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Rocks of ages

Cwm Idwal seen from beyond the Younger Dryas glacial advance limit. Llyn Idwal occupies the middle distance and just to the left of the lake, above the path, lies the edge of the 'Darwin boulders'.


As the Darwin Bicentenary draws near, Peter Worsley* reminds us all that Darwin was principally a geologist and describes Darwin's geologising visit to Cwm Idwal, North Wales, in June 1842

Geoscientist 18.11 November 2008

On the 13 February next year we shall be celebrating the bicentenary of the birth of Charles Darwin. The public are often unaware that during the first phase of his career, Charles Darwin regarded himself as a geologist rather than an evolutionary biologist. Sadly, his geological field research activities were effectively terminated when his mystery illness started seriously to restrict his mobility, and after 1842 (when still only 33 years old) he had to curtail thoughts of any new fieldwork.

Darwin's geological contributions as a whole were first reviewed on the centenary of his birth by Sir Archibald Geikie, distinguished geologist, historian of geology and past President of the Society¹. A splendid new and detailed appraisal of Charles Darwin as a geologist has been published by the American science historian Sandra Herbert ².

Recently I have highlighted the role of the Dano-Norwegian Jens Esmark as a pioneer in the formulation of the ‘Glacial Theory’. His ideas of palaeoglaciation were introduced to a Scottish audience by Robert Jameson shortly after their original publication and Darwin as a 17 year-old undergraduate at Edinburgh University in 1826, was probably a witness to this³. However, the year 1840 is generally regarded as seminal in British glacial geology due to the visit of Louis Agassiz and his forceful advocacy of a Swiss version of ‘Glacial Theory’ at the annual meeting of the British Association in Glasgow.

Subsequently, at the Geological Society, a heated debate raged - with the theory being enthusiastically championed by William Buckland, Reader in Geology and Mineralogy at Oxford University. As a result of this debate, Darwin’s interest was stimulated to such an extent that he decided to embark on his own fieldwork in order to test the new hypothesis. During the course of a journey through North Wales in 1842, he came across a distinctive erratic boulder group lying within the classic glacial cirque of Cwm Idwal. This he described and interpreted in a remarkably ‘modern’ manner in his benchmark account of North Wales in the Ice Age.
Fig 1. Map showing the location of the ‘Darwin Idwal boulders’ within the Cwm Idwal National Nature Reserve.


On 9 June 1834, from the deck of HMS Beagle, Darwin was able to observe the calving glacier termini along the Magellan Channel below Mount Sarmiento in Tierra del Fuego. He also noted the associated transport of glacier-borne debris by icebergs into the marine environment. This experience was to influence his conceptual understanding of glacial geological processes heavily. It appears that he never actually walked to a glacier margin; but he cannot have escaped seeing the distinctive glacial landforms adjacent to those ice margins and the impact of glacier variations on the pattern of plant colonisation.

Incidentally, those of us fortunate enough to participate in the Society’s Bicentenary ‘In the footsteps of Charles Darwin’ field excursion to South America (picture) found that glacier calving was extremely limited due to substantial glacier recession during the 1834 – 2007 period, with virtually all the glacier termini now being essentially terrestrial. Certainly, significant iceberg generation had ceased and with it the transport of ice-rafted debris.

After his return to Britain, Darwin undertook a detailed field study of the Parallel Roads of Glen Roy in 1838 and concluded that they were shorelines of marine origin - an interpretation consistent with the concept of a recent great submergence. This was in accord with his observations of the effects of recent tectonic uplift process along the coast of Chile.

From June to November in 1840, Darwin was resident at Maer Hall in northwest Staffordshire (the home of his in-laws, the Wedgwoods) and an early manifestation of his illness prevented him from attending the Glasgow meeting of the BA. When, later, Agassiz, Buckland and Lyell each addressed the Geological Society at a series of three meetings between November 4 and December2, Darwin travelled from Maer to London especially to be in attendance.

Technically he was still one of the two joint Secretaries of the Geological Society, although he had expressed a wish to resign. This became effective from February 1841. He was to witness firsthand the excitement and acrimony engendered by both Louis Agassiz and his principal supporting ‘convert’ William Buckland. In the next year, Buckland’s paper relating to his newly completed Snowdonia fieldwork was read (on December 15), but unfortunately illness again prevented Darwin from being present.

At the Geological Society, Darwin was not immediately persuaded by Agassiz’s and Buckland’s arguments for adopting a ‘glacialist’ interpretation of what had hitherto been mainly regarded as ‘diluvial’ phenomena. However, the subsequently published versions in the Society proceedings were sufficient to arouse his interest and he resolved to undertake a field test of the concept for himself in an area with which he was already familiar - North Wales. Indeed, he freely acknowledged that Buckland’s latest account acted as his ‘field guide’ when selecting localities worth visiting.

Thus, in June 1842 (just a month after completing his first written sketch of his ‘Origin of Species’ hypothesis at Maer and two months before he bought his new residence at Down House in Kent) he commenced a solo 10-day excursion to examine ‘glacial action’, with as his main objective the examination of evidence and interpretations presented by Buckland. He commenced his journey at Maer but first called at his birthplace and boyhood home, The Mount, in Shrewsbury.

His knowledge of north Welsh geology was in part due to the fact that in 1831, prior to his voyage in HMS Beagle, he had accompanied Adam Sedgwick to the principality. This earlier visit had been arranged so that Darwin might gain some supervised geological field training whilst Sedgwick was engaged in primary mapping.

Unlike the situation 11 years earlier, in 1842 ‘his eyes were now seeing what the mind was looking for’ and he was soon able to identify abundant landform and sedimentary evidence described by Buckland that was consistent with relatively recent glaciation. He almost instantly became a new member of the very select British group who supported the ‘Glacial Theory’ expounded by Agassiz. He stayed at the Royal Hotel (now the Plas y Brenin National Mountain Centre) in Capel Curig on 18 June and most probably from that base visited Cwm Idwal. The results of his new analysis of the north Welsh landscape were published in the same year4. For much of the period 1840 – 1875, those supporting the glacialist creed, envisaged a combination of localised mountain glaciation followed by a major phase of marine submergence with extensive iceberg rafting of glacial debris, and Darwin was no exception.

As already noted, he did enjoy the rare advantage of having observed contemporary South American glaciers calving into the sea. In the British geological community at large, there was a clear majority (including the influential Sir Roderick Murchison) who were highly sceptical of the Glacial Theory in any aspect. A swing in favour of glacial processes in the pendulum of opinion was boosted by an Arctic glaciological expedition from the British Geological Survey in 1865 led by Archibald Geikie5.

The 'Darwin boulders' viewed towards the south west. All 4 angular boulders (A-D) are on the left and behind D lies the non-angular boulder E. Lynn Idwal can be seen in the background.



The Cwm Idwal [spelt "Idwell" by Darwin] Site of Special Scientific Interest is renowned for its relict flora. Equally it is a locality of major glacial geological importance and is increasingly so as global climatic change becomes ever dominant in our lives. During the Younger Dryas some 11,000 years ago, it nourished a cirque glacier which probably terminated just beyond the northern Llyn Idwal lake shore; although unambiguous landform evidence is lacking.

Darwin, reported that "eighty yards west of the spot where the river escapes from the lake, through a low mound of detritus …. There is an example of a boulder broken into pieces." "The boulder now consists of four great tabular masses, two of which rest on their edges, and two have partly fallen over against a neighbouring boulder." It is certain that Darwin must have confused his compass bearings, since the only group of five related boulders observable in the field today that corresponds to his account lies some 100m southeast of where the Afon Idwal stream flows from Llyn Idwal at NGR SH 646 597 (picture). There is no boulder group that even remotely meets his description on the west bank. Apparently at other localities Darwin from time to time confused his compass directions.

Surprisingly, the identity of what will be termed ‘Darwin’s Idwal boulders’ appears to have been elusive in print. A recent examination of the boulder group, confirms that there are five members (picture). They all consist of light grey bedded volcanic pyroclastics with minor quartz veining. These will be designated A – E from left to right when viewed from the north (down glacier flow side). Boulder D is not evident from the south and is significantly smaller than the others. Boulders A – D inclusive are angular in shape and the faces between B – C and C – D appear to be inversions of each other, strongly suggesting that they were once part of the same piece of rock.

Boulder A may once have also been part of this same mass; but if this were the case then it has suffered major rotation and some of the intervening material has been removed. More likely, A is a separate fragment since the bedding in A lies near horizontal in contrast with the steeply dipping stratification in B - D. The outer (east) face of A probably reflects a master joint surface. No striations are visible on any of the A - D boulder surfaces consistent with their not having been subject to transport in a sub-glacial environment.

Boulder E (closest to the lake) also has no obvious striations but is quite different in morphology since it lacks any angularity. Rather it has a sub-rounded shape suggesting a rather different transport history. Like much glacially transported boulder-sized clastic material in the vicinity, it may well have been recycled from an earlier glacial transport event and probably has been subjected to a phase of rolling.

It is clear that Darwin appreciated that boulder E, with its distinctive morphology, had a different genesis from the others and when he described the occurrence of four angular boulders (i.e. A-D), he based his observations on what he saw when approaching the site from the north, as most visitors to the Cwm do today. Darwin specifically mentions four ‘tabular’ masses, two on their edges (these are A and B) and two against a neighbouring boulder – corresponding to C and D propped up by ‘non tabular’ E. Following from his observations on erratic boulders in South America, Darwin was fully aware of the significance of boulder shape as a clue to transport history and had earlier criticised as great an authority as Sir Henry de la Beche for failing to note the significance of the difference between angular and rounded mega-clasts.

In Sandra Herbert’s book, two photographs include part of the Cwm Idwal Darwin boulder group. She has commented that her choice of photographs was not accidental. Being a historian rather than a glacial geologist, she felt it better not to match these boulders to Darwin’s description positively. This is fully understandable, since previously the boulders had not been recognised as those described by Darwin by the many glacial specialists who have contributed to the abundant academic and popular literature relating to the Cwm’s glacial geological heritage.

Fig. 2. Photograph of the ‘Darwin Idwal boulders’ as viewed looking towards the north east. Note how the left hand boulder (E) does not have the angularity of the others (A – D). Llyn Idwal is in the background.



Darwin wrote "From the distance, though small in itself, at which the four pieces are separated from each other, they must have been pitched into their present position with great force; and as the two upright thin tabular pieces are placed transversely to the gentle slope on which they stand, it is scarcely possible to conceive that they could have been rolled down from the mountain behind them; one is led, therefore, to conclude that they were dropped nearly vertically from a height into their present places." He envisaged that they had fallen "through a crevice in the ice", a process which he must have learnt about from the writings of the two pioneer Swiss glacial geologists Charpentier and Agassiz.

In contemporary parlance, Darwin interpreted the boulders as supraglacial erratics - debris transported on the top of a glacier that had later slid down a crevasse to the glacier bed. Their characteristic angular form clearly suggested to Darwin that the fracturing occurred at the site, and his concept of fall-fracture accounting for their shape may well be correct. One alternative mechanism might be that the fracturing originated with a rock-fall from off the cirque backwall onto the glacier, followed by supraglacial transport, ice wastage and subsequent superimposition on to the former glacier bed.

Another may be attributed to post depositional periglacial weathering processes breaking down what was originally a single erratic introduced from supraglacial debris by melt-out. In modern proglacial zones, in situ rock disintegration of formerly supraglacial débris is common and is often facilitated by pre-existing microfractures. These can be inherited from pressure release joints related to repeated loading and unloading.


The ‘Darwin Idwal boulders’ are an excellent example of large angular clasts lying disconformably on a minor ‘morainic’ landform consisting of ill-sorted glacial sediment. Darwin’s conceptual interpretation of them as former supraglacial debris remains valid today and is the first recognition of a glacial feature of this type in Britain. It is remarkable that he was able to come to this conclusion at a time when the prevailing view of erratics was that they had been introduced by floating ice - a mechanism which he, under the influence of his Beagle days in Tierra del Fuego, still considered valid in many contexts. Hence historically, the locality is an important landmark in the global development of Quaternary science and being adjacent to the main tourist access footpath into the Cwm, it merits greater public recognition.


Grateful thanks are extended to Sandra Herbert and Michael Roberts for discussion of the site. Additional photography by Michael Glyn Williams.


  1. Geikie, A. 1909. Charles Darwin as geologist. Cambridge, Cambridge University Press, 91p.
  2. Herbert, S. 2005. Charles Darwin, geologist. Ithaca, Cornell University Press, 485p.
  3. Worsley, P. 2006. Jens Esmark, ‘Vassryggen’ and early British glacial theory. Mercian Geologist, 16, 161-172.
  4. Darwin, C. 1842a. Notes on the effects produced by the ancient glaciers of Caernarvonshire, and on the boulders transported by floating ice. The London, Edinburgh and Dublin philosophical magazine and journal of science, Series 3, 21, 180-188. (4 cont.) Darwin, C. 1842b. Notes on the effects produced by the ancient glaciers of Caernarvonshire, and on the boulders transported by floating ice. Edinburgh New Philosophical Journal, 23, 352-363.
  5. Worsley, P. 2007. The British Geological Survey's glaciological expedition to Arctic Norway in 1865. Mercian Geologist 16, 263-275.

* Emeritus Professor of Quaternary geology Geoscience Building School of Human and Environmental Sciences University of Reading