HYPOTHÈSES GLACIAIRES

Much of thisDrifty though rudely stratified, singularly ressemLlvi ordinary moraim malter in the appearance and quality of ilsmud. and the polùh y approocimate angiUarity^ scratched surfaces and >i2.'< of its stones. From circumstances presently to be meotioned, it i^ to me certain, that at a time when North Wales was so far subm.!ged that only the higher mountain tops rose as islands , — noo them more than about 2000 feet (600 mètres) high * some of th even then gave birth to glaciers that descended into the sea , ait: their ends breaking offin icebergs, floated hither and thither, vhi< L as they mehed, deposited their stony freights in the sea. The inio sity of the cold may be inferred from Ihis. The sea then flo\uthrough some of the greater valleys between the Menai Slraits ai Cardigan Bay, across the présent watersheds of the Passes**. Tb: principal of thèse are the Vale of Conway, and its upper branches t»

Capel Curig etc.; the Pass of Nant Francon , and its contiouali •:.

between Llyn Gwynant and Capel Curig ; the Pass ot Llanberis <ab .: iOOO feet (300 mètres) high at the watershed), opening into Cud Gwynant ; and the Valley of Afon Gain , between Gacmanron âi Beddgelert. The country was thus broken into a group of islanir . each one of which in great part had its coverîng of saow and kr. permanent till large cosmical changes produced a decided amtH ration of climate.

It is, therefore, not improbable that before this change i'-^ place, in other portions of thèse islands not possessed of the ft rm requisite to originate massive glaciers, snow and glacier ice iL^y nearly hâve covered their entire surfaces ; for, unless the cold wer sufTicient to produce such a resuit , it is dilBcult to understand li. *« on other parts of thèse small islands, good-sized glaciers, such jï then certainly fiUed the valleys, could bave been produced. But ii this covering of snow and ice did exist, it is very intelligible h:» the drift on the sides of the mountains is generally composed • ' stones from the hills close above, and is also more or less moraiolike in its character. In fact it is not till we reach the comparât! vt!<

* D. A. A une certaine époque , époque des glaciers monstres , toutes les socu. i> d'Angleterre étaient plus que probablement couverts de glace. " (?).

VALLÉE DE LLÏN CWELLYN ET LAC DE LLANBERIS. 591

distant low ground of Caernarvonshirc, near the sea, and the plains of Ângicsey, that far-travelled fragments from doubtful or unknown régions begin to occur in Drift deposits which are perfectly distinct from glacier moraines, although resting on ice-smoothed and strialed surfaces, the directions of the striations on which bear no ralation to the glacier valleys of Caernarvonshire , but run transver-

Sely to them , and were , in my opinion , produced by icebergs fioating , mostly from N. N. E. Unter any such circumstances , small icebergs and coast-ice grating

along the shores and sea bottoms, would in the course of time be suf&cient not only smooth and groove the rocks, but also to scratch blocks and stones that lay in the bottom of the sea or were imprisoned in the floating ire *.

Between the valley of Llyn Cwellyn (Cwm Seiont*) and the lakes v.iiée of Llanberis, there is a wild and high moorland tract almost enti-***"j^^j[*"[°J* ""^ rely covered by Drift, till we reach the solid rocks above Llyn Padarn. Just above either side of that lake, the hills are generally craggy, or else montœinéesy and clear of Drift; and beyond Clegyr towards Nant Francon, west of the mountains of Eiidyr fach and Bron-Ilwyd , there is a broad moor formed by Drifl of great thickness, and which, from an average height of about 1100 to 1300 feet (335 à 400 mètres), stretches eastward into the valley of Marchlyn-mawr, where it attains an élévation of about 2000 feet (600 mètres). Standing on this moor, above the left bank of the Ogwen, the eye easily detects on the opposite slopes a corresponding accumulation, stretching smothly up the higher valleys towards Aber, and bending on the east and south-east towards the valleys of Afon Berthan*, the Llafar', and Afon Gaseg*, streams thatrise in the higher recesses of Carnedd Dafydd aud Carnedd Llewelyn", on

their seaward flanks. in the vaUeys through which thèse streams flow*% the

Drift attains an élévation of about 2300 feet (700 mètres), stret-

* D. A. L'impression en petits caractères rappelle que les glaces flottantes ne polissent pas , ni strient , ni burinent les roches — 1862.

* SeUmt or Segontium.

* Berthan, a bush, « Bushy Brook.»

* A voice ; alluding to the fine echoes of this great semi-circular hollow.

* Gaseg, a mare

^ David's Carn and LleWélyn's Carn.

*' D. A. Système diluvien, qui n'est nullement confirmé. —^ Le système glaciaire explique tous les faits hypothétiques. — Voy. dans nos Matériaux preuves nonibreuses et positives.

592 GWM LLAFAR.

ching into their wide recesses with a smoolh ouUine, and 1 «as informed ^ Ihat part of thèse deposits contaîns shells al a height ùi about 1000 or 1200 feet (300 à 365 mèlres). cwm-Ltariir. Cwm-LIafar has an inleresting history, and is worth a visil, k ii.j easily reached from Bethesda or Glan Ogwen. A vasl semicirctilar hollow surrounded by tbe black crags (chiefly greenstone) of Cainedd Dafydd, Carnedd Llewelyn and Yr Elen', forros the uppereoi of the valley. In its botlom , belween the last-nanied peak and Hy nydd-du', the Drifl, thickly charged with angiUar boulders , okn of large size, forms a succession of terraces, the resuit of mariue denudation during pauses in the reelevalion of the country aller ib submersion to a deplh of more than 2000 feet (600 mètres). On ihe soulh-western side of Ihe valley there is a long and coroparati\eK narrow channel , through wich the brook runs , bare of Drifl a^d boiUderSy and having on the Mynydd-du side a well-marked ruLk moutonnée f sUiated in Ihe direction of the valley, thus oiarking tlie course of a narrow glacier about two miles in length, Ihai ploiujh-^' oui a long straight hollow in the Drifl. That this clearancc was ni efiected by the existing little strcam is évident ; for it is utterh i::adequate to remove the great blocks that form the bulk of the 1erraced drift. The removal by running water of the lighter delriius mixed with thèse would, indeed, hâve concentrated tbe bouldersi-ii the surface of the straight alluvial hollow, in the manner tbey m^^ are concentrated by river denudation on the banks of the far in«Hr: powerful Gorfai, two miles S. andS.E. ofCaernarvon*. As the Cwu Llafar glacier decreased in size , it deposited a minor terminal mi^ ravie where the brooks meet below Nant-bach and Nant-y-crai^ Wilhin this barrier a small lake seems to hâve been dammed u|>. tili the brook at the outlet eut a channel to the botlom of the muraine, and the water of the lake was drained away.

The same ploughing out of the Drift by glaciers on a rauch lancer scale is évident in the Passes of Nant Francon^ and LIanberis. lu

* By the late Mr. #Mhiui Trimmer.

* Elain , a fawn.

* The Black Mountain.

*0n a grander scale this concentration of boulders is well seeo in places oa the ltf«e^ terraces of the rivera St. Lawrence and Ottawa.