WHAT IS A GLACIER? 9 



ential motion, produces also a veined structure in the masses 

 themselves, at right angles to these marginal fissures. 



The surface of a glacier presents many interesting phe- 

 nomena. When the ice-stream is of sufficient size, the sur- 

 face is covered with a network of 

 small streams of water, flowing through 

 blue channels of ice sometimes many 

 yards in depth and width. But these 

 are destined eventually to encounter 

 some crevasse, where a circular shaft, 

 or moidin, as it is called, is formed, 

 opening a way to a subglacial chan- 

 nel, into which the Streams plunge Fig. 8. -Illustrates the forma- 

 • ,i i i -. ,-, 1,1 tion ol veined structure by 



witn a loud roar, and tne accumulated pressure at the junction of 



(. , , , ... two branches. 



waters may often be heard rushing 



onward hundreds of feet below the surface. During the 

 melting of a glacier, also, in the summer season, the surface 

 of the ice is frequently dotted with bowl-shaped depressions, 

 from one or two inches to many feet in depth, and filled 

 with beautiful clear water. The cause of this can not well 

 be conjectured. In Greenland, Nordenskiold attributed the 

 initial melting to accumulations of meteoric dust which he 

 named kryokonite. 



Glaciers in mountainous regions are also characterized by 

 lateral and medial moraines. Where the ice stream passes 

 by a mountain-peak, the falling rocks and the avalanches 

 started by streams of water, form along the edge of the gla- 

 cier a continuous line of debris, which is carried forward by 

 the moving ice, and constitutes what is called a lateral mo- 

 raine. If there be a current of ice on each side of the mount- 

 ain-peak, two of the lateral moraines will become joined be- 

 low the mountain, and will form what is called a medial 

 moraine, which will be carried along the back of the ice as 

 far as the motion continues. As the ice wastes away toward 

 the front, several medial moraines sometimes coalesce. This, 

 as will be seen, is finely shown in some glaciers of Alaska. 



A medial moraine, when of sufficient thickness, protects 



