THE WORK OF GLACIERS 



155 



may be exposed by the melting of the surface of the ice and 

 continue to the end of the glacier as a medial moraine. It will 

 readily be seen that the material of the various surface moraines 

 of a glacier may differ widely in composition, since they were 

 derived from the rocks of many parts of the valley. The Baltoro 

 Glacier of Hindu Kush has fifteen moraines of different colors. 

 (Bonnersheim.) 



Since the surface moraines are usually sufficiently thick to protect 

 from the sun's rays the ice upon which they rest, they are generally 

 situated on ridges of 

 ice, sometimes 50 to jM%, 



80 feet in height. f\ v M^ 

 After a time the ridges 

 become so high that 

 the morainic material 

 slips off, thus widen- 

 ing the morainic belt. 

 After several repeti- 

 tions of this process 

 the medial and lateral 

 moraines may cover 

 completely the lower 

 end of a glacier. 



The size of some 

 of the rock frag- 

 ments carried on the 

 surfaces of glaciers 

 is very great. One 

 such bowlder con- 

 tained 244,000 cubic 

 feet (Forbes), which 

 is equivalent to a 

 squared stone 1 22 feet 

 long, 50 feet wide, 

 and 3 6 feet high. The 

 weight of material which a glacier can carry on its surface is limited 

 only by what it may receive, and the very weight of the surface load 

 will hasten the movement of the glacier. Upon the disappearance 

 of a glacier, these great rock masses are often left in unstable posi- 

 tions and are then known as balanced bowlders, or rocking stones. 



Fig. 139. — Diagram and cross section of a mountain 

 glacier. Lateral moraines are seen to produce medial 

 moraines. The movement of the superglacial material to 

 form englacial and finally subglacial material is shown. 

 Icefalls occur near the confluence of the glaciers on the 

 right. 



