486 Alfred Hurler — Subaerial Erosion of Ski/e. 



thus attested is almost whollj^ referable to miiLlle and later Tortiary 

 times, the remaining and final touches being added b}'^ tlie ice and 

 frost of the Glacial period. Under existing conditions the processes 

 of degradation are practically at a standstill, and the total result of 

 siibaerial erosion since the disappearance of the glaciers has scarcely 

 left its mark even on the minor details of the surface-features. 



There is, of course, no anomaly in this, if we view it in 

 connection with the principle that the efficiency of the agents of 

 erosion is necessarily controlled by movements of upheaval and 

 depression in the area concerned, an aspect of the question which 

 has too often been neglected by geologists on this side of the 

 Atlantic. Although the latest movement of land relatively to sea, 

 of which we have direct evidence in the district, is one of elevation 

 to the extent of about 100 feet, it cannot be doubted that the land- 

 sculpture which gave the district its existing mountain and valley 

 system was effected at a time when the country stood at a con- 

 siderably higher level than at present. Desiring rather to record 

 observations of fact than to enter into theoi'etical discussions, I will 

 not pursue this subject. There is, however, one general question 

 upon which I feel constrained to make a few remarks, viz., the 

 share of ice-erosion in producing the existing surface-relief. 



The writings of some geologists during recent years show a 

 tendency to depreciate the importance of ice as an eroding agent. 

 How this attitude is to be justified on a priori grounds it is not 

 easy to understand. A pebble or a sand-grain held firmly in the 

 grip of a glacier must certainly be a more efficient graving-tool than 

 one rolled along the bed of a river, and the disparity in the same 

 •sense becomes more evident when we take into account the great 

 ]>ressure which must exist at every point of contact beneath a great 

 thickness of ice. We shall not, however, assign to ice-erosion its 

 due part in the total results produced unless we recognize that 



FIG. 1 



Fig. 1. — Transverse section of valley: ABCD initially, AEFC after modification by 

 glacial erosion. Longitudinal profile of tributary glen : GHB initially, GHE 

 after modification. 



its effects differ to a great extent in hind from those of water- 

 erosion. Some of these peculiarities I have specified in a recent 

 number of this Magazine (May, 1899, pp. 196-9) without attempting 

 to discuss their rationale. But, although a complete analytical 

 treatment of the subject would be difficult, it is easy to see that 

 the peculiarities in question connect themselves readily with the 



