THE MECHANICS OF GLACIERS 92 1 



the center of the glacier had suffered an upward displacement of 

 2.25 meters, and that the average upward displacement of the 

 whole section was 1.4 meters.^ These amounts are given after 

 allowing for the natural thickening of the ice due to winter pro- 

 gression parallel to the glacier's bed, without melting. There 

 is no apparent reason why the directions of flow in the winter 

 and in the summer should differ; so I think these observations 

 indicate a continuous movement of the ice toward the surface, 

 as we should expect. 



Professor Pfaff found in one of the reservoirs of the Aletsch 

 glacier, where the surface slope was 9°, that the direction of 

 motion made an angle of about 40° with the horizontal. This is 

 certainly a high angle and Professor Heim does not think the 

 method used sufficiently free of error to establish these "some- 

 what astonishing results."^ Here again, however, the observa- 

 tions are in harmony with theory. It is unfortunate that so 

 little has been done to determine the vertical component of the 

 motion. 



The correct observation of stratification is very difficult. It 

 is next to impossible to determine the surfaces of separation of 

 successive strata in the dissipator unless we find them marked 

 by a thin layer of debris. Aggassiz has made a drawing of sev- 

 eral such debris layers as they were exposed in a hole in the 

 upper part of the dissipator of the Unteraar glacier. 3 These 

 must be true surfaces of stratification ; they dip up stream at an 

 angle of about 30°. 



Professor Russell has described debris layers in the Sierra 

 Nevada glaciers, and suggests that they separate successive 

 strata ; they occur at fairly regular intervals, dip into the glaciers, 

 and were only seen below the neve-line. "* 



When the junction of two glaciers, which gives rise to a 



' Syst. Glac, pp. 559-563- 



'Heim, Gletscherkunde, p. 184. 



3 Syst. Glac, p. 260. Atlas PI. V, Fig. 15. 



■•The Glaciers of the United States. 5th An. Rep. U. S. Geol. Surv., 1883-4, PP- 

 316, 319. PI. XXXVII shows the outcrops of the layers in the Mt. Dana glacier. See 

 also Heim, Gletscherkunde, p. 131. 



