250 black. PERMAFROST [Ch. 14 



Asia, but as yet for only part of North America. Refinements in 

 delineations of these boundaries are being made each year. The south- 

 ern margin of permafrost is known only approximately, and additional 

 isolated bodies are being discovered as more detailed work is under- 

 taken. The southern margin of permafrost has receded northward 

 within the last century (Obruchev, 1946) . 



Permafrost is absent or thin under some of the existing glaciers, 

 and it may be absent in areas recently exhumed from ice cover. 



A greater extent of permafrost in the recent geologic past is known 

 by inference from phenomena now found to be associated with 

 permafrost (H. T. U. Smith, 1949b; Horberg, 1949; Richmond, 1949; 

 Schafer, 1949; Cailleux, 1948; Poser, 1947a, b; Troll, 1947, 1944; 

 Zeuner, 1945; Weinberger, 1944). Some of the more important phe- 

 nomena are fossil ground-ice wedges, solifluction deposits, block fields 

 and related features, involutions in the unconsolidated sediments, stone 

 rings, stone stripes and related features, and asymmetric valleys (H. T. 

 U. Smith, 1949b). The presence of permafrost in earlier geologic 

 periods can be inferred from the cold climates accompanying many 

 periods of glaciation and from fossil periglacial forms. 



In the Southern Hemisphere permafrost is extensive in Antarctica 

 and probably occurs locally in some of the higher mountains else- 

 where, but its actual extent is unknown. 



Thickness 



Permafrost attains its greatest known thickness of about 2,000 feet 

 (620 meters) at Nordvik in northern Siberia (I. V. Poire, oral com- 

 munication) . Werenskiold (1923) reports a thickness of 320 meters 

 (1,050 feet) in the Sveagruvan coal mine in Lowe Sound, Spitzbergen. 

 In Alaska its greatest known thickness is about 1,000 feet, south of 

 Barrow. 



Generally it can be said that the frozen zone thins abruptly to the 

 north under the Arctic Ocean. It breaks into discontinuous and 

 sporadic bodies as it gradually thins to the south (Fig. 2) (Muller, 

 1945; Taber, 1943a; Cressey, 1939). 



In areas of comparable climatic conditions today, permafrost is 

 much thinner in glaciated areas than in non-glaciated areas (Taber, 

 1943a). 



Unfrozen zones within perennially frozen ground are common near 

 the surface (Muller, 1945) and are reported to occur at depth (Taber, 

 1943a; Cressey, 1939). They have been interpreted as indicators of 

 climatic fluctuations (Muller, 1945; Cressey, 1939), or as permeable 

 water-bearing horizons (Taber, 1943a). 



