SPECIFIC DISCUSSION 1 9 



(11) The occurrence of rock-walled basins marked with ice-scorings, 

 proves that ice does excavate basins. Cirques are admittedly a quarrying 

 work of ice in combination with weathering. 



(12) The topographic features known as " hanging " valleys are not 

 recognized as normal product of weathering and stream work, but are 

 well explained by deep ice erosion in the main valleys. 



Specific discussion. — To the layman, and even to many geologists, the 

 points presented above would seem to establish a good case for the pos- 

 sibility, if not the actual occurrence, of ice-excavated valleys and basins. 

 They are certainly admirably adapted to serve as the foundation for 

 general assertion and assumption, since their indefiniteness is good 

 defense against direct attack. We will first consider the several points 

 separately and then discuss the matter as a whole. 



(1) To produce important erosional effects by abrasion, or the rasping- 

 process, exhibited on glaciated rock surfaces, would require a vastly 

 greater length of time than can be conceded. Probably millions of 

 years would be required to accomplish any considerable amount of 

 valley-cutting. The removal of rock by the slow process of glacial abra- 

 sion is so ineffective that it is practically a negligible factor in ice erosion. 

 The smoothing, polishing, or sandpapering of rock surfaces is rather an 

 argument against deep erosion, as it is such a slow process that it is 

 inconsistent with great excavation. Its impotency as a factor in valley- 

 cutting is proven by the freshly exposed beds of alpine glaciers where 

 the amount of erosion has been just about sufficient to prove that the 

 ice has been there. And yet these were vigorous glaciers, which have 

 been attacking their channels several thousands of years longer than 

 those glaciers which are assumed to have cut valleys hundreds or thou- 

 sands of feet deep. 



The glacial striae themselves supply one of the clearest proofs of the 

 slow and ineffective character of abrasion. Cross-strise are very common 

 phenomena, and may indicate different movements of the ice-body and 

 not merely varying currents. This certainly proves the weakness of the 

 later abrasion, for if general abrasion were such an effective process as to 

 cut hundreds of feet into crystalline rocks during the Pleistocene period 

 the rock should be removed so rapidly that double sets of strise would 

 be rare phenomena. 



The deep groovings and cornice-like flutings, such as shown on Kel- 

 leys Island, lake Erie, are by their rare occurrence good evidence of the 

 weakness of glacial erosion. If abrasion were effective, such planing 

 should be common. The curving channels evidently were not wholly 

 made by the ice, and the straight, cornice-like flutings represent some 

 rare combination of special and unusual conditions. It is illogical to 



