AQUEOUS AGENCIES. 5? 



might at first be supposed, but usually very rough. This 

 roughness is due partly to rock-fragments from the crum- 

 bling cliifs on each side, as will be presently explained ; 

 partly to the unequal melting of the ice by the sun, pro- 

 ducing pinnacles and hollows, as erosion produces hills 

 and valleys on land ; and partly to the crevasses. For 

 these reasons, the travel over the surface is often not only 

 difficult but dangerous, especially as the crevasses are 

 often concealed by recently fallen snow. 



Moraines ; Lateral Moraines. On each margin of a 

 glacier, near the bounding cliifs, is found a continuous 

 pile of debris, consisting of earth and rock-fragments of 

 all sizes up to many hundred tons weight. The pile may 

 be twenty to thirty feet high, and is itself raised on an 

 ice-ridge formed by the protection of the ice beneath 

 from the melting power of the sun. These two marginal 

 piles of debris are called the lateral moraines. They are 

 formed by the constant fall of rocks and earth from the 

 crumbling cliffs on each side. But as the cliifs are not 

 everywhere so steep that their fragments reach the glacier, 

 if the glacier were motionless the contributions would be 

 in isolated heaps only. But the motion of the glacier 

 converts these separate contributions into a continuous 

 ridge ; and, conversely, the continuity of the moraines is 

 a proof of the motion of the glacier. 



Metlial Moraine. When two tributary glaciers unite 

 to form a trunk-glacier, the two interior lateral moraines 

 of the tributaries unite, and from the angle between the 

 two tributaries will train off as a continuous ridge of 

 debris along the middle of the trunk-glacier to its point. 

 This is called a medial moraine. It is still more indispu- 

 table proof of the motion of the glacier, since it is obvi- 

 ously impossible for debris to reach the middle of a glacier 

 in any other way. The number of these medial moraines 

 will depend upon the complexity of the glacial system, for 

 there will be one for every tributary (Fig. 29). Even a 



