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Notes on the effects produced by the ancient glaciers of Caernarvonshire, and on the boulders transported by floating ice. By Charles Darwin Esq M.A., F.R.S. & F.G.S.─

Guided & taught by the abstract* of Dr. Bucklands memoir "On Diluvio-Glacial Phænomena in Snowdonia and the adjacent parts of North Wales"* I visited several of the localities there noticed, & having familiarised myself with some of the appearances described, I have been enabled to make a few additional observations.) ¶

Dr. Buckland states that the most distinct mountain, which he observed in North Wales is a bank

* Read before the Geological Society, December 15. 1841: & the abstract is published in the Athenæum 1842, p. 42.

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Dr Buckland has stated that a mile E of Lake Ogwyn there occurs a series of mounds, covered with hundreds of large blocks of stone, which approach nearer to the condition of an undisturbed morain, than any other mounds of detritus noticed by him in North Wales. By ascending these mounds it is indeed easy to imagine that they formed the N. Western lateral morain of a glacier descending in a NE line from the Glyndr Great Glyder mountain. On but at the southern end of Lake Idwell the phænomena of the phænomena some of morains are presented, though on a much smaller scale, are pres with perfect distinctness. On entering the wild amphitheatre in which Lake Idwell lies, some small conical & irregular little mounds may be seen at the further end, which would not attract anyone's scarcely might easily escape attention.

The principle ones chief ones lie on the west side of the grand great black perpendicular face of block rock, which forms (a)

On a nearer inspection they are found to consist of earth & stones, with many large boulder blocks on the surface (Back of Page)

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(a) the southern end boundary of the lake; other mounds are situated border on the eastern base of the precipice These mounds They have been intersected in many places by streams, & they are seen These mounds are seen to consist of earth & detritus, with great blocks of rock on their summits. R Until ascending them, They at first appear quite irregularly grouped, but immediately on ascending their further ends, they are instantly seem seem to fall into three (with traces of a fourth) narrow straight linear ridges. The one nearest the great precipice runs someway up the mountain, but the outer one ridge is longer & more perfect, it runs 10 to 15 and forms a trough with the mountain-side, from 10 to 15 feet deep. The outer ridges diverge more On the eastern and opposite side of the head lake, corresponding corresponding but less developed mounds of detritus detritus

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may be seen running a little way up the precip mountain.

It is, I think, impossible for any one to stand on who has read the descriptions of the morains in Alps, to stand on these mounds & for an instant to doubt here standing on that they are ancient morains; nor is it possible to conceive wh any other cause which could have abruptly thrown up these long narrow steep mounds of unstratified detritus abruptly against rubbish o the mountain-sides.

The I do not The three or four linear linear ridges evidently mark the principal stages in the retreat of the glacier; the outer one extends furthest is the longest & diverges most from the great wall of rock at the south end of the lake. The inner lines distinctly define the boundary of the glacier in during its last latter existence. & do not exaggerate when I At this period a small & distinct glacier seems to have descended from a narrow, but lofty gorge on the north-western end of the lake; & here tran remnants of a terminal a terminal morain may be traced in the little mounds, forming a broken semicircle round a rushy plain, scarcely more than a hundred yards in diameter. The rocks are smoothed, so mammillated & scored all round the lake, & at some little depth beneath the surface of the water, as I could both see & feel. Similar marks occur at great heights (where morain on b all sides, far above the limits of the morain just described, & were found produced at the time when the ice poured over

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the great rocky barrier which bounds the northern end of the lake. I may here mention that amphitheatre of Lake Idwell.

(I may here mention, that about 80 yards west of the spot where the river flows escapes from the lake, through a low mound of detritus, like a terminal morain, there is an example of the cases fact described by Agassiz by Charpentier & Agassiz of a great boulders broken into pieces, as if fallen from falling vertically down down through a crevice in the ice. It now consists of four great tabular masses, standing a little way apart, but with their sides, two of which stand on their edges, on a gentle slope & two have partly fallen over against a neighbouring boulder. The two upright pieces stand transversely. They must have

From the distance, at which the four pieces stand from each other. They must have been pitched into their present place position with great force; & as the two upright thin tabular pieces stand to the are placed transversely to the slope on which they rest, it is scarcely possible to believe that they could have been rolled down from the mountain behind them; & therefore must have been dropped nearly vertically.

The rocky & steep barrier (between 400 & 500 feet in height) over which the ice from glaciers folding up ice from the amphitheatre of Lake Idwell must have poured flowed into the valley of Nant-Francon, presents from its summit to its very foot (between 400 & 500 feet) present the most striking examples of the bos or dome-formed rocks (B) When two of them stand near are separated from each other by little gorges, their steep rounded sides are generally distinctly scored with lines, slightly dipping towards the great great valley in front. The summit of the summit bosses of rock are comparatively seldom scored; but close to the bridge over the

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(B) which might have served for models for some of the plates in most strikingly resembling probably (as remarked by Dr Buckland) some of the plates in Agassiz work on glaciers.─

Agassiz work on glaciers

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Ogwyn, I remarked some on singular zig-zag ores scores. At this spot the cleavage of the slate is nearly highly inclined, & owing apparently to the different degrees of hardness of the laminæ gentle furrows have been produced during by the grinding action of the ice, transversely to the part pass course of the ice, and scores, & to the probable course of the ice glacier. I here, as well as in some few one or two some other places noticed an appearance which made it vividly clear, that its formation of are very evident that these bosses could were not have been formed by aqueous had been effected by some process quite different from ordinary aqueous or atmospheric erosion; it is the abrupt projection to the length of one or two feet from the smooth surface of a smooth bos of a a pieces of rock a few yards square, & one or two feet in height, with its surface smoothed & scored like the bos fr on which it stands, but with its sides jagged rough : if a statuary were to cut a small figure out of a larger one, & to leave a small untouched before he quite completed his work, the abrupt projecting portions might be compared to these p masses of rock: how it comes that the glacier in grinding down a bos to a smaller size should ever leave a thin small portion apparently untouched, I do not understand.

On the summit of some of the bosses on this barrier there are perched boulders: but this phænomenon is seen far more strikingly close to Capel-Curig, where almost every dome of rock south of the Inn isl surmounted by one or more large large angular masses of foreign rock.

The contrast of the rude form & composition of these blocks of rocks, with the smooth mammillated domes on which they rest, struck me

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as one of the most remarkable of the effects produced by during the last retreat by the passage of the glaciers.─ (A)

¶ (Proceeding down the great great straight valley of Nant-Francon, which must formerly have conveyed have conveyed the united glaciers from Lakes Idwell & Ogwyn, we continue to meet with mammillated bos-formed rocks, till below the village of Bethesda; from this point towards Bangor thes these bos-formed rocks become rare; at least it is certain that a for a large number of irregular hummocks of rock with rugged surfaces project, whereas higher up in this valley, & in all the great central valleys of Snowdonia, such unground hummocks are not to be met with. At Bethesda, also beds of an unstratified masses of whitish or firm earth from ten to forty feet in thickness, full of [illeg] boulders mostly well rounded, & of but some angular, from one to four feet square, are first met with. This deposit is interesting from the boulders being deeply scored, like the rocks in situ over which a glacier has passed. The scores are sometimes irregular & crooked, but generally quite parallel, as I distinctly saw as one over the whole entire side of one large ore block. Some of the blocks were scored only on one side, others on two sides, but from the difficulty of turning over the larger ones, I do not know which case is most common. I saw one large block on which the scores on the opposite sides were quite all parallel; & another irregularly conical one, four feet in length, of which three fourths of the circumference were was marked with parallel striæ, converging towards the apex. (a) In two large blocks the one side of each of which

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A) on the sides of the valley mountains above Capel-Curig, I observed some boulders left sticking on very narrow shelf shelves of rocks, & other boulders of vast size scattered in groups. The largest boulder, I noticed there, was about 26 feet in length by t 12 in breadth & of unknown thickness.─)¶

(a) In the smaller elongated blocks, from six to twelve inches in diameter, I observed that the score or striæ were generally, if not always, parallel to their longer axis, of the stones, which shows that when subjected to the abrading abrading force, they arranged themselves in lines of least resistance.

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was exposed Out of three large blocks which chained remained were imbedded but partially exposed in a perpendicular cliff, the vertical sides of two were scored in a horizontal lines, of and of the third in an oblique direction. These several facts, especially the parallel scores striæ, on the upper & lower surfaces, show that the boulders were not scored on the spot, where they are now lie imbedded, as seems to have been the case with the boulders described by Mr. Maclaren* in the till near Edinburgh. The contrast is very striking in form of the state of the surface between of these boulders and those which lie scattered high up on the sides of the adjoining hills & of the central valley, from Caernarvonshire are or tall perched on the worn bosses; of rock there which were must have been undoubtedly carried on the surface of the glaciers to their present position is very striking or are perched on the worn bosses of naked rock; the boulders in these taller cases such boulders, as I particularly noticed, present no signs of attrition which accords with scores or striæ, but are with angular or as might have been anticipated from their being the s if as is supposed, they were transported on the surface of the glaciers.

In the quarries which I examined, namely, below the village of Bethesda, & at some little height on the eastern side of the village, the till rested on slate-rocks, not worn into bosses:─ I found, however, a rather smooth pap of greenstone marked with a few deeply scored deep scores.

The till forms, at the height probably of 600 feet above the sea, a little plain sloping seaward & between Bethesda & Bangor, there are other gently inclined

* Geology of Fife & the Lothians, p. 212.

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surfaces composed of till & stratified gravel. From the

Considering these proofs of recent elevation of this coast, hereafter to be mentioned, I cannot doubt that this till has been accumulated in a sloping sheet beneath the waters of the sea: in composition it resembles some of the beds of till in Tierra del Fuego, which have undoubtedly had this origin. I presume the scored, rounded & striated boulders were delivered pushed, in the form of a terminal moraine into the sea, by the great glacier which descended Nant-Francon.) Hence

(Mr. Trimmer* reports on the authority of some workmen, that sea-shells have been found on Moel Faban, two miles NE of Bethesda.— I ascended this & some neighbouring hills, but (of about but could find no trace of any deposit likely to include shells. This hill is stands isolated, out of the course of the glaciers from the central valleys; & it exceeds 1000 feet in height; its surface is jagged & presents not the smallest appearance of the passage of glaciers: but high up on its flanks (& probably perhaps on its very summit) there are large, angular & rounded boulders of foreign rocks.─

# G. Proceedings of the Geological Society, vol. I. p. 332.

(B) (note continued)

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(B) (note continued)

Mr. Trimmer was one of the earliest observers of the scores & other marks on the rocks of North Wales.

B. (note continued) Mr Trimmer has He has also, remarked that "some of the larger blocks amid the gravel have also deep scratches upon their surface." Mr. Trimmer himself found broken sea-shells in the diluvium at Beaumaris.

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Along the sea-coast Between Bangor & Caernarvon, and on the Caernarvonshire plain, I did not notice any bos-formed hillocks of rock. The whole country is in most places concealed by beds of till & stratified gravel, with scattered boulders on the surface: some of these boulders were scored. From the account given by Mr. Trimmer* of his remarkable discovery of broken fragments of Buccinum, Venus, Natica, & Turbo, beneath 20 feet of sand & gravel, on Moel Tryfan (SE of Caernarvon), I ascended this hill. Its height is 1192 feet*B to above the sea; it is strewed with boulders (not rested) of foreign rock, most of them apparently from the adjoining neighbouring mountains; & near the summit I found the rounded chalk-flint* (z) & pebbles small pieces of white granite alluded to by Dr. Buckland. The hill stands Its form is conical & it stands isolated: c wherever the bare rock protrudes, its surface is jagged, & shows no signs of being in any part bos worn into bosses.(P) A little way down the hill, there are many slate pits, & here the cliff bed of drift a bed two or three feet in thickness, of broken fragments of slate mixed with few imperfectly rounded pebbles & boulders of many kinds of rock, are seen in several places to rest on the slate, the upper surface of which, to the depth of several feet, has been disintegrated, shattered & contorted in a very curious manner. without the The laminated fragments having been removed and in some cases not entirely without their being, have not always been entirely displaced─ I did not succeed

*Ibid. / *B Murchison's Silurian System, p. 528.─

/ *z I may mention that at Little Madely, in Staffordshire, I have found chalk-flints in the gravel-beds, associated with existing species of sea-shells.—

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P. The contrast between the superficial slate of the bare rock on this hill & on Moel Faban, with that of the rocks in within the great central valleys of Caernarvonshire is very evident remarkable; it is a contrast of precisely the same kind as may be observed in these same valleys by ascending either side above the reach of the ancient glaciers.─

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in finding any fragments of shells, but near the summit of the hill on the eastern or inland side, th I found beds, at least twenty feet in thickness─ of irregularly stratified gravel & boulders, with distinct & quite defined layers of coarse yellow sand & others of an fine argillaceous nature nature. These beds so closely resemble those of Shropshire & Staffordshire, in which I have found are found (as I have myself observed in very many spots places) fragments of sea-shells, & which no every one now see, I believe, since the publication of Mr. Murchison's Work chapters on the Drift of these counties, admits were deposited amid the on the bed of the sea. are of submarine origin. It may, therefore be safely concluded the sea once rested as the close rule the shells, discovered by Mr Murchison on Moel Tryfan, were deposited the stratified deposits the boulders & pebbles of far-transported rocks beds which form one deposit, the layers sand, gravel & coarse & & argillaceous sand, & of gravel, [illeg] the boulders with far-transported pebbles & boulders, do not owe their origin to an inundation, but were deposited when Moel the summit of Moel Tryfan stood submerged beneath the sea & surface of the sea. Although I was unable to find any beds, likely to contain preserve sea-shells or Moel Faban, considering the number presence of boulder on it, together with the absence of mark. As there are no marks of the passage of glaciers over it this mountain (which indeed from its position could hardly have happened), we must suppose that the boulders were transported on floating ice; which and this accords with the remote origin of some of them, & with the presence of sea-shells of them. Within the central valleys of Snowdonia, the boulders appear to belong entirely to the

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rocks of the country. May we not conjecture that the icebergs, grating over the surface, & being lifted up & down by the tides, shattered & distorted pounded the surface of the soft slate-rocks in the same manner as they appear to have contorted the sedimentary beds of Norfolk the east coast of England (as shown by Mr. Lyell)*(a) Back of Page and of Tierra del Fuego. Although I was unable to find any beds on Moel Faban likely to preserve sea-shells, considering the absence of the marks of the passage of glaciers over it, I cannot doubt that the boulders on its surface were transported on floating ice. In a paper read before the geological society*(B) I have remarked that blocks of rock are transported by floating ice under very different conditions— 1st by the freezing of the sea, in countries even mountainous ones where glaciers do not descent to the coast— 2d by the descent of glaciers formation of icebergs by the descent of glaciers into the sea, from mountains not very lofty, where in latitudes (for instance in that of Geneva, or of the mouth of the Loire, in the southern hemisphere) where the surface of the sea does not never freezes & 3d, by these two agencies united. I here have further remarked that the condition & kind of the stones transported would often generally be influenced by the according to by the nature manner of production of the floating ice. From the nature of the Chalk formations In accordance with these views, I may remark that it does not seem probable from the low level of the Chalk-formation in Great Britain, that rounded chalk-flints could often have fallen on

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*(a. On the Boulder Formation of Eastern Norfolk London. Philosoph. Magazine. Vol. 16, (May) 1840, p. 351 &c

*(b) May 5th 1841. On the distribution of the Erratic Boulders, & on the contemporaneous unstratified deposits of S. America."

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the surface of glaciers, even in the coldest times, & therefore I conclude that such pebbles they were probably inclosed by the freezing of the water on the coast sea-shores. But we have the clearest proofs of the existence of glaciers in this country; & which in the vale of Nant-Francon seem to have reached the sea, when the land held a lower level & it appears that when the land stood at a lower level, some of the glaciers, as in Nant-Francon, reached the sea, we may therefore suppose that the great angular blocks of Welch rocks, scattered over Staffordshire over the central counties of England*(a) were transported on icebergs. I looked carefully in the the [illeg] valleys of Caernar near Capel-Curig & in Nant-Francon for beds of pebble, or other marks of marine erosion, but could not discover any: when, however, the Moel Tryfan & Faban stood beneath the level of the sea, inland creeks of salt-water must have extended far up nearly far up these valleys; & into which & on the surface of the water & where they were deep, the glaciers (as at present in Spitzbergen*(a)) would have floated on the surface, ready to become detached in large portions charged with. From the presence of bos-formed rocks low down down in the valley of Nant-Francon & on the shores of the Lakes of Llanberis (310 feet above the sea) it is evident glaciers filled the valleys, often the land had to risen to nearly its present height.

*B. Dr. Martens on the Glaciers of Spitzbergen, Edinburgh Phil. Journ. 1841, p. 288.

 

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*(a) on the summit of Ashley Heath in Staffordshire, there is many an angular round block of syenitic greenstone, four feet and a half into by four feet squar, & two feet in thickness. This point is 803 feet above the level of the sea. From this fact together with respect those relating to Moel Tryfan & Faban, we must, I think, conclude that the whole country of this part of England was, at the period of the floating ice, deeply submerged.

From the reasons given in my paper (Phil. Transact 1839.) I do not doubt that at this same period the central parts of Scotland stood at least 1300 feet beneath the present level, & that the its emergence has since been very slow. The boulder on Ashley Heath must have probably has been exposed to atmospheric agencies disintegration longer period than any other in this part of England. I was therefore, interested in comparing the state of it's the lower part, which was buried two feet deep in compact ferruginous sand (containing only quartz pebbles from the subjacent new red sandstone) with the upper part; but I could not, however, perceive the smallest difference in the preservation of the sharp outlines of its angles sides. I had a hole dug under another large boulder of dark green felspathic slaty rock, lying at a lower level; it was separated by 18 inches of sand, (containing one two pebbles of granite, & some angular & rounded masses of new red sandstone) from the surface of the new Red Sandstone. One of the rounded balls of this latter stone had been split into two & was deeply scored, evidently by the deposition of the boulder.

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& the these glaciers must have swept the valleys clean of all the rubbish left by the sea.—As far as these my very limited observations serve, I suspect that bos-formed that bos or dome-formed rocks will will serve as one of the best criterions between the effects produced by the passage of glaciers & of icebergs*(a)

Dr. Buckland has described in detail the marks of the passage of glaciers along nearly the whole course of the great central Welch valleys of Sandstone ; and I observed that these marks were evident at some hundred feet on the mountain-sides of the valleys hills above the watersheds dividing points watersheds, whence of division, whence where the water streams flowing into the sea at Conway, Bangor, Caernarvon, & Tremadoc Tremadoc, as the Bay of Cardigan divide: hence it appears that a person starting from any one of these four places (or from some way up the valley whence where the glacier ended), might formerly have walked on ice & without getting off the ice of the glacier ice , have come out at either of the other three places, or some till or above the or low down in the valleys in which they stand. The mountains at this period must have formed seperate islands, surrounded on one side separated from each other by rivers of ice & surrounded by the sea. The thickness of the ice in some several of the valleys must have been great. In the vale of Llanberis I ascended a very steep, abruptly [illeg] mountain ENE of the upper end of the upper lake, which slightly slightly projects a [illeg] where the valley slightly bends for

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* In the Appendix to my Journal of Researches, I endeavoured to show (in 1839) sometime since, from facts communicated to me by Dr. Richardson (p. 619) by the polished state of the rock in the rivers of North America which convey ice

that many of the appearances attributed to debacles & to the movements of glaciers on solid land, would in all probability be produced by the action of stranded icebergs. I have stated (p. 619), on the authority of Dr. Richardson that the rocky beds of the rivers in North America which convey ice, are smoothed & polished; & that (p. 620) the icebergs on the Arctic shore drive before them every pebble from the bare & leave the submarine ledges of rock absolutely bare.—

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the lower 1000 feet (estimated, I think, correctly) the marks left by the glacier were very distinct, especially near the upper limit, In where there in scores under are were boulders perched on the bosses of rocks & where the scores on the nearly vertical faces of rock where were, I think, more distinct than I anywhere else observed there others, which I saw. This shoulder of the mountain seems to have been opposed to the the course of the ice-stream & glaciers & the ice

The scores on some of the almost vertical faces of rock might have been seen 50 or 60 yards at a consi

These scores on the nearly vertical face of rock are generally slightly inclined but at various angles downward seaward, like as the surface parallel to the bed of the valley; in some spots however the do of the glacier must have been. on some few faces of rock this inclination must have been as much as ten degrees. But on one particular face of rock, much scored, inclined the sloping & inclined at an angle of somewhere about 50 degrees.) well marked & continuous lines, well-marked sloped were inclined sloped upwards (in a contrary sense to e the surface of the glacier at an angle of 18°, with the horizon. I laid down marked copied the angle score on a large sheet of paper, & afterwards I allowed for the inclination of the rock. This cause of this appears to have been

This face of rock did not lie parallel to the sides of the main valley, but formed one side of the sloping end of the mountain, over & round which the ice appears to have swept & with prodigious force, to expanding its stream laterally after being closely confined by the shoulder above mentioned. At this point, where the glacier has burst laterally swept to the westward, over & has expanded, its height surface seems in a short space to have to have rather sensibly decreased in a short space.  For on a

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sensi declined much: for on a hill with lying N.W of the shoulder & forming a lower part of the same range (it stands SSE of the Victoria Inn, & has a reddish summit) the marks of the passage of the glacier are at a considerably lower level, although. At the very summit, however, of this hill, several large blocks of rock of rock have been moved from their places, as if the ice had occasionally passed over the summit, but not for long enough periods to wear it the rocks smooth.—

I cannot imagine a more instructive & interesting lesson for anyone who wishes (as I did) to learn the effects produced by gla the ancient passage of glaciers, than to ascend a mountain like one of those [illeg] run south of the upper lake of Llanberis, which are constituted of the same kind of rock, similarly stratified, from top to bottom. At The lower portions consist entirely of convex domes or bosses of vert naked rock, generally smoothed, & here or there but with their steep faces often deeply scored in nearly horizontal lines, & with their summits occasionally crowned supporting surmounted by perched boulders of some different variety of foreign foreign rock; T the upper portions, on the other hand, are less naked and the are generally more thickly covered with turf, through which the jagged ends of the slaty rocks project through the turf in irregular hummocks;— these no ha smooth bosses, no scored surfaces, no boulders are

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M. S. & Mear…. Heights

on N. Wales. — Glacier

Paper August 1842.

Original notes

Notes on forms of

Rocks in eddies of

Welch. Torrents.