26 H. L. FAIRCHILD — ICE EROSION THEORY A FALLACY 



It is evident that any deepening of glacial channels must be chiefly 

 by abrasion, and that this must be proportionate to the two factors of 

 velocity and pressure, other factors remaining uniform. With this in 

 mind, let us briefly consider the accepted principles enumerated above. 



The slower movement at the bottom of the glacier causes a corre- 

 sponding reduction in one of the factors of abrasion. The reduced rate 

 of flow may be due to either of the three causes noted above (a, 6, c,) or 

 to all combined, and the reduction by a heavy load of debris and by 

 shearing may amount to practical stagnation. We find here a most 

 important principle in its bearing on deep erosion. Rapid corrasion by 

 the ice is a self checking process. To the degree that the bottom ice is 

 receiving rock debris, either by its own wear or otherwise, its flow is 

 checked, while at the same time the subglacial rubbish serves as a shield 

 and protection to the bed-rock. The product of abrasion is a rock-slime, 

 which has in itself no helpfulness in erosion, but serves instead as a 

 lubricant to prevent erosion. It seems impossible for the rock-flour to 

 be removed as rapidly as produced, if produced in large amount. Its 

 production is inconsistent with free circulation of water beneath the ice, 

 and the estimated rate of bottom melting, due to earth heat, amounting 

 to one-fourth of an inch per year, can have no important effect in wash- 

 ing away debris. The advocates of enormous erosion are requested to 

 answer this question: If any glacier sawed down its bed " thousands of 

 feet " during Pleistocene time, how did the saw clear itself for continuous 

 cutting ? 



The checking of erosion by its own product implies that abrasion 

 should be freer near the head of the glacier and diminishing toward the 

 end as the subglacial load increases. This agrees with observation and 

 contradicts the ice-erosion theory for the Swiss lakes and the Norwegian 

 fiords, where it is claimed that the ultimate sections of the valley are 

 the deeper. 



Two more elements connected with the abrasion by the glacier are of 

 interest and importance here. These are the apparent viscosity and the 

 practical rigidity — two contradictory principles which seem, neverthe- 

 less, to act together * To the degree that the glacier moves as a rigid 

 mass or bolt or plane its effectiveness is gone when its bed has been 

 smoothed so as to offer little resistance to the ice plane. To the degree 

 that viscosity is effective the abrasion is lessened by the failure of the 

 ice to hold its cutting tools up rigidly and effectively to their work. 

 Both elements are unfavorable to unlimited erosion. 



The work of the ice plane is rarely illustrated by straight groovings, 



*The latest discussion of this complex and much disputed problem is in Geology, I, by Cham- 

 berlin and Salisbury, pp. 294-308. 



