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“408 DYNAMICAL GEOLOGY, aS : [Boox HL 
buried under an ice-cap, which ranges in thickness up to a depth (in 4 
the South Polar circle) of 10,000 feet (2 miles) or even more. 
Greenland lies under such a pall of snow that all its inequalities, 
save the mere steep mountain crests and peaks near the coast, are 

Fic. 139.—SEcTION SHOWING THE PropvcTION OF ICEBERGS AT THE Foor OF THE 
JOKULS FJORD GLACIER. 
concealed. The snow creeping down the slopes, and mounting over 
the minor hills, passes beneath by pressure into compact ice. From 
the main valleys great glaciers like vast tongues of ice, 2000 or 8000 
feet thick, and sometimes 50 miles or more in breadth, push out to 
sea, where they break off in huge fragments, which float away as 
icebergs. As far back as 1777, Captain Cook gave interesting 
descriptions of the glaciers of South Georgia (Lat. 54° S.), which 
reach the sea in a line of cliffs (Fig. 140). 
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Fic. 140.—View or GLacier 1N Possession Bay, SoutH GEORGIA. 
Work done by Glaciers.—Glaciers have two important 
geological tasks to perform—(1) to carry the débris of the mountains 
down to lower levels; and (2) to erode their beds, ; 
(a) Transport.—This takes place chiefly on the surface of the ice. 
Desceniling its valley, the glacier receives and bears along on its margin 
