308 



PROF. W. MORRIS DAVIS OK 



[Aug. 1909, 



chiefly protective, or that glaciers are actively destructive. What- 

 ever difficulties and risks may attend this double enquiry, they will 



Pig. 7. — Diagram of the present features of the Snowdon mass, 

 based on a slcetch from the flank of Mynydd Mavir, looking 

 eastwards. 



be lessened by following each of its two courses carefully, step by 

 step, rather than by trying to leap at once from the beginning 

 to the end. 



XVIII. Glaciers as Protective Agencies. 



Let it be assumed that small glaciers come, in an early stage of 

 the Glacial Period, to occupy the upper valleys of a group of subdued 

 mountains, as illustrated in fig. 8 (below). The creeping ice will 



Pig. 8. — Diagram of protective 

 glaciers in the valleys of a 

 subdued mountain. 



Fig. 9. — Diagram of the <? 

 of normal erosion on fig 



slowly scrape away the pre-Glacial rock-waste and will scour the 

 underlying un weathered rocks into moutonnee form; then 

 further glacial erosion is assumed to be insignificant. But, at the 



