GROUND-ICE WEDGES 



647 



As the growing vein of ice becomes more wedge-like in form, the 

 pressure exerts a vertical component against the sides of the wedge. 

 This tends to force the wedge upward. If an upward movement 

 should occur, the ice would carry its protective covering with it and 

 be able to exist level with or even somewhat above the general sur- 

 face of the block. Since a bulging of the block by the growing 

 wedge seems necessary, some upward motion of the wedge may 

 have taken place without bringing the top of the wedge up to the 



Fig. 15. — Photograph of exposure shown in Fig. 



14 



general level. In the depressed blocks it is to be noticed (Figs. 7 

 and 8) that the surface of the ground between the parallel ridges 

 (probably underlain by ice) is higher than that of the blocks on 

 either side. No exposures were found illustrating this case, so it 

 is impossible to say whether the surface of the ice is actually higher 

 than that of the blocks. 



The usual covering for the ice is muck capped by turf, or peat 

 capped by growing sphagnum ( ?) moss. As the thickness of this 

 mantle increases by surface growth, the limit of the summer's thaw- 

 ing should rise, thus allowing a constant upward extension of the 

 surface of the ice wedge at the locus of growth. Only two or three 

 cases of apparent upward growth of the surface were seen. In one 



