6S2 



E. DE K. LEFFINGWELL 



RATE OF GROWTH OP WEDGES 



Fresh ice-filled cracks 8-10 mm. wide have been observed in the 

 ground immediately above the ice wedges. This may be put as the 

 maximum width of the crack. Open cracks about 5 mm. wide have 

 been found in the ice itself near the upper surface. The width, of 

 course, diminishes downward. If 5 mm. is assumed as the width 

 at the top, it would require only 600 years to build up a wedge 3 

 meters wide, which is about the maximum width seen in the region. 



Fig. 21. — A large tundra mass broken off along the sides of a polygon block. The 

 ice has mostly melted away. 



If the cracks do not all open every winter, this period must be mul- 

 tiplied by some factor. The writer had frequently observed open 

 cracks during the previous years, but not realizing their bearing, 

 did not keep any record of their abundance. About 1,000 years 

 seems to be the order of age of the largest wedges. Unless some 

 unknown cause prevents a greater growth, the temperature could 

 not have been sufficiently low to bring them into existence at an 

 earher date, or else the coastal plain has not been elevated above 

 sea-level for a longer period. 



If we assume that the elevated blocks are bulged by the growing 

 ice, the amount of general elevation of the surface of the tundra can 



