346 GLACIAL EROSION IN NORTH WALES. [Aug. I909, 



suggestions made many years ago by Prof. W. Salomon regarding a 

 possible mode of ice-erosion. The changes of pressure following on 

 the movement of the ice bring about variations in the freezing-point 

 of water, and corresponding alternations of frost and thaw. By this 

 means the rock is attacked along clefts and joint-planes, and 

 detached fragments are carried forward in the flow of the ice. 



As regarded the formation of cwms and corries, the speaker 

 noted that the independent glaciation of this region required during 

 the greatest cold a snow-line very much below the highest ridges 

 and summits. The hill-slopes above this level carrying firn only, 

 escaped ice-erosion. As the snow-line rose during a retreat the 

 glaciers broke up into smaller tongues, and higher surfaces came 

 under the action of ice. It was thought not improbable that small 

 corries had been excavated by the action of short tongues of ice, 

 which formed when the snow-line persisted at a level much higher 

 than that at which it stood during the period of maximum 

 glaciation, and after the retreat of the ice from the larger valleys. 



Prof. W. W. Watts agreed that the pre-Glacial form of Snowdon, 

 which the Author had restored with great skill, was probably 

 correct, as was indicated not only by the form of mountains on the 

 west but also on the north-east. The real difficulties of the question 

 had not been solved by the Author. These were (i) the steep heads 

 of the cwms, (ii) the steps of the cwms, and (iii) the existence of 

 lakes both in cwms and valleys. If the steep cvvm-heads were due 

 to frost, glaciers may have acted by transporting ; but the speaker 

 believed that frost work might go on under a considerable depth of 

 scree, such as the Author believed to be now filling up the cwms. 

 If the ' steps ' were due to cutting back at some point of rock- 

 difference, such difference ought to be still observable, and the 

 Author had proceeded on the statement that such differences were 

 now ineffective. As to lakes, if a glacier were steadily scouring a 

 valley down, every movement, whether upward or downward during 

 the epoch of glaciation, would result in the formation of a lake ; 

 such movement in complicated rock-structure would not necessarily 

 be recognizable by its effect on structure. 



Prof. Garwood wished to associate himself very heartily with 

 Dr. Marr in welcoming the Author to a meeting of the Society, 

 and congratulated the Fellows on the opportunity that had been 

 afforded them of listening to a most interesting communication. 

 He thoroughly agreed with the Author that the assemblage 

 of features described were essentially characteristic of glaciated 

 countries ; but, while never doubting that ice was an erosive agent, 

 he had found it difficult to account for certain features in the Alps by 

 simple differential ice-erosion, and had been driven to consider the 

 possibility of differential erosion as between ice and water. Was 

 it not possible that ice, being relatively a protective agent, might 

 have retarded the erosion of those portions of the surface on which 

 it still remained after it had retreated from other parts of the 

 district ? The speaker referred to such cases as the Albigna Valley 

 below the Maloja Pass, which was still occupied by a large glacier, 



