SNOWDON. CENTRE DE SIX ANCIENS GLACIERS

the top of Ihe valley ; but débris and scattered blocks are numeruu? , and one of tmw^ical size bas been left by the retreating ice just abcvthe gorge, suowdon R. 40. Such is a brief account of some of the more strîking y/o^ /"/

ccDiro e iix anciens ^^^^^^g^^ Tound SnowdoH. The mountain formed the centre of sii

glaciers that flowed from thef direction of the peak down the vallt}? of Cwm-Brwynog , Cwm-y-clogwyn , Cwm-y-Llan, Llyn Uydaw. and Cwm-dyli, Cwm-glas , and Cwm-glas-bach. Two of thèse, from Cwm-dyli and Cwm-y-Lland , flowed into the larger glacier-volhj of Nant Gwynant, two passed out to the west, and the glacier A Cwm-glas-bach and Cwm-glas contributed to swellthe mass oftiie great glacier that descended the Passof Llanberis. — Afler a fewdav? spent in exploring thèse recesses , the mind readily embraces th» whole subject, and a short walk from Llanberis on the heigbts "i Snowdon , along the ridges from Llechog to Crib-y-ddysg}ll ani Blaen-y-maen S enablesthe glacialist mentally to restore the n-hJ.-: séries of glaciers that radiated from the central peak. Série d'obiemuon» R. 1 1 . Tlic fact that the valleys of North Wales were once ti>

pour déterminer la ^qysbù bv vanishcd qlociers implies that the ice must hâve passeJ

plus giande épaiueur o «/ & s

que pouTent avoir les tlirOUgh SCVCral pliaSCS.

eiaciers dans le pat- geforc thc commcncement of the Newer Pliocène period , aiiii

sage de Liant eris.

aller the greater contours of the country were much as they arcnow, there were , probably, for a time no glaciers hère. After llii> they began , attained a maximum size , dwindled , and disappeareii. Reflecting on thèse things , it became an object with me y in tl)^ year 1854, to endeavour to ascertain what might hâve beentli* greatest thickness of the ice in the Pass of Llanberis. For this purpose I made a séries of observations with the sympiesomeler, t.* détermine the height above the bottom of the valley of the hi'jh ^^ striatïons that run south-east and north-west, or in the directiL of the fbw of the glacier. The method was simple : first, by raapping the striœ I rendered myself familiar with the glacial phaimena ofthe valley. l'then selected ten or twelve favourable points, and at each made first an observation with the instrument in tli-: bottom of the valley, and then ascending the hills on both sides a:

* Front of the Stone.

SÉRIES d'observations. 579

right angles to it, I determined the height above the bottom at which striations were found running along the hills in the direction of the gênerai fall of the Pass thus * :

If , as I believe , the striations at 6, c were made by a glacier that flowed down the Pass , then the Une a , d may represent approximalely the thickitess of the ice at one period of its history ; and uniess the glacier, by long continued érosion of its rocky floor, contributed tnuch to deepeix the vdlley**^ the average thickness of the ice was from 1400 to 4300 feet (335 à 395 mètres). Buth the surface of th^e ice at b, c was higher than the présent watershed of Gorphwysfa at the upper end of the Pass , and to produce a glacier-stream flowing down the valley and filUng it to heights so gi^at half way down the Pass, at the présent watershed there must bave been a pressure of accumulated ice , not less than about 500 feet (450 mètres) thick. That glaciers must deepen their vaUeys is self-evident , and therefore the above estimate cannot be strictiy correct. But it is nevertheless of suf&cient value to warrant the conclusion, that when the Welsh glaciers attained their largest size , the ice was of very great thickness ; and therefore , — considering the shape of the ground and the ice marks, — of a length so great, that it flowed weli d own across the lower hills that skirt Llyn Padarn , in the manner shown by the striations above that lake on the raap***.

It would be easy to give descriptions of other old glacier vaUeys in many respects as remarkable as those of Snowdon , more especially of Cwm-du beneath Mynydd-mawr^ about a mile west of Llyn- Cwellyn ; of part of the country between the river Conwy, Afon Llugwy S and Nant Francon ; of the vaUeys leading up from Traeth

" D. Â. Illustration du passage de Uanberis.

** D. A. L'action des glaciers sur les surfaces de roches qui les encaissent est très-considérable et infiniment (incomparablement) plus forte que celle faite par les courants d'eau.

**' D. A. Les roches moutonnées, usées, polies et striées sont des preuves de Tancienne extension des glaciers à une certaine époque. — Ces limites ne sont pas le maximum de la puissance des anciens glaciers monstres. (Voy. dans nos matériaux : Glaciers en activité),

* Afon means «a river, » and Llugwy <the outbreack of the water.» The word is very appUcable when taken in connection with the source of the river; Ffynnon Llugwy ^ the « spring» or « source of the Llugwy.» The moraine mounds Ihat dam the lake bave been eut through by the river in a manner suggestive of the bursting of a dam, though in reality it was probably only deepened gradually by the natural outflow of the water.

Contrée

entra Bangor et Be-

tfaesda.