254 black. PERMAFROST [Ch. 14 



generally have lower permafrost tables than undisturbed natural areas 

 adjacent to them. Structures above the ground and insulated from the 

 ground protect the surface from insolation and commonly produce 

 higher permafrost tables. 



Origin 



The origin of perennially frozen ground is discussed by Muller 

 (1945), Zeuner (1945), Taber (1943a), Cressey (1939), Nikiforoff 

 (1932), Leffmgwell (1919), and others. Generally it can be stated 

 that most sporadic bodies of permafrost are relics of colder climates. 

 Discontinuous bodies of permafrost are largely relics, but under favor- 

 able conditions they can grow in size, and new deposits are being per- 

 ennially frozen. In areas of continuous permafrost, heat is being dis- 

 sipated actively from the surface of the earth to the atmosphere, and 

 new deltas, bars, landslides, mine tailings, and other deposits are being 

 pergelated (incorporated in the permafrost) (Bryan, 1946a). 



Local surface evidences indicate that heat, in some places at least, 

 is being absorbed at the base of permafrost faster than it is being 

 dissipated at the surface (Hopkins, 1949; Young, 1918). Hence the 

 cold reserve is being lessened, and the thickness of permafrost is de- 

 creasing from the base upward. 



The mean annual air temperature required to produce permafrost 

 undoubtedly varies many degrees because of local conditions. Gener- 

 ally it is given as 30° to 24° F. ; theoretically permafrost can form above 

 32° F. (Theis, unpublished manuscript) , and apparently it is doing so 

 locally in parts of southwest Alaska where poor drainage, abundant 

 vegetation, cloudy summers, and low insolation are found (S. Abraham- 

 son, oral communication). 



The relative effects of past climates have been inferred qualitatively 

 from present temperature profiles and indirectly through a study of 

 past deposits, pollen analysis, vegetal changes, structural soils, block- 

 fields, etc. 



The origin of large, clear ice masses in the permafrost is a special 

 problem in itself. Numerous theories are extant, and one or more may 

 apply to a particular mass of ice (Taber, 1943a; Leffmgwell, 1919). 



GEOLOGIC RAMIFICATIONS 



Throughout the Arctic and sub-Arctic the role of permafrost is ex- 

 tremely important. As an impervious horizon in continuous perma- 

 frost zones, it exerts a drastic influence on surface waters, completely 

 prevents precipitation from entering the natural ground-water reser- 



