6 MAN AXD THE GLACIAL PERIOD. 



Such a line of debris is called a moraine. When it 

 forms along the edge of the ice, it is called a lateral 

 moraine. It is easy to see that, where glaciers come out 

 from two valleys which are tributary to a larger valley, 

 their inner sides must coalesce below the separating prom- 

 ontory, and the two lateral moraines will become united 

 and will move onward in the middle of the surface of the 

 glacier. Such lines of debris are called medial moraines. 

 These are characteristic of all extensive glaciers formed by 

 the union of tributaries. There is no limit to the number 

 of medial moraines, except in the number of tributaries. 



A medial moraine, when of sufficient thickness, pro- 

 tects the ice underneath it from melting ; so that the 

 moraine will often appear to be much larger than it really 

 is : what seems to be a ridge of earthy material being in 

 reality a long ridge of ice, thinly covered with earthy debris, 

 sliding down the slanting sides as the ice slowly wastes 

 away Large blocks of stone in the same manner protect 

 the ice from melting underneath, and are found standing 

 on pedestals of ice, often several feet in height. An in- 

 teresting feature of these blocks is that, when the pedestal 

 fails, the block uniformly falls towards the sun, since that 

 is the side on which the melting has proceeded most 

 rapidly. 



If the meteorological forces are so balanced that the 

 foot of a glacier remains at the same place for any great 

 length of time, there must be a great accumulation of 

 earthy debris at the stationary point, since the motion of 

 the ice is constantly bearing its lines of lateral and medial 

 moraine downwards to be deposited, year by year, at the 

 melting line along the front. 



Such accumulations are called terminal moraines, and 

 the process of their formation may be seen at the foot of 

 almost any large glacier. The pile of material thus con- 

 fusedly heaped up in front of some of the larger glaciers 

 of the world is enormous. 



