Ch. 14] CHARACTER 251 



Temperature 



The temperature of perennially frozen ground below the depth of 

 seasonal change (level of zero annual amplitude) (Muller, 1945) 

 ranges from slightly less than 0° C. to about —12° C. (I. V. Poire, oral 

 communication) . In Alaska the minimum temperature found to date 

 is —9.6° C. at a depth of 100 to 200 feet in a well about 40 miles south- 

 west of Barrow (J. H. Swartz, 1948, written communication) . Repre- 

 sentative temperature profiles in areas of (1) continuous permafrost are 

 shown in Fig. 3a; of (2) discontinuous permafrost in Fig. 3b; and of 

 (3) sporadic bodies of permafrost in Fig. 3c. 



Temperature gradients from the base of permafrost up to the depth 

 of minimum temperature vary from place to place and from time to 

 time. Measurement of four wells in northern Alaska resulted in gradi- 

 ents between 120 and 215 feet per centigrade degree (data of J. H. 

 Swartz, G. R. MacCarthy, and R. F. Black). 



The shape of a temperature curve indicates pergelation or depergela- 

 tion (aggradation or degradation of permafrost) (Muller, 1945; Taber, 

 1943a) . Some deep temperature profiles have been considered by Rus- 

 sian workers to reflect climatic fluctuations in the recent geologic past. 

 No known comprehensive mathematical approach has been attempted 

 to interpret past climates from these profiles, although it seems 

 feasible. Some of the effects of Pleistocene climatic variations on geo- 

 thermal gradients are discussed by Birch (1948). 



Character 



As permafrost is defined as a temperature phenomenon, it may en- 

 compass any type of natural or artificial material, whether organic or 

 inorganic. Generally, permafrost consists of variable thicknesses of 

 perennially frozen surficial unconsolidated materials, bedrock, and ice. 

 Physical, chemical, or organic composition, degree of induration, 

 texture, structure, water content, and the like, range widely and are 

 limited only by the extremes of nature or the caprice of mankind. 

 For example, perennially frozen mammals, rodents, bacteria, artifacts, 

 beds of sand and silt, lenses of ice, beds of peat, and varied junk piles, 

 such as kitchen-middens, mine dumps, and ships' refuse heaps, are in- 

 dividual items that collectively can be lumped under the term perma- 

 frost. 



Ground perennially below freezing but containing no ice has been 

 called "dry permafrost" (Muller, 1945). 



Permafrost containing much ice is abundant, particularly in poorly 

 drained fine-grained materials. The ice forms thin films, grains, fill- 



