THE GLACIER'S SURFACE FEATURES 



395 



and above that size their action is always to protect the glacier 

 from the sun. This nice adjustment to the size of the rock frag- 

 ments will be clear from examination of Fig. 422, for rock is a 

 poor conductor of heat, and in even the longest summer day a 

 thin outer layer only is appreci- 

 ably warmed. Large rock blocks, 

 grouped in the medial and lateral 

 moraines, hold back the process of 

 lowering the glacier surface during 

 the summer, so that late in the 

 season these moraines stand fifty 

 feet or more above the glacier as 

 armored ice ridges. 



Isolated and large rock slabs, as 

 the season advances, may come to 

 form the capping of an ice pedestal 

 which they overhang and are known 

 as glacier tables (Fig. 423). Such 



FIG. 423. Small glacier table upon 

 the surface of the Great Aletsch 

 glacier in 1908. 



tables the sun attacks more upon one side than upon the other, 

 so that the slab inclines more and more to the south and may 

 eventually slip down until its edges rest against the glacier sur- 

 face. Rounded bowlders, which less frequently become perched 

 upon ice pedestals, may, from a similar process, slide down upon 

 the southern side and leave a pyramid of ice furrowed upon this 

 side and known as an ice pyramid. 



Fine dirt when scattered over the glacier surface is, on the other 

 hand, most effective in lowering its level by melting. Use was 

 made of this knowledge to lower the great drifts of snow which 

 had to be removed each season during the construction of the 

 new Bergen railway of southern Norway. Each dirt particle, 

 being warmed throughout by the sun's rays, melts its way rapidly 

 into the glacier surface until the dust well which it has formed is 

 so deep that the slanting rays of the sun no longer reach it. When 

 the dirt particles are near together, the thin walls which separate 

 the dust wells are attacked from the sides in the warm air of sum- 

 mer days, thus producing from a patch of dirt upon the glacier 

 surface a bath tub (Fig. 424 d) . At night the water which fills these 

 basins is frozen to form a lining of ice needles projecting inward 

 from the wall, and this, repeated ha succeeding nights, may 



