Ch. 14] RELATION TO TERRAIN FEATURES 253 



ings, veinlets, large horizontal sheets, large vertical wedge-shaped 

 masses, and irregular masses of all sizes. Many masses of clear ice are 

 arranged in geometric patterns near the surface, that is, polygonal 

 ground and honeycomb structure. The ice may be clear, colorless, 

 yellow, or brown. In many places it contains numerous air bubbles, 

 oriented or unoriented, and silt, clay, or organic materials. Size, shape, 

 and orientation of the ice crystals differ widely. Discordant structures 

 in sediments around large masses of ice are evidences of growth 

 (Taber, 1943a; Leffingwell, 1919). 



Relation to Terrain Features 



In the continuous zone of permafrost the upper limit (permafrost 

 table, Muller, 1945) is generally within a few inches or 2 feet of the 

 surface. Large lakes and a few large rivers lie in thawed areas slightly 

 larger than the basins they occupy (Black and Barksdale, 1949; 

 Muller, 1945). Well-drained coarse-grained materials may thaw an- 

 nually to a depth of 6 feet. Poorly drained fine-grained materials pro- 

 tected from insolation and insulated with moss and other vegetation 

 may thaw annually to a depth of only 4 inches. 



In the discontinuous zone most major rivers and lakes are not under- 

 lain by permafrost, and it may be absent in the tops of some well- 

 drained low hills. Seasonal thaw (active layer, Muller, 1945) pene- 

 trates 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 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 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). 



Works of man commonly upset the natural thermal equilibrium 

 and may tend to destroy permafrost or to aid in building it up. Most 

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



