646 



E. DE K. LEFFINGWELL 



The typical formation associated with ice wedges in the region 

 under discussion is muck, a black mud containing much vegetable 

 matter more or less decomposed. It varies from a peaty detritus, 

 which shows signs of having been waterlaid, to sand or mud mixed 

 with a varying amount of decaying vegetation. Undisturbed sec- 

 tions of this muck will usually show horizontal bedding. Occa- 

 sionally sand or a slimy clay was seen under the muck where a good 

 exposure revealed the lower strata (Fig. 12). As the ice wedge 

 grows in thickness and presses against the edges of the cleaved muck 

 and sand beds, they may become upturned and in time bent to the 



tr> 



Fig. 14. — Two joining wedges; the one on the right is cut obliquely. The dotted 

 lines represent lines of air bubbles within the ice. a, muck and clay, much disturbed; 

 b, sand. See Fig. 15. 



vertical or even beyond (Figs. 12, 17 and 18), causing the ridges 

 which often run along either side of the frost crack in "depressed 

 block" areas. In "elevated block" areas, the process is not so 

 easily understood. It may be that the block as a whole has bulged 

 sufficiently to bring its surface up to the general level, or else a 

 central depression has been filled by growth and capped by turf. 



The writer's observations were insufficient to disclose the factors 

 which control the character of block development. The "elevated 

 blocks" are characteristic of drained areas and are nearly constant 

 features near banks. The "depressed blocks" are associated in the 

 writer's mind with flat, marshy country. This, however, may be 

 the effect rather than the cause of the difference in character of the 

 blocks. The network of depressions drains the elevated blocks, 

 but the ridges form dams which interfere with surface drainage. 



