Geology Reference
In-Depth Information
6.2.3. Development of the Polygon Net
Thermal-contraction cracks form polygonal, chiefl y tetragonal, nets that cover extensive
areas of the Arctic and sub-arctic. Typically, the average dimensions of the polygons that
develop in unconsolidated sediments range from 15 m to 40 m. In bedrock, the polygons
or nets are less well developed but, where they do occur, they are smaller, usually 5-15 m
in diameter.
The theoretical development of the polygonal pattern was examined fi rst by
A. Lachenbruch (1962, 1966) and then by S. E. Grechishchev (1970). It was concluded
that the angular intersection of the polygonal net should exhibit a preferred tendency
towards an orthogonal (i.e. right angle) pattern (Figure 6.3). However, this contrasts with
the many fi eld descriptions of polygonal ground in which hexagonal or angular junctions
dominate (Black, 1952; Leffi ngwell, 1919). One implication of the hexagonal net, and of
Figure 6.3.
Types of polygonal nets in permafrost terrain.
