GROUND-ICE WEDGES 



649 



Where the protective covering is of muck, creeping of the soil 

 will tend to close up the open frost crack. This will thin the cover- 

 ing, and if the rate of surface growth is not sufficient to counteract 

 the resulting decrease in thickness, the upper surface of the ice will 

 be lowered by melting. The increased slopes will cause side 

 material to creep down over the ice, thus keeping the protective 

 mantle up to the required thickness. A shallow depression will 

 thus be formed whose slopes are of the proper angle to cause the 

 proper amount of creeping. 



Fig. 18. — An ice wedge in muck showing upturned strata 



In the case of sphagnum ( ?) moss the case may be somewhat 

 similar. The moss and subjacent peat may also close the crack by 

 creeping. At the same time the bed will become thinner, but 

 growth of the moss will soon thicken it again. If the moss grows 

 too fast, the depression will be filled and the conditions of moisture 

 favorable to growth will cease. Thus it is possible for the growing 

 ice wedge to have a covering of peat of constant thickness. 



When a bank is undercut by wave or river action, large masses 

 of tundra often break off. Since the ice wedges are planes of weak- 

 ness, the break is along the edges of the polygon blocks, whenever 

 this is possible. This is especially true of high banks, where whole 

 blocks will break out, leaving many re-entrant angles in the result- 

 ant bank, along which a nearly continuous exposure of ice may be 

 seen. The impression is carried away of a heavy horizontal sheet 



