becomes subject to hydrostatic pressure. In many cases, the water 

 under pressure is forced to the surface where it may form seep- 

 ages and ice sheets, icing-mounds, blisters or bursts. Or, the water 

 may become frozen within the active layer in large solid masses, 

 sheets, wedges, veins, lenses or minute grains causing a swelling 

 of the surface of the ground. In a few instances, when the water 

 was strongly mineralized, it has remained in a fluid state although 

 the surrounding ground was frozen. Water exists below the 

 permafrost layer. It is always fluid, and usually is under con- 

 siderable pressure. 



Soil that is saturated by melt-water during a part of the summer 

 is really half afloat, and on the slopes this soil slips slowly down- 

 ward. Such solifluction causes a certain amount of sorting of the 

 material, making a striped arrangement of the soil with alter- 

 nating coarser and finer particles. These stripes take the form of 

 small irregular terraces. On level ground the form taken by the 

 water-soaked soil is an irregular hexagonal pattern where the 

 centers are gravel and larger stones, and the interspaces are finer 

 sand. These are the polygonal soils of the polar areas. Vege- 

 tation commonly concentrates in the perimeters of the hexagons. 



The existence of permafrost has an effect on the vegetation. 

 The excessive moisture in the active layer above the impervious 

 frozen ground inhibits the growth of some trees. Bogs and 

 marshes are characteristic of wet permafrost areas. Trees with 

 shallow roots such as the larch, spruce, tamarack, balsam poplar 

 and dwarf birch can thrive in areas, where the active layer is as 

 shallow as 12 to 18 inches. Jack pine has a prominent tap root, 

 and thus cannot grow in permafrost areas. Jack pine forests 

 are a good indication that permafrost is either very far below 

 the surface or completely absent. 



DRAINAGE 



Permafrost is in part the cause of the haphazard, poorly defined 

 drainage of much of the artic regions. The inability of the sur- 

 face water to seep into the frozen ground forces a considerable 

 amount of the scant precipitation to remain on or near the surface. 

 Glaciation, of course, is primarily responsible for the disruption of 

 the pattern of drainage in the vast areas that were covered by the 

 ice sheets. There has not been sufficient time since the recession 

 of the ice for well-defined drainage to be established. Thus, much 

 of the relatively flat land of the Arctic is studded with innumerable 

 shallow lakes connected by sluggish streams that seem to wander 

 aimlessly. There are, however, some prominent rivers which flow 



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