650 



E. DE K. LEFFINGWELL 



of ice of great thickness, whereas, in fact, Httle of it extends back 

 more than a few feet from the face of the exposure (Figs. 19, 20, 

 and 21). 



In Fig. 22 is shown a plane-table map of an area of frost cracks, 

 and a sketch of the exposures of ground-ice in the bank immediately 

 below. The polygon blocks were of the elevated type, but the 

 rehef was very faint, being somewhat obscured by sand which had 

 drifted up from a sand spit on the left. The exposure was some- 

 what slumped, but gave sufficient details to illustrate the case. 



Fig. 19. — Exposure of ice extending some distance inland. A tundra block is 

 breaking off along an ice wedge. 



The heavy lines on the map show open frost cracks; the dotted 

 lines, the evident loci of cracks. Where there was no surface indi- 

 cation, no lines were drawn. The shaded areas are supposedly 

 underlain by ice wedges. 



Ground-ice occurs at the intersection of every crack with the 

 bank, where the details were not masked by slumping. 



The average diameter of eleven blocks shown on the map is 

 about II meters. The largest block is about 11 by 15 meters, and 

 the smallest 5 by 8 meters. The largest wedges of ice are about 

 2 . 5 meters wide at the top, and this width has been used in indi- 

 cating the areas probably underlain by ice, except where surface 



