Abstract
The altitudinal zone affected by periglacial processes is classified into periglacial and transitional zones. The periglacial zone is the optimal zone of zonal periglacial processes and/or of periglacial denudation-smooth slopes, and is generally defined as the zone between snowline and lower boundary of solifluction. The transitional zone is the zone where extrazonal periglacial processes occur above and below the periglacial zone. There are a number of unclarified points regarding the altitudinal positions of the periglacial and the transitional zones, and their relations to the permafrost zone. The present paper examines the altitudinal positions of the periglacial, transitional, and permafrost zones in Japan, and tries to clarify the altitudinal relationships among them, as well as the genetic relationships between periglacial processes and permafrost.
In Japan, the alpine zone can be substituted for the periglacial zone whose altitudinal extent is ca. 1, 000 m. Its upper boundary is expressed by the snowline, while its lower boundary is represented by the scrubline which correlates well with the warmest month isotherm of ca. 10°C. On the other hand, the altitudinal extent of the transitional zone below the periglacial zone reaches ca. 2, 000 m. The lower boundary of this transitional zone is represented by the lower boundary of the Fagus crenata forest zone, i. e., the 21.1°C mean summer isotherm. The lower boundaries of continuous and discontinuous permafrost zones are represented by the-6°C and the 21.1°C annual mean isotherms, respectively. The 10°C isotherm for the warmest month (August) descends north ward with the gradient of ca.-70 m/°N, while the gradients of the above-noted-6°C and the-2°C annual mean isotherms attain ca.-160 m/°N. Because the 10°C August isotherm intersects the-6°C and the-2°C annual mean isotherms, the Japanese periglacial and the transitional zones are classified into the following types (I a to II c) based on their altitudinal relations to permafrost zone:
(Ia) The periglacial zone occupied by only continuous permafrost. This zone occurs on the Hokkaido mountains higher than ca. 2, 250 m, north of ca. 42°N.
(Ib) The periglacial zone occupied by both continuous and discontinuous permafrost. This zone occurs in the Japanese Alps and on Mt. Fuji above 2, 900 to 3, 100 m in altitude.
(IIa) The transitional zone whose upper part is characterized by the presence of both continuous and discontinuous permafrost. This zone includes the Hokkaido district north of ca. 42°N.
(IIb) The transitional zone whose upper part is occupied by only discontinuous permafrost. Its distributional range is ca. 33° to 42°N in latitude. Most of high mountains of the Tohoku and Chugoku districts are included in this zone.
(IIc) The transitional zone where no permafrost occurs. All the high mountains of the Kyushu district are included in this zone.
Periglacial processes are classified into the following two types (A and B), based on their relations to permafrost:
(A) The periglacial processes that form small-scale landforms and/or-phenomena (e. g., patterned ground, solifluction lobe, involution, etc). This type of periglacial process is not controlled by permafrost.
(B) The periglacial processes that form large-scale landforms (e. g., block stream, periglacial denudation-smooth slope). This type of periglacial process is controlled by permafrost.
These results obtained in this paper are indispensable for reconstructing change in the paleoenvironment in Japan.
