ERRATIC BOULDERS 



155 



produces and which have escaped the washing of the sub-glacial 

 stream. 



When a glacier is retreating, it may build up a new terminal 

 moraine at each point of arrested withdrawal, or if the retreat is 

 gradual and steady, the ground in front of the ice will be covered 

 with moraine material, spread out in a sheet, not heaped up in a 



Fig. 59. — Perched block near the Yellowstone Cation, National Park. 



(U. S.G. S.) 



SU 



t*\_ w~^ 



moraine or mound. The retreat of the glacier may leave behind 

 it isolated masses of ice deeply buried in the debris of the terminal 

 moraine ; when such masses melt they form depressions in the 

 mound and give rise to the "kettle moraines." A shrinking glacier 

 will contract laterally and in depth, as well as longitudinally, and 

 in this way the blocks of the lateral moraine will be left stranded at 

 intervals over the former glacial bed. Such blocks and boulders 

 are known as erratics, or perched blocks, and when their parent 



