GLACIERS AS A GEOLOGICAL AGENT. 57 



scratches, are very characteristic of glacial action. We will call such 

 surfaces glaciated, and the process glaciation. 



3. The turbidity of ordinary rivers is usually yellowish, the turbidity 

 of glacial rivers is always milky. The one is due to sediments derived 

 from soil, and therefore oxidized ; the other is due to ground-up sound 

 rock or rock-meal. 



Transportation. — The transporting power of glaciers follows no law 

 similar to that pointed out under rivers — in fact, it has no relation at 

 all to velocity. The reason is, that the stone rests on the surface as a 

 floating hody. There is, therefore, no limit to the transporting power. 

 Bowlders of 250,000 cubic feet are carried with the same ease and the 

 same velocity as the finest dust. 



Deposit — Balanced Stones. — A water-current carrying stones bruises 

 and rounds their corners, and deposits them always in the most secure 

 positions ; but glaciers often deposit huge angular fragments of rock 

 in the most insecure positions — so nicely balanced, sometimes, that a 

 touch of the hand will dislodge them. The reason is, they are set 



Fig. 46.— Glacial Scorings (after Agassiz). 



down by the gradually melting ice with inconceivable gentleness. Thus 

 balanced stones, rocking-stones, etc., are common in glacial regions. 

 In using these as a sign of glacial action, however, we must recollect 

 that a bowlder dropped by any agent, or even a bowlder of disintegra- 

 tion (p. 6), may in time become a rocking-stone, by slow but irregular 

 disintegration changing the position of the center of gravity. But angu- 

 lar erratics in insecure positions are very characteristic of glacial action. 

 Material of the Terminal Moraine.— The material of the terminal 

 moraine is very characteristic : 1. It consists of fragments of every con- 



