14 



PLIOCENE GLACIERS 



Upper 

 Gallery. 



Table-case 

 a Recess 41 



in another instance generally to the west in the vast rocky 

 amphitheatre of Glaslyn and Llyn Llydaw. In the higher 

 _part of such tributary valleys, such as Cwm-y-Llan "the 

 grooves converge towards the hollows at acute angles to the 

 main direction of the valleys, in the manner that might be 

 expected from ice of considerable thickness pressing down- 

 ward, while by the weight and partial tension of the whole 

 mass, it was at the same time pushed and dragged onward to 

 feed the main icy stream, a movement and a result necessarily 

 aided by the fact that glacier ice flows faster at the centre 

 and slowerat the sides. On the sides of thePasses of Llanberis 

 and Nant Francon longitudinal grooves are found running in 

 the directions of the main valleys, at a height of 1,300 feet 

 above the bottom, sometimes quite across and transverse to 

 the mouths of the tributary valleys that enter these passes ; 

 and unless they were actually much deepened by the glaciers' 

 it follows that at one period the ice of the Pass of Llanberis 

 was 1,300 feet thick. If this were the case, then, as the 

 watershed at the top of the pass is only 1,000 feet above 

 the sea, it follows that the accumulation of snow and ice 

 above that point must have been very great, so as partly to 

 feed the glacier and produce that pressure from above that 

 aided the ice on its course. When by degrees the glaciers 

 diminished in size, then the minor tributary glaciers were 

 no longer over-ridden by the chief glacier, but each valley 

 poured in its tributary stream of ice ; and thus it happens 

 that in some cases, when carefully looked for at the mouths 

 of tributary valleys, transverse striations are found cross- 

 mg each other, one set true to the course of the original 

 great glacier, and others formed by minor tributary streams 

 of ice that moulded themselves to the branching valleys 

 when the supply of snow had declined or else the average 

 temperature had risen. By degrees these results were 

 clearly produced by amelioration of the climate, for in 

 Wales there is perfect evidence of the gradual decline of 

 the glaciers in. the retreating moraines concentrically ar- 

 ranged one within another,— as, for instance, in three or 



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