654 E. DE K. LEFFINGWELL 



The principle of the development of ground-ice wedges is capable 

 of widespread action throughout the region of permanently frozen 

 ground. It is so persistent on the north shore of Alaska that it is 

 to be expected to come into play in similar regions elsewhere. The 

 writer is inclined to believe that much of the ground-ice described 

 in the literature as in horizontal sheets may turn out to be in ver- 

 tical wedges. In the classical locality at Eschscholtz Bay, no one 

 observer agrees with the others. One says that there is a solid 

 mountain of ice, while a second finds only a thin veneer of ice 

 against the face of the bank. A more careful observer finds scat- 

 tered outcrops of ice, including at least one vertical dike. In other 

 regions "polygon marks" and vertical markings upon whitish ice 

 are mentioned. 



Fig. 23. — Hypothetical section of ice wedges and depressed polygon block 



SUMMARY 



The permanently frozen ground contracts in the cold Arctic 

 winter and cracks are formed, which divide the surface of the ground 

 into polygonal blocks. In the spring these frost cracks become 

 filled with surface water which immediately freezes. In the expan- 

 sion of the frozen ground as its temperature rises in summer, the 

 vein of ice being more rigid than the country formation, the read- 

 justment takes place in the latter. The result is to bulge up the 

 inclosed block either bodily or else locally along the sides of the ice. 

 During the next winter's cold wave, a new crack forms at the same 

 locus, so that a continually growing wedge of ground-ice is formed 

 (Fig. 23). Thus the tundra becomes underlain by a network of 

 ice wedges, which inclose bodies of the original formation. 



