Geology Reference
In-Depth Information
but they seem to form in poorly-drained areas that have recently experienced local lower-
ing of the drainage base level.
8.5.2. High-Centered Polygons
The change from low-centered to high-centered polygons is usually brought about by an
improvement in drainage, often associated with stream incision along the lines of the
bordering ice wedges. On well-drained uplands, the preferential melt of ice wedges often
leads to the formation of high-centered polygon terrain consisting of gentle convexities or
mounds. In lowland areas, where low-centered polygons are more common, the raised
rims progressively disappear as the trough broadens and the central depressed areas of
(A)
(B)
Figure
8.8.
Evolution of low-centered polygon relief. (A) Stages of evolution towards formation
of thermokarst (“fortress”) polygons. (B) Tundra terrain showing “fortress” polygons, Sachs River
valley, southern Banks Island, Canada.
