travelled appeared to confirm nicely their role as the means

whereby erratics were transported (23). This then was one contribution made by Darwin's voyage to the theory.

There were others. Unlike Lyell, Darwin had no hesitation in attributing the

Jura erratics to the combined effects of icebergs and marine submergence. He did so arguing largely from the basis of a parallel he observed between the Jura fronting the Alps across the valley of Switzerland, and the island of Chiloe fronting the Andes across a strait. He argued that by far the most likely explanation of the Chiloe erratics, of which he observed vast numbers, was that they had been carried there by icebergs. He had already inferred that - like Scandinavia - the southern parts of South America were rising relative to the sea. Should, in the future, the rise be sufficient for thz, present sea-floor to appear above sea-level, an exact parallel would be established with the Jura and the Alps, Chiloe would face the Andes separated from it by a great valley. Surely then, Darwin concluded, an identical process must account for the Jura erratics (24).

There was other evidence also which seemed to Darwin to confirm the iceberg hypothesis. Agassiz had claimed that glaciers and ice sheets could best explain the occurrence on rocks and pebbles of parallel striations which could be found continuing sometimes over considerable distances. Darwin was the first to argue that such marks could be explained equally well by the scratching



II4

of rock surfaces by fragments of material only partially enclosed in icebergs. The constancy of direction of such striations was to be attributed to the

constancy of the marine currents which impelled the icebergs (25). The comparative scarcity of erratics to be found in the valley of Switzerland was only to be expected given that boulders would be most likely to be deposited where the icebergs carrying them were grounded near to land (26). The

tendency for particle size to diminish away from the materials was also easily understood, since the finer particles would naturally be carried further since smaller ice floes with smaller draughts could carry them (27). Agassiz's crucial argument against the iceberg theory had been that it could not explain the super-position of large boulders on top of gravel on top of sand, but this was exactly what Darwin did expect given his assumption of a gradually rising land surface. As the land was slowly uplifted, he wrote:

. . . the same forces which carried the large pebbles to a certain distance from the beach, and the smaller ones to a still further distance, will, after each little elevation, carry them somewhat further: a layer of little pebbles thus covering the sand, and a layer of large pebbles the smaller ones. Hence, when the part near the shore is converted into dry land, a section of the bed which was originally the bottom of the sea will necessarily show solid rock covered by sand, this by fine pebbles, and these again by others, gradually increasing in size. (28)

The achievement of which Darwin was probably most proud in this context, was that he was able to envisage a means whereby all these processes might occur without needing to invoke any general cooling of the Earth. That both iceberg and glacier theories seemed to require a much colder climate in the past constituted a major obstacle to their general acceptance (29). This is because at that time it was almost universally believed that so far from being colder in the past, the climate had been warmer; this being believed both for reasons of physics and because it was thought to be indicated by the fossil record. In one of his most effective passages, Darwin hypothetically inverts the hemispheres so that the northern hemisphere has a distribution of land and sea similar to that of South America and adjacent islands. Following Lyell's theory of climatic change, outlined in the first edition of the Principles, he argues that this alone could account for the descent of glaciers to sea-level and the formation there of icebergs, whereas in close juxtaposition could also be found semi-tropical vegetation of the kind indicated for the Pleistocene period by then known fossils. The passage is worthy of being quoted at length:



II5

We are all so much better acquainted with the position of places in our own, than in any other quarter of the globe, that I will recapitulate what is actually taking place in the southern hemisphere, only transporting in imagination each part to a corresponding latitude in the north. On this supposition, in the southern provinces of France, magnificent forests, intwined by arborescent grasses, and the trees loaded with parasitical plants, would cover the face of the country. In the latitude of Mont Blanc, but on an island as far eastward as central Siberia, tree-ferns and parasitical orchidae would thrive amidst the thick woods. Even as far north as central Denmark, humming-birds might be seen fluttering about delicate flowers, and parrots feeding amidst the evergreen woods, with which the mountains would be clothed down to the water's edge. Nevertheless, the southern part of Scotland (only removed twice as far to the westward) would present an 'island almost wholly covered with everlasting snow', and

having each bay terminated by ice-cliffs, from which great masses yearly detached, would sometimes bear with them fragments of rock.... A chain of mountains, which we will call the Cordillera, running north and south

through the Alps (but having an altitude much inferior to the latter), would connect them with the central part of Denmark. Along this whole line nearly every deep sound would end in 'bold and astonishing glaciers'. In the Alps themselves (with their altitude reduced by about half) we should find proofs of recent elevations, and occasionally terrible earthquakes would cause such masses of ice to be precipitated into the sea, that waves

tearing all before them, would heap together enormous fragments, and pile them up in the corners of valleys. At other times, icebergs 'charged with no inconsiderable blocks of granite', would be floated from the flanks of Mont Blanc, and then stranded on the outlying islands of theJura. Who then will deny the possibility of these things having actually taken place in Europe during a former period, and under circumstances known to be different from the present, when on merely looking to the other hemisphere, we see they are among the daily order of events? (30)

Together with his account of the voyage, in I839 Darwin published two articles which related to the iceberg theory. One was a brief note on a rock seen on an iceberg which confirmed that icebergs did carry boulders over great distances (3 I). The other - on the Parallel Roads of Glen Roy (32) - included a brief reference to Darwin's preference for icebergs as the means of transporting erratics (33), as well as an interesting footnote which quotes Sir James Hall's report of the movement by floating ice of a large block in the



Ii6

Moray Firth (34) which had left inscribed in the sand a deep furrow as it moved. Darwin asked:

What would have been the effect if this large and heavy block had been

pushed over a surface of solid rock instead of sand? This question will recall to the mind of those who have read the late papers of Messrs Charpentier, Venetz, and Agassiz, the case of the longitudinally and obliquely scratched rocks of the Alps. (35)

The paper Darwin addressed to the Geological Society in 1841, 'On the distribution of the erratic boulders, and on the contemporaneous unstratified deposits of South America' (36), was devoted almost entirely to the iceberg theory, and introduced an important distinction, which shall be discussed later, between icebergs and coast-ice as means of transporting materials (37). It also offered a possible explanation of what Lyell, in particular, came later to see as

perhaps the greatest difficulty for the theory: namely the scarcity of marine fossils in drift supposedly dropped from icebergs at a time of deep submergence. Darwin's suggestion was that their absence might be accounted for by the ploughing up of the sea bottom by stranded icebergs, thus producing conditions in which such creatures could not exist (38).

This paper made but scant reference to the glacier theory, which is perhaps surprising given that it was delivered so soon after Agassiz's famous visit to Britain, and especially since Darwin's next glacial paper, on the ancient

glaciers of Caernarvonshire (39), shows him to have been most ready to accept Agassiz's demonstration of the former existence of glaciers in many parts of Britain. To my awareness, the exact relationship of Darwin to the glacier theory has never been sufficiently investigated.

Our study of what Darwin had written in the Voyage of the Beagle should convince us that he was well-prepared for the discovery of former glacier action in the upland areas of Britain. Indeed this is very much what he had

predicted in the extensive quotation cited above (40). To accept as much did not for him imply that there was no role also for icebergs. Indeed unless the action of icebergs was to be invoked a number of problems remained unsurmounted. Thus in his study of the South American erratics he attributed all to the action of icebergs since the land surface had clearly been modelled by the action of the sea and sloped too gently - he believed - for glacier movement to be a possibility. Similarly, the absence of moraines and the characteristic angularity of the erratics also appeared to him to argue conclusively against glaciers (4I). Darwin, in general, was prepared to accept the existence and operation of valley glaciers in areas no longer exposed to



II7

glaciation, but he did not accept continental ice sheets and only in the late 1860's did Croll convince him of past climates being sufficiently cold to allow so great an expanse of ice (42). The title of one of his papers sums up his

position; there he is to study 'the ancient glaciers of Caernarvonshire' together with 'the boulders transported by floating ice' (43). For Darwin, the iceberg theory possessed great attractions, some, as we shall see, personal to himself. It was not lightly to be abandoned. Not only was much of it his own work, but it had the additional virtue of being indisputably uniformitarian. It presumed the operation of no processes which could not be observed to occur in the

present. Also under-studied, and of equal interest, is the question of Darwin's

possible role in the abandonment of the glacier theory by Lyell shortly after his conversion to it. On this there is only a little evidence but, such as there is, it is

intriguing. Lyell's enthusiasm for the glacier theory can have lasted no more than seven months at most. His first change of mind occurred when Buckland convinced him that even his home county of Forfarshire possessed features which could not be adequately explained except by resort to the action of

glaciers. It was on this subject that Lyell presented his one paper in favour of the glacier theory at the famous meeting of the Geological Society of November 1834 (44). And yet, by the time of his publication of the 2nd edition of The Elements of Geology (45), he had reverted more or less

completely to the iceberg theory, a theory which he was subsequently never fully to abandon (46). What happened?

Unfortunately, only a few letters of Lyell survive from the crucial years, 1840 and 1841, and none makes any reference to icebergs (47). Similarly, very few for this period survive from Darwin but there is one highly interesting letter, undated but undoubtedly of 1841 (48). This apparently was originally enclosed with Lyell's copy of Agassiz's Etudes sur les glaciers which - an

interesting circumstance in itself- Darwin had borrowed from him. The full text of this letter, in so far as it relates to the glacier/iceberg debate, is given in the notes at the end of this paper (49). Darwin begins by referring to a

mysterious 'extract' presumeably sent by Lyell for Darwin's study but of which there appears no other record. Later he expresses his admiration for

Agissiz's book:

What a capital book Agassiz's is. In [reading] all the early part I gave up entirely the Jura blocks, and was heartily ashamed of my appendix (and am so still of the manner in which I presumptuously speak of Agassiz), but it seems by his own confession that ordinary glaciers could not have



II8

transported the blocks there, and if an hypothesis is to be introduced the sea is much simpler; floating ice seems to me to account for everything as well as, and sometimes better than the solid glaciers. The hollows, however, formed by the ice-cascades appear to me the strongest hostile fact.

Here Darwin is putting forward two arguments against Agassiz. The first is that he has to invoke the operation not of 'ordinary glaciers' but of ice sheets, with all their attendant difficulties, in order to explain the Jura erratics. The second refers to the funnel-shaped cavities (lapiaz or couloirs) which Agassiz accounted for as eroded by cascades falling from high up in the glacier. This Darwin could not understand since, given that the glacier moved, surely not a circular hollow but a longitudinal slit should be formed, assuming that there was sufficient time for erosion to occur at any one place, of which he was

sceptical. As he wrote, this latter Darwin considered to be the strongest fact

against glaciers. The strongest factfor icebergs, he thought to be the tails of drift left behind certain Scottish crags. How, he asked, were these to be explained if not by icebergs?

No reply to this letter survives but given the close links between Lyell and Darwin at this time and the fact that the iceberg theory was very much the result of their collaboration, it seems not unlikely that, although perhaps not motivated by Darwin's precise objections, Lyell's subsequent abandonment of the glacier theory may well have been encouraged by Darwin's scepticism. Both after this date held very similar views, being prepared to accept a much wider extension of glaciers in the past, and their existence in areas no longer subject to glaciation, but, for the rest, viewing marine submergence and the action of icebergs as much more likely than continental glaciation. Lyell was never to accept the reality of continental glaciation (so).

After 1842, Darwin was increasingly preoccupied with his work on the transmutation of species, so much so that when, in 1848, he returned to the iceberg theory, he viewed it very much as an interruption and one which he was only provoked into making by an article by Milne which treated the theory 'as almost absurd' (51). In particular, Milne had argued that resort to icebergs failed to explain the movement of erratics not just laterally but also vertically, sometimes as much as by several thousand feet (52). Darwin's counter-argument was ingenious (53). In 1841, he had distinguished between transportation by icebergs and transportation by coast-ice. The two processes would operate differentially in different climates - the latter occurring only where winters were extremely cold - and with different effects - materials carried by icebergs would remain uneroded and therefore angular whereas



II9

those repeatedly shifted by the freezing and thawing of coast-ice would be progressively rounded (54). Darwin now argued that the raising of erratics by large amounts could be explained by a combination of coast-ice transportation and continental subsidence. This latter he believed to have occurred at a rate possibly of a few inches or even feet each year in Britain and North America, with the effect that boulders frozen into coast-ice, since the level of the sea remained the same, would on thawing be deposited a few inches or feet higher each year. If such a process were to continue over many years, the boulders would be raised to considerable heights. One inference to be drawn from this was that boulders raised by the greatest amount should also be those most rounded by erosion. Darwin suggested that this should be tested (55).

Darwin's contribution to the Admiralty handbook A manual of scientific enquiry, published in I849, contained a particularly revealing section on the action of ice (56). This manual, as its title implies, was intended as a guide to travellers, naval and otherwise, so that their studies overseas might prove most useful to the advancement of science. Darwin used the opportunity to

encourage observations which would help decide between the glacier and

iceberg theories. He wrote:

The voyager in the Polar Seas would render an excellent service to geology by observing all the effects which icebergs produce in rounding, polishing, scoring, and shattering solid rocks, and likewise in transporting gravel and boulders.... All facts illustrating the difference in the results produced by coast-ice and true icebergs would be very valuable. Do the boulders fixed on coast-ice, when driven over rocky shoals, become themselves scored? Wherever there was reason to believe that a surface has been scored by recent ice-action, a minute description and drawings ought to be made of the depth, length, width, and direction of the grooves; and even large slabs

brought home. On true icebergs are the fragments of rock generally fixed or loose; when icebergs turn over, are fragments frequently seen embedded in that part which was under water; and how were they fixed there? The nature, number, size, form, and frequency of occurrence of all fragments of rock seen on floating ice ought to be recorded, and the distance from their

probable source. A polar shore, known from upraised organic remains to have been lately elevated, would be eminently instructive. Do grey icebergs force up the mud and gravel at the bottom of the sea in ridges like the moraines of glaciers? Can shells, or marine animals, live in a shallow sea, often ploughed up and rendered turbid by the stranding of icebergs?... The means to distinguish the effects of ancient floating ice



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from those produced by ancient glaciers is, at present, a great desideratum in geology. (57)

It is a mark of how open Darwin's mind was that, despite his own attachment to the iceberg theory, he should have recommended 'M. Agassiz' work on Glaciers ... [for] any one going to the colder regions' (58).

Darwin's final paper on the theory, in I855, although ingenious, was founded unfortunately on a mistaken understanding of the work of Rendu and Forbes on glacier motion (59). Here his problem was to demonstrate how

icebergs could explain the 'rectilinear, uniformly-directed grooves' found on rock pavements. This had been proposed as a crucial objection to the iceberg theory for how could scratching by materials carried in icebergs explain deeply scored furrows which went up and down over the contours of the land often for long distances. At most, surely such scratching could account only for the occasional scored hill-top (60)? Darwin believed that he was able to provide a satisfactory explanation. Wrongly, he argued that the plasticity of ice was related not to the pressure placed upon it but to the degree to which it was

charged with water. If this was so, icebergs being more waterlogged than

glaciers and, since water-supported, being under less pressure, would, like

glaciers but more so, be capable of moulding themselves to the undulations of the surface over which they moved. He wrote:

In short, if in our mind's eye we look at an iceberg, not as a rigid body ... which would be deflected or broken up when driven against any submarine obstacle, but as a huge semi-viscid, or at least flexible mass floating on the water, I believe much of the difficulty will be removed which some have

experienced in understanding how rectilinear grooves could be formed continuously running, as if regardless of the outline of the surface ..... (61)

Such iceberg flow would also force in front of it banks of detritus including boulders. Darwin continues:

Although a fragment of rock or an irregularly formed moraine may by any one iceberg be propelled for only a very short distance, yet in the course of years the transportal can hardly fail to become far extended, the boulders being rolled over large inequalities of surface, and even up heights by the action of successively smaller bergs: an abyss, however, deeper than the deepest floating iceberg would, of course, absolutely stop this rolling or

pushing action. (62)

Here then we have a fascinating explanation of moraines as pushed underwater by iceberg bottoms.



121

By I860, it was clear that the majority of geological opinion was moving towards the glacier school. It was, for example, in this year, that Ramsay, who was to make so important a contribution to glacial studies, finally abandoned

icebergs for glaciers (63). Both Darwin and Lyell, however, resisted this trend and although Darwin wrote no further papers devoted to this subject, that he retained his interest in icebergs is clear from a number of his letters, including that with which I began this paper, as also from a paragraph on the subject to be found in On the origin of species (64). In I872, he wrote to Lyell:

As for glaciation, I will not venture to express any opinion, for when in South America I knew nothing about glaciers, and perhaps attributed much to icebergs which ought to be attributed to glaciers. On the other hand, Agassiz seems to me mad about glaciers, and apparently never thinks of drift ice. (65)

In I879, Darwin wrote to Mackintosh to congratulate him on his recently published work on erratics, and remarks:

I have thought for some years that the agency of floating ice, which nearly half a century ago was overrated, has of late been underrated. (66)

Finally, in I882, the year of his death, Darwin wrote again to Mackintosh and

regretted that Geikie's reply to Mackintosh's paper had 'underrated the

importance of floating ice' (67).

CONCLUSION

Rudwick (68) has demonstrated convincingly how central to his overall research programme was Darwin's explanation of the Parallel Roads of Glen Roy, since it appeared to provide conclusive evidence in favour of his tectonic

theory of oscillating crustal blocks. This, Rudwick argues, to have been Darwin's fundamental geological theory and one which buttressed his ideas on the transmutation of species. I believe that Rudwick (69), however, underestimates the significance of Darwin's studies of erratics. Like Glen Roy, erratics - as explained by the iceberg theory - demanded that large areas of the Earth's surface should have experienced both subsidence and subsequent uplift, often of considerable dimensions. Certainly, Darwin was at least as tenacious - indeed, even more so - in his support for the iceberg theory as he was in relation to Glen Roy, and such tenaciousness, in the face of the carefully reasoned objections of such asJamieson (70), is rendered more comprehensible if the theory performed a more central role in Darwin's work than it initially appears to do.



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Darwin's conceptual structure indeed bears comparison to an iceberg in that at any one time only a small portion was ever rendered visible. His theories of Glen Roy, coral reefs and erratics all demanded, and provided support for, his tectonic theory which envisaged the Earth as subdivided into

parallel zones of uplift and subsidence in ceaseless motion as fluid magma flowed in and out under them (71). In his ignorance of continental drift, Darwin needed this theory, in turn, in order to explain the known facts

concerning the development and dispersal of species (72). According to this theory, oceans could no longer bar the diffusion of plants and even animals, but at most constituted only temporary barriers as islands periodically arose and sank in their midst. For Darwin there was thus an indissoluble link forged between the occurrence of an erratic, such as the 'bell-stone' at Shrewsbury which had first awakened his interest in rocks, and the entire panoply of the organic world as revealed in The origin of species. Not for Darwin were there neat divisions to be drawn between his 'geological' and 'biological' work. His aim was to erect an interlocking series of theories spanning the whole of natural history. His preference for, and stubborn adherence to, the iceberg theory is thus to be explained quite simply: it fitted, whereas the glacier theory did not.

NOTES

(I) F. Darwin ed. The life and letters of Charles Darwin, including an autobiographical chapter, London (1887) (hereafter cited- 'F. Darwin (I887)') Vol. 2, p. 112.

(2) Darwin's letter to Lyell, written on the same day, makes still more of the value of Hartung's find as a conclusive test case.

I am extremely much obliged to you for sending me Hartung's interesting letter. The erratic boulders are splendid. It is a grand case of floating ice versus glaciers. He ought to have compared the northern and southern shores of the islands. It is extremely interesting to me, for I have written a very long chapter on the subject, collecting briefly all the geological evidence of glacial action in different parts of the world, and then at great lengths (on the theory of species changing) I have discussed the migration and modification of plants and animals, in sea and land, over a large part of the world. To my mind, it throws a flood of light on the whole subject of distribution, if combined with the modification of species. (F. Darwin (1887) Vol. 2, . 113)

The chapter referred to here is Chapter II, 'Geographical distribution', in C. Darwin, On the origin of species, London (I859). See p. 363 for a paragraph on the role of icebergs as a means of dispersing species, which mentions Hartung's find.



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(3) D. Tilas, Strenrikets Historia, udford i det wid Praesid: i afflaggande haldne Talet in for Kongl. Swenska Wetenskaps-Academien den 14. April 1742, Stockholm (1742).

(4) J.J. Ferber, 'Einige Anmerkungen zur physichen Erdbeschreibung von Kurland', Appendix to: J. B. Fischer, Zusatse zu seinem Versuch einer Naturgesichte von Livland, Riga (I784). On Tilas and Ferber, see H. Qdum, 'Contributions to the literature on erratic boulders' Meddelelser fra Dansk Geologisk Forening, 10, 499-506 (I945). On Wrede, see note 6. On Gruithuisen, see K. A. von Zittel.

History of geology and palaeontology: to the end of the nineteenth century (trans. M. M. Ogilvie-Gordon), London (1901), p. 229. J. Hall, 'On the revolutions of the Earth's surface' Trans. R. Soc. Edin., 7, 139-212 (I812). On Goethe, see Cameron, D. 'Goethe - discoverer of the ice age' J. glaciol., 5, 751-754 (1964-5). P. Dobson, 'Remarks on bowlders' Amer.J. Sci., o1, 217-218 (1826).

(5) C. Darwin, Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle between the years 1826 and 1836, London (1839) (hereafter cited - 'Darwin (I839a)') Vol. 3, p. 622.

(6) Wrede's otherwise obscure publication gained an international audience through being criticized by J. A. De Luc, Geological travels. Travels in the north of Europe (trans.) London (i8io-I), and by Hall (1812). It was from his reading of Hall that Darwin first learnt of Wrede.

(7) C. Lyell, 'On the proofs of a gradual rising of the land in certain parts of Sweden' Phil. Trans. R. Soc. Lond, I25, 1-38 (1835).

(8) See F. J. North, 'Centenary of the glacial theory' Proc. Geol. Assoc., 54, I-28 (I943).

(9)J. A. De Luc, An elementary treatise on geology (trans. H. de la Fite), London (1809) (Io) At the famous meeting of the Geological Society in November 1834, Agassiz,

Buckland and Lyell all read papers advocating the glacier theory. L. Agassiz, 'On glaciers, and the evidence of their having once existed in Scotland, Ireland and England' Proc. Geol. Soc. Lond., 3, 327-332 (1840-1); W. Buckland, 'On the evidence of glaciers in Scotland and the North of England' Proc. Geol. Soc. Lond., 3, 332-337, 345-348 (I840-I); C. Lyell, 'On the geological evidence of the former existence of glaciers in Forfarshire' Proc. Geol. Soc. Lond., 3, 337-345 (1840-1).

(i ) See R. J. Chorley et al. The history of the study of landforms or the development of geomorphology. Vol. I. Geomorphology before Davis, London (1964) pp. 210-234, 330-339.

(12) F. Darwin (1887) Vol. I., p. 41. (13) Darwin (i839a) vii. (14) C. Lyell, The Principles of geology (hereafter cited - 'Lyell (P.G.)'), ist ed. Vol.

I (1830) pp. 299-300. (IS) C. Darwin (1859), op. cit. note 2, p. 363. (16) Lyell (P. G.) Ist ed. Vol. 3 (1833) pp. 149-5IO.

(17) Darwin (I839a) p. 291 footnote. (18) C. Lyell (1835), op. cit. note 7, pp. 20-21.

(19) Darwin (i839a) pp. 615-625. See also pp. 284-293. (20) The date of these papers, during the most fertile period of Darwin's career, lends

them an especial interest. They constituted an important element in the



I24

'public' aspect of his work at this time. See M. J. S. Rudwick, 'Charles Darwin in London: the integration of public and private science' Isis, 73, I86-206 (1982).

(21) C. Darwin, Charles Darwin's diary of the voyage of H.M.S. Beagle (ed. N. Barlow), Cambridge (I933).

(22) Darwin (I839a) 288-290. (23) This finding was frequently cited by other geologists, e.g., C. Lyell, The elements

of geology, ISt ed. (1838) p. 137; A. Sedgwick in a letter to Agassiz, dated 5 March 1838 (J. W. Clark and T. M. Hughes, The life and letters of the Rev. Adam Sedgwick, Cambridge (1890) Vol. I, pp. 503-504); R. I. Murchison, The Silurian system, London (1839) p. 543; J. L. Hayes, 'Probable influence of icebergs upon drift' BostonJ. nat. hist., 4, 426-452 (1844) pp. 444-446.

(24) Darwin (I839a) pp. 286, 292, 619-621. (25) Darwin (I839a) pp. 620-625. This explanation rapidly became commonplace and

my attribution of its origin to Darwin may be controversial. Hall (1812), op. cit. note 4, adopted a diluvial explanation of such scratches despite his adoption of the iceberg hypothesis as the means of transporting erratics. Lyell (P. G.) ISt ed., Vol. 3, pp. I47-I48, also adopts a diluvial explanation. He first

explains striations by iceberg scratching in P.G., 6th ed., Vol. I., p. 265 (I840). Murchison does not employ this explanation in 1839, op. cit. note 23, but does so in his later publications, e.g., R. I. Murchison and M. E. de Verneuil, 'On the geological structure of the northern and central regions of Russia in Europe' Proc. Geol. Soc. Lond., 3, 398-408 (1840-I), 406-407; R. I. Murchison, 'Anniversary address of the President' Proc. Geol. Soc. Lond., 3, 637-687 (1842), 68o-68I. Buckland provided what he considered to be conclusive evidence against the explanation in 1840-I, op. cit. note 10, pp. 336-337.

(26) Darwin (I839a) pp. 620-621.

(27) Darwin (I839a) p. 621. Lyell adopted this explanation in The elements of geology, 2nd ed. London (1841) Vol. I, pp. 239_240.

(28) Darwin (I839a) p. 621. Agassiz's criticism may be found in 'Etudes sur les glaciers, Neuchatel (1840) pp. 283-284.

(29) On the early nineteenth century 'directionalist' consensus see M. J. S. Rudwick, 'The glacial theory' Hist. Sci., 8, 136-157 (1970).

(30) Darwin (I839a) pp. 291-292. Murchison (1839), op. cit. note 23, fully accepts Darwin's views here and quotes the whole passage and more (pp. 542-543). Lyell (1841), op. cit. note 27, also exploits these ideas (pp. 259-260).

(31) C. Darwin, 'Note on a rock seen on an iceberg in 61° South latitude'J. R. Geogr. Soc. Lond., 9, 528-529 (1839). Quoted by Lyell (P. G.), 6th ed., Vol. I, pp. 379-380 (1840).

(32) C. Darwin, 'Observations on the Parallel Roads of Glen Roy, and of other parts of Lochaber in Scotland, with an attempt to prove that they are of marine origin' Phil. Trans. R. Soc. Lond., I29, 39-8I (I839).

(33) Darwin (1839), op. cit. note 32, p. 71. (34) Not 'Solway Firth' as Darwin mistakenly writes.

(35) Darwin (1839), op. cit. note 32, pp. 71-72 footnote.

(36) C. Darwin, 'On the distribution of the erratic boulders and on the con-



I25

temporaneous unstratified deposits of South America' Trans. Geol. Soc. Lond. 2nd ser. 5, 601-631 (1842).

(37) Darwin (1842), op. cit. note 36, p. 430. (38) Darwin (1842), op. cit. note 36, p. 421. Murchison (1842), op. cit. note 25,

adopted this as a partial explanation (p. 681). (39) C. Darwin, 'Notes on the effects produced by the ancient glaciers of

Caernarvonshire, and on the boulders transported by floating ice' Phil. mag., N.S. 2I, 180-i88 (1842).

(40) Darwin (I839a) pp. 291-292. (41) Darwin (1842), op. cit. note 36, pp. 429-431. (42) F. Darwin and A. C. Seward eds. More letters of Charles Darwin: a record of his work

in a series of hitherto unpublished letters, London (1903) pp. 161-164. (43) Darwin (1842), op. cit. note 39. (44) Lyell (1840-1), op. cit. note Io. (45) Lyell (1841), op. cit. note 27. (46) See G. L. Davies, The earth in decay: a history of British geomorphology 1578-1878,

London (1968) pp. 291-293.

(47) K. M. Lyell ed. Life, letters andjournals of Sir Charles Lyell, Bart, London (1881). (48) F. Darwin and A. C. Seward eds. (1903), op. cit. note 42, pp. 148-150. (49) Letter 499.

Your extract has sent me puzzling very much, and as I find I am better at

present for not going out, you must let me unload my mind on paper. I

thought everything so beautifully clear about glaciers, but now your case and Agassiz's statement about the cavities in the rock formed by cascades in the glaciers, shows me I don't understand their structure at all. I wish out of pure curiosity I could make it out.

If the glacier travelled on (and it certainly does travel on), and the water kept cutting back over the edge of the ice, there would be a great slit in front of the cascade; if the water did not cut back, the whole hollow & cascade, as you say must travel on; and do you suppose the next season it falls down some crevice higher up? in any case, how in the name of Heaven can it make a hollow in solid rock, which surely must be the work of many years. I must point out another fact which Agassiz does not, as it appears to me, leave very clear. He says all the blocks on the surface of the glaciers are angular, and those in the moraines rounded, yet he says the medial moraines whence the surface rocks come & are part [of], are only two lateral moraines united. Can he refer to terminal moraines alone when he says fragments in moraines are rounded? What a capital book Agassiz's is....

[See text] I am glad to observe that Agassiz does not pretend that direction of

scratches is hostile to floating ice. By the way, how do you and Buckland account for the 'tails' of diluvium in Scotland. I thought in my appendix this made out the strongest argument for rocks having been scratched by floating ice.

Some facts about boulders in Chiloe will, I think, in a very small degree elucidate some parts of the Jura case. What a grand new feature all this ice



I26

work is in Geology! How old Hutton would have stared! This letter presents a number of puzzles. What was the nature of the 'extract' sent to Darwin and what was the 'case' which it seems to have contained? Did this 'case' worry Lyell as it did Darwin and, if so, did it indicate that he was already wobbling in his faith in the glacier theory? Darwin, later in the letter, asks 'how do you and Buckland account for .. .', so Darwin clearly saw Lyell as still proposing that theory. All in all, it is far from certain whether 'Lyell here was attempting to convert Darwin (he did after all lend him his copy of Agassiz), whether he was becoming increasingly uncertain and was seeking Darwin's opinion, or whether Darwin had initiated the exchange - possibly in response to Lyell's pro-glacier theory of 1840- and was seeking to persuade Lyell that acceptance of a role for glaciers did not necessarily imply that icebergs had to be abandoned. Since, in geology, it is usually assumed that the relationship between Lyell and Darwin was essentially one way with Darwin very much the junior, this last is certainly the most interesting possibility, but the case is far from proved. What is certain, however, is that Darwin's opinion was important to Lyell. See notes 23, 25, 27, 30 and 31 for examples of Darwin's influence on Lyell in this area.

(50) On Lyell's later views, see Chorley et al. (1964), op. cit. note II, pp. 232-233, 330-339; and Davies (1968), op. cit. note 46, pp. 291-293.

(51) F. Darwin (1887) Vol. I, p. 328. (52) D. Milne. On polished and striated rocks, lately discovered on Arthur's Seat, and

other places near Edinburgh. Edin. new phil. j., 42, 154-172 (1847).

(53) C. Darwin. On the transportal of erratic blocks from a lower to a higher level. Qtly. j. Geol. Soc. Lond., 4, 315-323 (1848).

(54) Darwin (1842), op. cit. note 36, p. 430. (55) The ideas expressed in this paper were embraced by H. T. De la Beche

'Anniversary address of the President', Qtly. J. Geol. Soc. Lond., 5, xix-cxvi (1849) xxviii-xxx; and by J. Smith, 'On the scratched boulders and rocks of the coalfield of Scotland' Qtly. J. Geol. Soc. Lond., 5, (1849) p. 19. T. F. Jamieson rejected Darwin's theory on the grounds that: no water-rolled pebbles accompanied the boulders; on Craig Dhu, the highest erratics, so far from being the most rounded, were the most angular; and, finally, the scoring of subjacent rocks should not have been as horizontal as he found them to be, but should - on Darwin's theory - have run uphill 'from the blocks being driven on it by the surf'. 'On the ice-worn rocks of Scotland' Qtly. J. Geol. Soc. Lond., I8, 164-184 (1862) p. 175.

(56) C. Darwin, 'Geology' in J. F. W. Herschel, ed. A manual of scientific enquiry; prepared for the use of Her Majesty's Navy: and adaptedfor travellers in general, London (1849) pp. 156-I95.

(57) Darwin (1849), op. cit. note 56, pp. I79-I8I. (58) Darwin (1849), op. cit. note 56, p. 181. (59) C. Darwin, 'On the power of icebergs to make' rectilinear, uniformly-directed

grooves across a submarine undulatory surface' Phil. mag., 4th ser. IO, 96-98 (I855).

(60) As Hayes (1844), for example, had remarked, op. cit. note 23, p. 451.



127

(61) Darwin (I855), op. cit. note 59, P. 97. (62) Darwin (I855), op. cit. note 59, p. 98. (63) A. C. Ramsey, 'On the glacial origin of certain lakes in Switzerland, the Black

Forest, Great Britain, Sweden, North America, and elsewhere' Qtly. J. Geol. Soc. Lond., I8, 185-204 (1862) p. 186.

(64) Darwin (1859), op. cit. note 2, p. 363. (65) Darwin and Seward (1903), op. cit. note 48, vol. 2, pp. I64-I65. (66) F. Darwin (1887), op. cit. note I, p. 235. D. Mackintosh. Results of a systematic

survey, in 1878, of the directions and limits of dispersion, mode of occurrence, and relation to drift-deposits of the erratic blocks or boulders of the west of England and east of Wales, including a revision of many years' previous observations. Qtly. J. Geol. Soc. Lond., 35, 425-455 (I879).

(67) Darwin and Seward (1903), op. cit. note 48, Vol. 2, p. I70. J. Geikie, 'The

intercrossing of erratics in glacial deposits' Scottish naturalist, 6, 241-254 (I88I). (68) M. J. S. Rudwick, 'Darwin and Glen Roy: a "Great failure" in scientific method?'

Studies in history and philosophy of science, 5, 97-I85 (1974) pp. I6I-I65.

(69) M.J. S. Rudwick (1970), op. cit. note 29, pp. 200-201.

(70) T. F.Jamieson (1862), op. cit. note 55. (71) C. Darwin, 'On certain areas of elevation and subsidence in the Pacific and

Indian Oceans, as deduced from the study of coral formations', Proc. Geol. Soc. Lond., 2, 552-554 (1838).

(72) Darwin (1838), op. cit. note 71.



darwin and the iceberg theory

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