GROUND-ICE WEDGES 



6S3 



readily be calculated. If 20 per cent is taken for the surface com- 

 pression of the block, the average compression will be 10 per cent. 

 An average block 11 meters in diameter will be compressed hori- 

 zontally I . I meters to an assumed depth of from 8 to 10 meters. 

 This will cause an increase in a vertical direction of about i meter. 







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Fig. 22. — Planetable map of frost cracks on the tundra, with a sketch of the 

 exposures of ground-ice in the bank at one edge of the mapped area. 



The heavy lines on the map represent open frost cracks in July. The dotted 

 lines indicate evident frost-crack loci. The stipple marks show the areas probably 

 underlain by ground-ice. 



In the section below, the map, the white spaces represent ground-ice, the dotted 

 spaces, sand. The rest of the exposure has slumped. 



The case is different with the depressed blocks, where the adjust- 

 ment is concentrated into the ridges that surround them . The depres- 

 sions are being continually filled with growing vegetation, as well as 

 picking up wind-blown material, thus forming muck beds. In this 

 way a much greater general elevation of the surface of the tundra 

 is possible. Much of the muck of the region may have been 

 developed in this way. 



