28 J. E. Hobbie 



a depth of 100 cm (Walker 1973). At the Barrow site, the maximum depth 

 of thaw is around 25 cm below the grassy tundra while in the sediments of 

 the ponds it may vary from 20 to 50 cm. Large rivers and lakes of this 

 region may be underlain by extensive thawed areas. Brewer (1958) found 

 60 m of thawed material beneath Imikpuk Lake. 



The presence of permafrost has important biological effects. Because 

 the permafrost is impervious, the water cannot drain away, and the low- 

 lying soils are saturated. Roots are restricted to the upper, thawed layer of 

 soil which limits the total quantity of nutrients available. Finally, nutrients 

 and energy are removed from circulation either when the permafrost level 

 rises or when soil and sediments accumulate and become part of the 

 permafrost. 



The upper layers of permafrost on the coastal plain contain large 

 quantities of ice. One form of this occurs as interstitially segregated ice (up 

 to 80% of the top 3 or 4 m of permafrost) (Sellmann and Brown 1965). 

 When this melts, due to disturbance of the plant cover or to heat transfer 

 by flowing water, a depression is formed that may result in a pond. 

 Another form of ice occurs when water runs into cracks formed by the 

 winter contraction of the frozen tundra. The resulting buried ice takes the 

 form of ice wedges that can range from a few centimeters to 8 m in width. 

 Over many years, the wedges grow and eventually a network of ice wedges 

 is formed. Sometimes these wedges are expressed on the surface as 

 polygonal ground (Figure 2-3) caused by heaving or other surface 

 processes that form troughs and ridges. Typically, these polygons may be 

 20 to 50 m across; polygonal ground covers almost the entire coastal plain. 



In the early stages of growth of polygonal ground, the polygons are 

 low-centered and often contain small ponds. The water changes the 

 insulating properties of the surface and also traps heat so that the upper 

 layers of the permafrost thaw, subsidence occurs as the ice melts, and a 

 basin up to 0.5 m in depth is formed. These ponds frequently coalesce and 

 may form a lake. Eventually, the lake may grow enough that a drainage 

 divide is breached. Then the lake may drain and the polygonal ground 

 start to form again; the whole process has been called the thaw-lake cycle 

 (Britton 1957). 



Many of the larger thaw lakes of the coastal plain display a striking 

 elliptical shape with an elongated north-south axis. The exact reason for 

 the orientation has been the subject of a number of studies and theories 

 (Black and Barksdale 1949, Livingstone 1963a, Carson and Hussey 1960), 

 but it is evident that differential erosion is still occurring today. For 

 example, Lewellen (1972) measured a rate of elongation of 1.3 m per year 

 in Twin Lakes. In similar lakes, Hussey and his co-workers measured 

 currents at the ends of the lakes of up to 61 cm per second, which 

 Livingstone (1963a) believed to be adequate to account for the elongation 

 of the lake basins. However, Walker (1973) believes that the precise 

 mechanism of elongation is still unexplained. 



