PERMAFROST — BLACK 279 



(Black and Barksdale, 1949; Muller, 1945). Well-drained coarse- 

 grained materials may thaw annually to a depth of 6 feet. Poorly 

 drained fine-grained materials protected from solar radiation and 

 insulated with moss and other vegetation may thaw annually to a 

 depth of only 4 inches. 



In the discontinuous zone permafrost is absent under most major 

 rivers and lakes. It may be absent in the tops of some well-drained 

 low hills. Seasonal thaw (active layer, Muller, 1945) penetrates 1 

 foot to 10 feet, depending on insulation, insolation, drainage, and type 

 of material. 



Sporadic bodies of permafrost may be relics below the active layer 

 or may be forming in favorable situations in poorly drained fine- 

 grained materials on north-facing slopes. In the zone of sporadic 

 permafrost the active layer may or may not reach the permafrost 

 table, and it ranges between 2 and 14 feet in thickness. 



Generally the depth of thaw is at a minimum in northern latitudes 

 and increases to the south. It is at a minimum in peat or highly or- 

 ganic sediments and increases successively in clay, silt, and sand to a 

 maximum in gravelly ground or exposed bedrock. It is less at high 

 altitudes than at low altitudes ; less in poorly drained ground than in 

 dry well-drained ground ; at a minimum under certain types of tundra 

 and increases successively in thickness under areas of bog shrubs, black 

 spruce, larch, white spruce, birch, aspen, and poplar to a maximum 

 under tall pines. It is less in areas of heavy snowfall ; less in areas 

 with cloudy summers; and less on north-facing slopes (Muller, 1945; 

 Troll, 1944; Taber, 1943a; and others). 



Works of man commonly upset the natural thermal equilibrium 

 and may tend to destroy permafrost or to aid in its formation. Most 

 roads, runways, and other structures on the surface of or in the ground 

 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 solar radiation and commonly 

 produce higher permafrost tables. 



Origin. — The origin of perennially frozen ground is discussed by 

 Jenness (1949), Muller (1945), Zeuner (1945), Taber (1943a), Cressey 

 (1939), Nikiforoff (1932), Leffingwell (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 favorable conditions may grow in size, and new de- 

 posits are being perennially frozen. In areas of continuous perma- 

 frost, heat is being dissipated 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). 



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