416 
of this kind, known as “ giants’ kettles” (Fig. 153), exist in great | ‘ 
numbers. ‘There can be little doubt that they have had an origin 
t 
DYNAMICAL GEOLOGY. 
-[Boox IIT. 



























































































































































































































































































































































































Fic. 152.—Ice-worn SURFACE OF Rook, SHOWING PoLisuH, STRI@, AND GROOVINGS, . 
under the massive ice-cover which once spread over that peninsula, 

Fig. 153.— SecTION OF 
“ GIANTS’ Kerrues,” NEAR 
CuURISTIANIA. 
1 Brégger and Reusch, Q. J. Geol. Soc. xxx. 750, r 
The Greenland ice-sheet is traversed in sum- 
mer by powerful rivers which are swallowed 
up in the crevasses. Excavations of the same 
nature are no doubt also in progress there.? 
As rocks present great diversities of struc- 
ture and hardness, and consequently vary 
much in the resistance they offer to denuda- 
tion, they are necessarily worn down un- 
equally. The softer, more easily eroded 
portions are scooped out by the grinding 
action of the ice, and basin-shaped or various 
irregular cavities are dug out below the level 
of the general surface. Similar effects may 
be produced by a local augmentation of the 
excavating power of a glacier, as where the 
ice is strangled in some narrow part of a 
valley, or where, from change in declivity, 
it is allowed to accumulate in greater mass as 
it moves more slowly onward: Such hollows, 
on the retirement of the ice, become recepta- 
cles for water, and form pools, tarns, or lakes, 
~ 


