THE WORK OF GLACIERS 



189 



Fig. 178. 



Size and Work of Icebergs. — Bergs from Greenland seldom stand 

 200 feet out of water, and a height of 100 feet is more usual; but 

 icebergs have been reported from the Antarctic which are of great 

 size, being several miles long and 500 feet or more high. Icebergs 

 vary greatly in shape (Figs. 181, 182), 

 those of the Antarctic regions being 

 frequently of a tabular form, while 

 those from Greenland are usually 

 picturesquely irregular. If icebergs 

 were regular in shape and without 

 debris their thickness could be easily 

 determined, since in the case of solid 

 ice the part which appears above the 

 water is only one ninth of the mass. 



The principal geological effects of 

 icebergs are two : they abrade the 

 bottoms of the shallow seas where 

 they strand, and they transport their 

 load of debris until it is dropped as 

 the ice melts. Most of the load is 

 lost before it. has been carried 100 

 miles, but some of the debris of Green- 

 land icebergs is deposited on the New- 

 foundland Banks. It is stated that 

 in the Baltic Sea bowlders which have 

 been dropped from icebergs are often 

 found upon vessels which have been 

 sunk a few years. 



Glacial Movement 



Fig. 180. 



Figs. 178-180. — Diagrams illus- 

 trating the theories of the forma- 

 tion of icebergs. Fig. 178. — Ice- 

 bergs formed by the breaking off 

 and floating of the foot of a glacier 

 as the upper portion is eroded and 

 melted back by the waves. Fig. 179. 

 — Icebergs formed by gravity, since 

 it is held that the upper part of a 

 glacier will project beyond the lower 

 part, both because of the more rapid 

 movement of the top and because of 

 the melting of the ice. Fig. 180. — 

 Icebergs broken from the glacier as 

 it enters the sea, by the buoyancy 

 of the water. 



There is great difference of opinion 

 concerning the mechanics of glacial 

 movement, and the problem may be 

 considered as one yet to be solved. 



(j) Viscosity Theory. — One of the 

 early theories held that the motion of glaciers is due to the semiplastic 

 or viscous nature of ice (Forbes), which permits it to move upon a 

 slope very much as do such substances as thick tar or sealing wax, 

 the force which urges it forward being its own weight. Experiments 

 have been performed which appear to show that, in small masses, ice 



