MA LA SPINA GLACIER. 227 



broken into finer and finer fragments and are reduced in part 

 to the condition of sand and clay. When the debris is suf- 

 ficiently comminuted it is sometimes carried away by surface 

 streams and washed into crevasses and moulins. Not all of 

 the turbidity of the subglacial streams can be charged to the 

 grinding of the glacier over the rocks on which it rests, as a lim- 

 ited portion of it certainly comes from the crushing of the surface 

 moraines during their frequent changes of position. 



Isolated blocks of stone lying on the glacier, when of sufifi- 

 cient size not to be warmed through by the sun's heat in a single 

 day, also protect the ice beneath and retain their position as the 

 adjacent surface melts, so as to rest on pedestals frequently sev- 

 eral feet high. These elevated blocks are usually flat, angular 

 masses, sometimes 20 feet or more in diameter. Owing to the 

 greater effect of the sun on the southern side of the columns 

 which support them, the tables are frequently inclined south- 

 ward, and ultimately slide off their pedestals in that ■ direction. 

 No sooner has a block fallen from its support, however, than 

 the process is again initiated, and it is again left in relief as 

 the adjacent surface melts. The many falls which the larger blocks 

 receive in this manner cause them to become broken, thus illus 

 trating another phase of the process of comminution to which sur 

 face moraines are subjected. On Malaspina glacier the formation 

 of orlacial tables is confined to the summer season. In winter 

 the surface of the glacier is snow -covered and differential melt- 

 ing can not be marked. The fact that glacial tables are seldom 

 seen just after the snows of winter disappear suggest that winter 

 melting takes place to some extent, but in a different manner 

 from what it does in the summer. Just how the blocks are dis- 

 lodged from the pedestals in winter has not been observed. 



While large objects lying on the surface of the glacier are 

 elevated on pedestals in the manner just described, smaller ones, 

 as is well known and especially those of dark color, become 

 heated by the sun, and, melting the ice beneath, sink into it. 

 When small stones and dirt are gathered in depressions on 

 the surface of the glacier, or, on a large scale, when moulins 



