﻿458 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



ice beneath the line of plates. The work with the level gave a dif- 

 ference of 393 ft. between the base of this ice at the margin and the 

 plate which gave the maximum surface melting and forward move- 

 ment. This would leave but 15 ft. for the rise in the valley floor 

 in the 2,200 ft., an amount which is very probably too small and it 

 is likely that the surface melting becomes less toward the lower 

 margin owing to the concentration of the debris. 



5. Shearing. — The steeply inclined ice front at the western side 

 of the valley shows a succession of ice strata, more or less well 

 defined, which dip back into the glacier at rather a steep angle. At 

 the mouth of an abandoned drainage tunnel, seen in pi. lxi, a, these 

 strata have a dip of 26 , which is somewhat below the actual dip 

 when measured at right angles to the strike. There is some sand, 

 a little fine gravel and, occasionally, a cobble-stone between these 

 strata but the amount of foreign material is small and inconspicuous. 

 A few consecutive days' visits to this part of the glacier, in early 

 July, showed that a differential movement between adjacent strata 

 seemed to be taking place (fig. 66). In order to test whether or not 

 such was actually the case a point was selected upon the face 52 

 ft. above the valley floor, the ice slope being 45 °, and six heavy spikes 

 driven into the ice until their heads were flush with the surface. 

 Three were thus placed in the base of the upper stratum and three 

 corresponding ones in the upper part of the subjacent layer, the 

 former projecting beyond the latter 19.7 in. July 21. Two days 

 later, July 2^, it was evident that the melting was greater upon the 

 face of the upper stratum, in spite of which this now projected 24.4 

 in. beyond the lower. The spikes were now visited regularly each 

 morning until August 3 and then reset after the amount of melting 

 and apparent differential movement had been determined. The 

 measurements were necessarily rough but they showed each day that 

 the melting was greater upon the upper stratum, the average amount 

 for each spike being 1.75 in., while that for the lower series was 

 1.53 in., or nearly T 4 in. in excess. Some sand and fine gravel, 

 washed down from above, daily accumulated in the lee of the pro- 

 jecting upper layer and gave the appearance of a concentration of 

 dirt in the upper part of the lower (fig. 66). When this dirt was 

 small in amount it was observed that melting was accelerated; when 

 greater, that melting was retarded. The upper stratum continued 

 to gain slowly upon the lower and reached a maximum of 26.6 in. 

 on July 27, after which it fluctuated slightly, the observations closing 

 with it at 25.6 in. Time did not permit the verification of these 

 results at other points where the same thing appeared to be occurring. 



