640 



E. DE K. LEFFINGWELL 



seldom a relief of more than one foot, but that of the "depressed 

 blocks" may be twice as much. 



The parallel ridges form shallow reservoirs, very similar to those 

 of the block system of irrigation, especially when they take a rec- 

 tangular form, as is often 

 the case. They often 

 contain ponds and are 

 always swampy, so that 

 , one keeps to the ridges 



for dry footing when 

 crossing such an area 

 _^ ^ ^ - (Figs. 7 and 8). 



FORMATION OF ICE 

 WEDGES 



The open frost crack 

 is in a favorable posi- 

 tion for being filled with 

 water during the melting 

 of the snow, for most of 

 them He in depressions 

 upon a flat surface. 

 Even those that by 

 chance get no water 

 probably become filled 

 with ice crystals de- 

 posited by the damp air, 

 by internal "breathing." 

 The crack, being filled 

 with soHd ice from the 

 freezing of the water, or 

 containing much ice in the form of frost crystals, thus contains 

 a narrow vein of true ground-ice in the portion which lies below 

 the depth reached by the annual thawing. When the frozen 

 ground expands under the summer's heat, the readjustment to 

 the strain may take place in four ways: (i) The pressure may 

 melt the ice, so that the crack is closed again. (2) The formation 



Fig. 6. — A frost crack on the surface of a recently 

 drained area. 



