PEOF. W. MOEEIS DAVIS ON 



[Aug. 1909, 



it was suffering an active attack by the ice. The second example 

 occurs where a porphyry-spur of Mynydd Mawr has been truncated 

 from its former projection into the valley of Afon Gwarfai : its 

 form is bold and rugged, to a degree that is altogether inappro- 

 priate to the work of normal erosion on the side of an open-floored 

 valley, a third of a mile wide ; the widening of such a valley by 

 normal erosion proceeds gently, but these cliffs have been shaped 

 by some severe agency. The lower part of the cliffs is simply the 

 unfinished or immature side of an overdeepened glacial channel ; 

 immature because the rock there is more resistant than the slates 

 next up-stream by Llyn Cwellyn, where the channel is beautifully 

 smooth and mature, as already noted. The higher part of these 

 cliffs rose above ice-level and is due to sapping of the lower part 

 by the ice. 



The cliffs on the south side of Mynydd Mawr (fig. 32) and on the 



JFig. 32. — Sketch of the Craig y Bere, on the south side of Mynydd 

 Maivr, looking north-eastwards from a cwm south of Nantlle 

 Valley. 



east side of Moel y Cynghorion stand high above the valley- floors, 

 to which the descent is made by long talus-clad slopes : hence their 

 upper parts were probably for much of the Glacial Period exposed 

 to superglacial weathering, rather than to direct glacial erosion. 

 But they are situated where strong glacial erosion of the lower 

 slopes is expectable : in the first example, because of glacial over- 

 flow into Nantlle Valley, as will be explained below ; in the second 

 example, because of a sharp northward turn of the glacier from 

 Cwm Dur-arddu. The upper parts of these cliffs may therefore 

 be ascribed to the sapping of the slopes below them by glacial 

 erosion, and not to direct ice-action. Similar but smaller sapped 

 cliffs occur on the north side of Y Gam, facing those on the south 

 side of Mynydd Mawr. 



A paragraph may be given to the great cliffs which rise on each 

 side of the deep Llanberis ' pass ' or valley, next north of Snowdon. 

 Under the theory of protective glaciers, Llanberis Valley must 

 be regarded as of pre -Glacial origin, deepened as a result of 

 the successive uplifts of the district which gave rise to the steps 

 in the neighbouring valleys. But as thus explained, it is not 



