20 J. Brown et al. 



accumulation. The rims of low-centered polygons, the tops of high- 

 centered polygons, and particularly the outer edges of both are more ex- 

 posed to summer and winter microchmatic extremes. Since polygonal 

 terrain forms the basis for much of the biological and pedological varia- 

 tion found in wet coastal tundra, a brief review of its development 

 follows. 



Shallow, narrow troughs averaging 0.5 m deep by 1 to 2 m wide and 

 underlain by ice wedges form the outlines of polygons having diameters 

 that range from a few meters to more than 30 m (average 12 m). Ice 

 wedges develop as a result of cracking of the ground due to contraction 

 during periods of intense and rapid winter cooling. The narrow thermal 

 cracks are subsequently filled by ice in the form of winter hoarfrost or 

 from spring meltwater that freezes. The cracking and ice filling, repeated 

 over many centuries, results in the growth of vertical wedge-shaped 

 masses of ice which penetrate many meters deep and may expand to 

 several tens of meters wide (Lachenbruch 1962). 



The increase in volume caused by the expanding ice wedges produces 

 buckling or heaving of the surfaces on either side of the wedges and par- 

 allel to them. The ridges or rims so produced, together with melting of 

 the tops of the wedges, cause depressions or troughs immediately above 

 the wedges that further define the polygonal surface pattern (Figure 

 1-10). As a rule, polygonal terrain becomes more deformed and elevated 

 with the passage of time, as ice wedges expand laterally and polygons 

 subdivide, i.e. secondary ice wedge cracks form within the polygons. 

 Troughs and central basins may deepen as the underlying wedges melt in 

 response to climatic or microclimatic changes. Many of the basins may 

 remain filled with water to form permanent or seasonal ponds. 



Where the drainage level has been lowered, as on stream banks and 

 the shores of drained lakes, the trough produced by thawing of ice, ther- 

 mokarst, and thermal erosion may result in topographic reversal of low- 

 centered polygons. That is, a polygon center that was once low becomes 

 elevated and better-drained with respect to the deepening troughs, and 

 the low-centered polygon is thus converted to a high-centered polygon 

 (Figure 1-10). In extreme cases, particularly where the underlying miner- 

 al soil is sandy, the depression of the troughs may be more than 1 m, and 

 the raised centers then consist of dry, peaty soil. Because of their elevated 

 position, the raised areas are vulnerable to desiccation and wind erosion 

 as well as to slumping into the troughs. These processes appear to be ac- 

 centuated by lemmings, which find the features ideal nest sites and riddle 

 them with burrows, causing further desiccation and oxidation of the 

 peat. In situations where organic materials are removed altogether, the 

 depth of summer thaw increases and frost heaving may convert the sur- 

 face to small, bare frost scars or boils, or into hummocks. 



The sequence of events just described produces the contrasting types 



