VALLÉE DE LLYN IDWAL

Twll-du S famous for ils ferns and saxifrages (the relies of an Alpine floia), closes the valley on the south-west, above which, by Llyn-ycwn*, there is an upland valley, covered with angular drift^ ieading to the Pass of Llanheris. Below Twll-du,in the bottom, lie the sombre waters of Llyn Idwal, the depth of which I do not hnow, but which are partly dammed up by a terminal moraine y lowest in the middle, thegreater amount of débris having, — as is usiial with short glaciers — been brought down at the sides of the ice , where, under the circumstances, the subséquent removal o{ moraine matier was also probably less. The water is also, I incline to think, partly retained because it lies in a rock basin ground out by the old glacier. Below the moraine ^ ail the way down to the Ogwen, the rocks are strikingly moutonnées , and when undecayed they are covered wilh striXy which gradually curve round to take the direction of the main valley. Between Llyn Idwal and Cwm Cywion, on the top of the dividing ridge, high above the north-western angle of the lake, I found grooves running right across the watershed in the direction of Nant Francon ; as if Cwm Idwal, before the glacier dwindled to form the still-existing moraines, had at one time been full of ice j at least to the height of thèse striationSy thus overfloving the barrier that divides it from the west side of Nant Francon. If so , it may be that at this period , or when still larger, it abutted on the opposite mountain lelow Braich-du', thus producing the singular smoothing of the rocks well up the clifT, and striating them transversely to the slope of the hill , in the direction in which the ice would unter the circumstances bave escaped down the valley.

As the glacier decreased in size it deposited the moraines that now skirt Llyn Idwal, its progressive diminution being marked on the i^estern side of the lake by four moraines arrangea in long symmetricai mounds one within another. There is also some appearance of an irmer terminal moraine towards the southern end of the lake , where it narrows ; and on the east there are patches of moraine tnatter and ice-sm^oothed bosses of rock rising through the soil *.

* The Black Hole.

* The Hounds' Lake.

> The Black Arm.

* D. A. Illustration : Section à travers la moraine de Uyn Idwal.

568 LLYN OGWEN (CWM BOCHLWYD).

Descending lo the outlet of Llyn Ogwen , very perfect mamitillated roches moutonnées more or less form the hill side and lie m the bottom , covered with slraight grooves , numing in the direction of the valley. Similar signs mark the lower part of the monotain on the angle of Craig-yr-hysfa, and ice-smoothed rocks, perftC or decaying, rise so high up the mountain towards Braich-du. that , to appreciate them , it is necessary to scranible along aod Dp and down the face of the mountain almost from bottom lo t:p

Some of thèse were probably made by grating icebergs during the partial submerfa.'» of the country.

uyn oriren. Cwm- Abovc Llytt Ogwen , in Cwm Bochlwyd , there are again am] le "p^lrlt proofs of glacier action. A little lake lies in the hollow of this, ol.^ d'ancieni giaderf. of tho wildest valleys in North Wales, apparently slightly dammr Lac barré ^^ ^ moraim matter; and just beyond the upper end of the lake, there is an immense moraine heap, running nearly across the ?alic} and formed of angular blocks of stone^ some of them from six to ten yards in length. Occasionally , thèse are marked by straight striù\ which do not cross each other; showing that thèse blocks once f :>rmed part of them , rent by the weather , fell on the ice , and were finally floated down to be deposited on this moraine. Fréquent ^«i perchés hère , as at Llyn Idwal , dot the roches moutonnées , and it b important to note that above Llyn Bochlwyd , striatians run southeasterly right up to the summit of the steep and shattered ridgetlu. unités Y Glyder-fach to Y Tryfan; so that hère, on the old snowshùK it would appear that the ice pressed downwards in opposite directions into Cwm Bochlwyd and Cwm Tryfan, and afterwards ciurii^* round , helped to form the glaciers that flowed northerly through both valleys on opposite sides of the sharp peak of Y Tryfan. In tLis, on a small scale, it ressembled some Alpine passes where, standiic: on a gently sloping snowshed, such as that of the Upper-Aar glacier, you look down upon opposite Systems of drainage.

Tontes lei monta. To avoid further statements similar to those ahready made , it is gnea. Tdiée. et lac. jj^^ euough to say that in the wild , little visited mountain tract,

entre Bangor, Capel c? j /

curig et conway ren- that Ucs bctwccn Baugor, Capcl Curig, and Conway, thei^e is not i ferment des prearei mountaiu, vallcy, or lakc that to the instructed eye does not shi-w

do l'exiatence dan- ^ j7 j

tiens glaciers. sigus of tho vifluencc of glaciets ; or, more strongly still, ofthe is

sea of the Drift ; and I shall now tum to the considération of il.^

relations of the glaciers of North Wales to thèse remarkable marine deposits.

R. 13. Every one who has given any attention to Tertiary geology phénomènei is aware that a large portion of the low country of the north of *""***"" " **"*"• • Europe , from the Ural to the Atlantic , is more or less covered by what is technically termed Drift ; that is to say, of loose superûcial accumulations of sand , gravel , and clay , charged with rounded ^ subangular, and angular stones and boulders, many of which bave travelled for hundreds of miles. Thus the great plains south of the Baltic are covered with fragments of granité and gneiss derived from the Scandinavian chain , an instance of which will occur to many , who remember opposite the Muséum at Berlin the great vase or tazza ofpolished granité, made from a boulder found in the neighbourhood. The plains of Siberia are in great part formed of diluvial deposits j which, according to Cltiiiaeiieir, pénétrâtes the mouths of the valleys on the northern flanks of the Altai , and shows many analogies with the Drift, even though it is doubtfuUy said to contain no erratic boulders; and ail the broad champaigns and tablelands of North America^ from the Laurentian chain of mountains to the Ohio, are covered by gravel and clay , well sown with bovlders , many of which bave travelled hundreds of miles from the gneissic mountains beyond the St. Lawrence. The east coast of England, from Yorkshire to Essex , is in like manner covered by Drift, partly of Scandinavian origin, and ail the solid formations of the central plains , from Leicester sbire to Worceslershire and Ihe borders of North Wales, are more or less obscured by détritus well charged with boulders, many of which bave been transported from Gumberland , and perhaps from Scotland, and Wales itself.

For long it was a favourite dogma that thèse heterogeneous mixtures of near- and far-transported material were scatlered over the northern continents by great sea waves , which , rushing from the north, strewed half a world with rubbish, and polished and striated the rocky surfaces over which the débris passed. But now that spasmodic geology is at a discount , more sober imaginations believe that the blocks of stone that strew our continents and islands were cliiefly dropped where they lie, by the same agency — that of icebergs — that is now sowing the Western Atlantic with earth , and

erralic boiUderSj derived from the moun tains and coasts of Gr: rland, where glaciers descend to the sea. The question then na!] rally arises.

Giader. jj ^^ j^f^^ ^^ ^^ rdatiou of the Drift to the glaciers that .

Diai^riaai erratiques, cxkted îti OUT owu moufUain vcgions?

First it must be stated , that the occurence of this Drifl high .: the mountains^ introduces an élément of great difSculty inaccijsting for some of the détails of the glaciation of a mountain lanJ Ll

North WaleS ; in so far that it is sometimes impossible to décide wfaicfa of tb« stjtions were produced by glaciers, and which by icebergs*; for when the C*^\lir

was deeply submerged , and the mountain tops formed a groiij'

islands , striations must bave been formed by the grating bergs on the sid«< ': channels (nOW clevatcd valleys) through which the icebergs floated, aiH] h é ^possible always to separate thèse from striations formed by glaciers of laler date. 1 r.3<

be, as I hâve elsewhere stated*, that an earlier set of glaciers p ceded the déposition of the Drift, but whether or not this vî^ï*'" case , it does not interfère with the fact , that after the re-elevnt of the Drift covered country, the greater glaciers ploughed ibe L out of some of the larger valleys , and during that process prod j ec those striations that still seam théier sides. An instance of the liiàculty occurs in the valley between Llyn Ogv^en and CapelCurçaD' between Capel Curig and Pen-y-Gwryd. In the former case, ihe liLher striations on the flanks of Carnedd Dalydd may bave been prodoH :

icebergs during the submersion of the country, and at a time when Drilt^^

deposited in the valley the Llugwy, and probably also in Nanl fn^ con. But al a later date , after re-elevation , this drift has b^t ploughed out of Nant Francon by a large glacier , while it slill remains in the valley of the Llugwy. The slope of the country stvx to bave sent the moviug ice that gathered in Cwm Idwal and C^c Bochlwyd, ail down the valley of Nant Francon, leaving, in 'k

* D. A. Les glaces flottantes ne strient pas les roches. Les glaciers terreskrei «n >" vite nous disent qu'ils sont les auteurs des surfaces polies , burinées , rayées et str^ et que leurs ancêtres , les glaciers monstres autrefois en activité et plein de vie. '>saient de même. Voilà positivement ce que nous disent les glaciers en adivité , ^ •>* nous le répètent tous les ans , depuis 1842 jusqu'en 1868 indusivemeot ; et ils iic''-> ^ adressez-nous des demandes et nous répondrons carément.

'Geol Joum., vol. vii, p. 371.