1 66 WORK OF SNOW AND ICE 



by its movements, are less perfectly and regularly stratified than 

 valley trains and outwash plains. 



ICEBERGS 



Where glaciers advance into water the depth of which approaches 

 their thickness, their ends are broken off (Fig. 172), and the de- 

 tached masses float away as icebergs (Fig. 173). Many of the bergs 



Fig. 173. An iceberg. (Robin.) 



are overturned, or at least tilted, as they set sail. If this does not 

 happen at the outset, it may later, as the result of melting, wave- 

 cutting, etc., which shift the centers of gravity of the bergs. The 

 great majority of them do not float far before losing all trace of 

 stony and earthy debris; but the finding of glaciated pebbles in 

 dredgings far south of all glaciers shows that bergs occasionally 

 carry stones far from land. The importance of icebergs as agents 

 of transportation has been greatly exaggerated, and the assignment 

 of shoals, like the Banks of Newfoundland, to them, is without 

 foundation. 



Map work. See Interpretation of Topographic Maps, Exercises XI to XIII, 

 and Plates XCV to CXXIX, Professional Paper 60, U. S. Geological Survey. 



