RATE OF GLACIAL MOTION. 247 



the lake reached its maxiinuni height, accepting the evidence that there 

 was at least one such. In this case the summit of the delta would 

 doubtless be somewhat lower than it would have been if its formation had 

 been continued until the Herman beach was raised. But even in this case 

 the shore drift and the wind action that followed would modify the original 

 form and altitude of the delta, possibly to an extent sufficient to obscure 

 such limited differences as might distinguish the two hypotheses. It does 

 not seem, thei-efore, to the present writer that an argument of a demonstra- 

 tive nature can be based upon the deltas, because its validity must rest 

 upon the rather elusive differences in the heights of the deltas, since it 

 does not appear that it can rest upon the mere existence of the deltas. 

 Indeed, so far as the magnitude of the deltas is concerned, that hypothesis 

 which postulates the greater length of time in the formation of the sui- 

 rounding glacial drift is the more favorable to large deltas, especially to 

 large deltas of sand as distinguished from" deltas of coarser material that 

 would be the normal result of exceptionally rapid melting. 



The present writer does not agree with Mr. Upham in regard to a high 

 rate of glacial movement, although he formerly entertained much the same 

 view. A high rate of motion enters into the interpretation of the phe- 

 nomena under consideration as a factor in the explanation of the supposed 

 rapid production of the till under Mr. Upham's interpretation. If the 

 alternative interpretation be adopted, it is relatively immaterial what rate 

 of motion prevailed. 



It is to be observed that a rapid movement of the ice is antagonistic 

 to a rapid retrea't, because the two are directly opposed to each other. 

 With given conditions of wastage the slower the ice movement the more 

 rapid the retreat. A rapid motion, however, would probably be helpful in 

 the rapid production and transportation of glacial debris, and so it might 

 shorten the time required to form the till sheets and the moraines. 



In respect to the evidence drawn from existing Alaskan glaciers in 

 support of rapid motion, the present writer regards the measurement first 

 cited, assigning a rate of 70 feet per day, as untrustworthy. The later 

 measurement, by Dr. Reid, giving a rate of 7 feet per day, is believed to 

 be entitled to confidence, but even this rate is a maximum. The average 



