276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



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 perma- 

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

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

 1944; Zeuner, 1945; Weinberger, 1944, and others). Some of the 

 more important phenomena are fossil ground-ice wedges, solifluction 

 deposits, block fields and related features, involutions in the uncon- 

 solidated 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 known 

 facts of former periods of glaciation and from fossil periglacial forms. 



In the Southern Hemisphere permafrost is extensive in Antarctica. 

 It probably occurs locally in some of the higher mountains elsewhere, 

 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 communication). Werenskiold (1923) reports a thickness 

 of 320 meters (1,050 feet) in the Sveagruvan coal mine in Lowe 

 Sound, Spitsbergen. In Alaska its greatest known thickness is about 

 1,000 feet, south of Barrow. 



Generally the permafrost thins abruptly to the north under the 

 Arctic Ocean. It breaks into discontinuous and sporadic bodies as 

 it gradually tliins to the south (fig. 2) (Muller, 1945; Taber, 1943a; 

 Cressey, 1939; and others). 



In areas of comparable climatic conditions today, permafrost is 

 much thinner in glaciated areas than in nonglaciated 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). 



Temperature. — The temperature of perennially frozen ground be- 

 low 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 tempera- 

 ture recorded to date is -9.6° C. at a depth of 100 to 200 feet in a 

 well about 40 miles southwest of Barrow (J. H. Swartz, 1948, written 

 communication) . Representative temperature profiles in areas of ( 1 ) 

 continuous permafrost are shown in figure 3, a; of (2) discontinuous 

 permafrost, figure 3, 6; and of (3) sporadic bodies of permafrost, 

 figure 3, €. 



