648 



E. DE K. LEFFINGWELL 



case (Fig. 9) there is an upward projection of ice above the general 

 surface of the wedge, indicating a sudden change of the Kmit of 

 thawing. The others showed a dome-shaped surface, indicating a 

 gradual change. Since the majority of exposures show the surface 

 of the wedges to be nearly parallel with the surface of the ground, 



Fig, 16. — ^An ice wedge in sand. A tunnel has been cut in the ice by drainage of 

 surface waters through the frost crack. The sand on either side is apparently bulged. 



Fig. 17. — Section perpendicular to a bank, showing upturned muck beds in a 

 tundra block which has broken off along an ice wedge. 



it seems that a balance must be maintained between the thickness 

 of the covering and the increase in area to be covered, as the wedge 

 becomes wider. 



The rate of growth of turf must be very slow in this region, for 

 there are many half-buried bowlders on the surface of the tundra, 

 which have been there since glacial times, or at least since the 

 coastal plain emerged from the sea. Many bare spots exist where 

 the turf has not been able to get a footing. 



