NANT FRANCON ET SES VALLÉES

marked by scratches oflen as fine as if made with the point of a knife. Those in the upper part of Ihe valleys always follow the gênerai slope, but unfoilunately when I examinée! North Wales, having other geological objects more especially in view, I became at length so familiar with striations , that I often neglected to record them , and I hesitate to draw them on the map , lesl they should be ont of place.

Taking Nant Francon in connection with its branching valleys , an attentive considération of ail the circumstances bas led me to think that it was so far filled with ice, that the mouths of the minor valleys to a height of from 700 to 4000 feet (215 à 305 mètres) above the river, were over-ridden by the main stream ofice, which flowed across the lower ends of the spurs that branch from the crested ridge on the west. But , when by amélioration of climate the great glacier diminished^ the lower spurs that bound the minor valleys stood out, partly denuded of tc^, while the upland hollows still contained minor glaciers that left markings on the rocks more or less transverse to those that were formed when , from side to side, the lower part of Nant Francon was full of ice. The same is the case w ith the existing glaciers of the Aar.

In none of the tributary valleys north of Lly Idwal , are the signs cwm-Gniinog. of a small glacier so distinct as in Cwm-graianog % which , on this account, is well worth a visit. From Bethesda it is easily reached from below, and from Llanberis the quickest route is to go up Cwm-dudodyn , and down what some would consider the perilous slope of Moel Perfedd in a sort of couloir at the west end of the valley. The pedestrian then finds himself in a small craggy valley over lialf a mile in length , looking across Nant Francon. On the east the felspathic porphyry of Moel Perfedd " rises in a rough peak , and on Ihe west the great bare ripple-marked strata of the Lingula grits dip towards the hoUow at an angle of 48"* or 50<>.

At the mouth of the valley obove the steeper descent to Nant Francon, a small but beautifully-symmetrical terminal moraine crosses the valley in a long crescent-shaped curve, that once passed

Nant Francon.

Moraine terminale.

' Gravelly, or Stony Valley. See the description of the groiind inside the moraine. * Perfedd , the middle point.

584 VALLÉE DE LLTN IDWAL.

from 300 to 300 yards (180 à 275 mètres) up the eastem side ofthe glacier. On this side, as might be expected, almost every stone of tti^ moraine is a fragment of the feispatic rock of Moel Perfedd , liavii^ been shed from the edge of the glacier by a part of the ice that haï that mountain as its source. Further west along the moraine , the material becomes mixed with fragments of grit and slaty sandstone; and by degrees, passing to the western side ofthe valley, ihemoraifw malter consists entirely of pièces of the Lingula beds that fonn ttK crags of Carnedd-y f]liast\ on the right ofthe drawiog. This will Le appreciated by any one who bas studied lithologically the soîtrces of moraines^ and the graduai mingling of material in the do^uwanl flow of some of the Alpine glaciers. In Cwm-graianog the vrhole is formed of large angular loose stones mixed with smaller débris, through which the drainage percolates. The largest of thèse liesoo the top of the moraine, from 450 to 500 feet (435 à 150 mètres > above Nant Francon. It was originally eleven yards long, nine broad, and about one and a half high (130 mètres cubes), and when entire must bave weighed nearlySOO tons ; but it bas been broken intofour pièces by the frost. Two smali lines of stones descend from the lower side ofthe moraine, lookingas ifiheyhdidheen shed by ice from Cynuigraianog, even though where they lie below the greal moraine the rock stiir shows the north-westerly 5/nœ formed probably at an earlier date by the great Nant Francon glacier. Inside the moraine^ a gredt part of the bottom of the valley is covered with rubbish and heaps of loose blocks, underneath which, in places , the running water is heard flowing with a tinkling sound. Doubtless much ofthe smaller débris bas been carried away by the water, and the mounds roay he partly the resuit of the falling in of some of the material. vaiié0 xhe valley of Llyn Idwal bas long been classic ground to geolo-

'^ ^" ^' ' gists, having been the first case in which a spécial glacier valley in Wales was described in détail'. Its features are simple. The rocks of the valley consist of interbedded masses of grit and feispathic trap, lying in the form of a synclinal curve , and forming on either side the tall clifiTs of Y Garn and Y Glyder-fawr. The black wall of

* The Carn of the Greyhound-bitch.

*D. A. Illustration: Cwm-Graianog.

* Sec Cluirles Darwls , Phil. Mag. Ser. itî , vol. xxî.