434 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May 28, 



whence the greatest body of ice comes ; and consequently the en- 

 closing wall most readily surmounted will be that opposite to this 

 direction. 



VIII. Conclusions. 



At the commencement of the cold period the glaciers formed small 

 terminal moraines high up the valleys. As the cold increased, the 

 glaciers enlarged, the old moraine matter was partly pushed on- 

 wards, partly overridden ; at last on the cold attaining its maxi- 

 mum, most of the glaciers were united to form a more or less con- 

 tinuous ice-sheet. 



1. Ice-sheet or confluent-glacier Period. — This is distinguished by 

 the ice not being strictly confined to the valley system in which it 

 originated, but occasionally being thrust by lateral pressure and 

 pressure from behind over watersheds. During this period the ice 

 from the upper reaches of Borrowdale was alone sufficient to fill 

 the valley, and it passed over Castle Crag (900 feet) and exerted 

 much abrading force on squeezing through this the narrowest part 

 of the vale. This great Borrowdale glacier was also continuous 

 with another great ice-sheet which swept in a N.N.W. direction 

 across the Watendlath and Grange Fells, being, it would seem, 

 partly reinforced by ice pressed over from the Thirlmere valley 

 across the watershed by Blea Tarn. 



The ice at the head of the present Derwent "Water was thus in 

 such quantity that the western part of the Borrowdale glacier was 

 caused to overlap the ridge of Cat Bells, and partly occupy the Yale 

 of Newlands, just as part of the modern Aletsch glacier overlaps a 

 bounding wall and occupies a side valley. Great ice-sheets also 

 came down the JNewlands valley and its tributaries to swell the 

 size of the Borrowdale glacier. 



The ice in the Thirlmere valley was of such a thickness and so 

 pressed against the western side by the great supplies off the long 

 Helvellyn range, that, as already noticed, it partly escaped across 

 the western watershed south of Armboth Pell, and also took, in great 

 part, a north-westerly course on reaching the lower end of the 

 valley. It would seem, indeed, that the sheets of ice from the 

 Thirlmere and Borrowdale valleys were united in the low ground 

 north of Castlerigg Fell, and that the whole of Keswick Yale from 

 Threlkeld to Bassenthwaite was filled with ice, which abutted against 

 the flanks of Skiddaw, and perhaps of Blencathra, just as the old 

 Rhone glacier abutted against the flanks of the Jura. Like the 

 great old Swiss ice- sheet also, this probably sought an exit from the 

 vale in two directions, one to the east, beneath the slopes of Blen- 

 cathra, and the other and main one to the west, towards the low 

 ground below the present Bassenthwaite Lake. This mass of ice 

 in Keswick Yale may also have been increased by sheets from the 

 southern slopes of Skiddaw, Blencathra, and the intervening valley 

 of the Glenderaterra. 



The ice, sufficient in quantity to block up Keswick Yale where 

 widest, had, on the west, to be squeezed through the narrow neck, 



