- The Association of Japanese Geographers
- 地理学評論 (ISSN:13479555)
- vol.80, no.5, pp.246-258, 2007-04-01 (Released:2010-03-12)
Morphology of hummocky patterned ground (hummocks) reflects various physiographic factors such as frost conditions, surface gradient and hydrological environments. Hummocks in a seasonally frozen mire in the Nemuro peninsula include two types in terms of the presence of live sedgy-tussock. Sedged forms (Type Y) concentrate in a waterlogged condition, while sedge-free forms (Type T) are densely or sparsely distributed in a shallow (including waterlogged) or deep groundwater condition, respectively. Morphometric analyses indicate the contrast between high and horizontally circular Type Y hummocks, and wide and vertically half-circular (flatter) Type T hummocks. The distribution and geometry imply that the waterlogged condition promotes hummock development by differential frost penetration (cryoturbation) in combination with deep seasonal frost and sedge-controlled uneven vegetation. The hummock development produces a thermal contrast between the apex and the depression, which enhances differential frost heave. Assisted by flat terrain, these processes produce closely-spaced high hummocks, up to 52cm in height. These results demonstrate that relatively large hummocks intensively occur even in a climatically marginal region for periglacial hummocks (mean annual air temperature=6°C), where flat and waterlogged mire environments with deep seasonal frost enhance periglacial processes.