Geology Reference
In-Depth Information
An earlier suggestion that the size of the polygonal net refl ects the severity of the
climate (Dostovalov and Popov, 1966) appears, at fi rst sight, to be logical, with smaller
polygons refl ecting increasingly severe winters (Figure 6.7A) . However, observations made
upon the lateral propagation of cracks that developed on the newly-exposed lake fl oor at
Illisarvik do not support a fi ssure hierarchy (Figure 6.7B). As Mackay (1986d, p. 1784;
Mackay and Burn, 2002) points out, the continued development of the crack is limited by
the infl ux of vegetation to the site and the trapping of snow such that, with 60 cm snow
cover, the main and lateral crack systems became inactive after the fourth winter.
Figure 6.7.
The growth of the polygonal net. (A) According to Dostovalov and Popov (1966), there
is a successive growth of fi ssures of higher orders. (B) Plan of the new thermal-contraction-crack
system at Illisarvik, one year after lake drainage, does not support the simple hierarchical model.
From Mackay (1986d).
