GROUND-ICE WEDGES 



639 



nomenon to the cracking of the frozen ground during the winter's 

 contraction. The writer has spent six winters in the region under 

 discussion, living most of the time upon the tundra, which is chiefly 

 underlain by muck. Frequent camps have been made upon other 

 formations, such as sands and silts, and the impression carried away 

 is that the sound of the cracking ground was heard everywhere. 

 This has been confirmed by a prospector who has lived nearly 

 thirty years in the country. 



It was at first thought that the reports were caused by the crack- 

 ing of hard snowdrifts, but the fresh cracks in these drifts were seen 

 to run into the ground below. When the snow melts in the summer, 



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Fig. 5. — A large ice wedge in peat beds. The peat beds are not upturned, 

 ice is capped by growing moss. 



The 



fresh open cracks can be seen cutting across all the tundra forma- 

 tions, even mud and growing moss beds, and dividing the surface 

 into polygonal blocks; these cracks resemble mud cracks but are 

 of a larger size. The blocks have an estimated average diameter 

 of about 15 meters, and have a tendency toward the hexagonal 

 form, although rectangles and pentagons are often developed. 



Occasionally a crack is seen to run across a flat surface with no 

 associated features, as is illustrated in Fig. 6, but usually it is accom- 

 panied by a distinct topography. Either the crack lies in a gentle 

 depression which surrounds an elevated polygon block, or it runs 

 between two parallel ridges which surround a depressed block. 

 These features do not vary from block to block, but each is locally 

 developed over a considerable area. The "elevated blocks" have 



