STRUCTURE OF GLACIAL ICE 327 



stratified structure. This may not always Ijc visible, but, on the 

 other hand, it may be very prominent. It is especially well marked 

 where layers of wind-blown dust or sand were spread over the 

 older layer before the deposition of the new one. 



Fig. 62. Diagram illustrating lateral upturning of ice layers in a glacier. 

 The bottom line is sea level. (Chamberlin and Salisbury.) 



Shear Zones and Flozv Structure. Instead of moving uni- 

 formly as a mass, lines of more rapid movement within the ice 

 may develop, as the result of which a shear structure will come 

 into existence. This may simulate stratification, especially when, as 

 is commonly the case, debris from the bottom is carried up into 

 the ice along the shear zones. Though a secondary structure, it 

 may be mentioned here for purposes of comparison. 



Fig. 62,. Diagram illustrating compound or double glacier, with debris-laden 

 layers rising laterally and medially, forming corresponding 

 moraines. The bottom line is sea-level. (Chamberlin and Salis- 

 bury.) 



Flow structure is developed within many glaciers. This is most 

 pronounced near the front of the ice and along its sides. The 

 layers near the front turn up and the debris within the ice thus 

 gradually reaches the surface. Along the sides the movement is 

 also from below upward, and where two valley glaciers have joined 

 into a single trunk glacier, the current of each still remains dis- 

 tinct, and we have upward moving currents from both sides in the 

 center as w^ell as on each side. (Figs. 62, 63.) 



BIBLIOGRAPHY VIII. 



1. AMUNDSEN, ROALD. 1913. The South Pole. 2 volumes. London' 



John Murray. New York, Lee Keedick. 



2. CHAMBERLIN, T. C. 1885-1886. The Rock Scouring of the Great Ice 



Invasion. Seventh Annual Report of the Director of the U. S. Geological 

 Survey, pp. 155-254. 



3. CHAMBERLIN and SALISBURY. 1909. Geology, Vol. I. 2nd edition, 



pp. 321-22. Extended bibliography on glacier motion, with summary. 



