Gregory — Progress in Interpretation of Land Forms. 127 



"The amount of glacial erosion in the central districts has 

 been very considerable, but not greatly in excess of pre-glacial 

 soils and old talus and alluvial deposits. Most of the solid rock 

 that was carried away came from ledges rather than from val- 

 leys; and glaciers had in general a smoothing rather than a 

 roughening effect. In the outer areas on which the ice advanced 

 it only rubbed down the projecting points; here it acted more 

 frequently as a depositing than as an eroding agent. ' ' 



During the past quarter-century the cleavage in the 

 ranks of geologists, brought about by Ramsay's classic- 

 paper, has remained. Fairchild and others in America, 

 Heim, Bonney, and Garwood in Europe argue for insig- 

 nificant erosion by glaciers ; and Gannet, Davis, Gilbert, 

 Tarr in America followed by Austrian workers present 

 evidence for erosion on a gigantic scale. A perusal of 

 the voluminous literature in the Journal and elsewdiere 

 shows that the difference of opinion is in part one of 

 terms, the amount of erosion rather than the fact of 

 erosion; it also arises from failure to differentiate the 

 work of mountain glaciers and continental ice sheets, of 

 Pleistocene glaciers and their present diminished repre- 

 sentatives. The irrelevant contribution of physicists has 

 also made for confusion. 



It is interesting to note that the criteria for erosion 

 of valleys by glaciers has long been established and 

 by workers in different countries. Ramsay (1862) in 

 England outlined the problem and presented generalized 

 evidence. Hector (1863) in New 7 Zealand pointed out 

 the significance of discordant drainage, the "hanging 

 valleys" of Gilbert, The U-form, the broad lake-dotted 

 floor, and the presence of cirques and the process of 

 plucking w T ere probably first described by LeConte 

 (1873) in America. The truncation of valley spurs by 

 glaciers pointed out by Studer in the Kerguelen Islands 

 (1878) w T as used by Chamberlin (1883) as evidence of 

 glacial scouring. 



Conclusion. 



During the past century many principles of land 

 sculpture have emerged from the fog of intellectual 

 speculation and unorganized observation and taken their 

 place among generally accepted truths. Many "of them 

 are no longer subjects of controversy. Erosion has 



