The writer examined linear depressions and asymmetrical ridges frequent on the ridges of Mt. Iide (Fig. 1). “Linear depression” was named first by Suzuki, I. (1974) in place of double ridges so far ...used. As an important forming factor of linear depression, frost shattering of base rock under cold environment is presumed, but such a process is thought to be feeble at present. On the other hand some linear depressions seem to be formed by the fault activity. In the examined area most of the linear depressions have small rills through which melt water from snow runs out, and the linear depressions on the main ridge often present asymmetrical form. Heavy snow drift filling the depression is thought to have modified the linear depression to such forms. At present, most of the linear depressions have been filled up gradually, but the inclined bottom in the direction of axis of linear depression has been eroded by rill water. In the formation process of asymmetrical ridges, fragments produced by frost shattering was carried downward by solifluction on the windward gentle slope, but at present it is stabilized by vegetation cover, while the leeward steep slope has been eroded by nivation, which is still active. These observations are supported by the difference in drainage density of valley on windward and leeward slopes of the main ridge (Table 2).
Well- developed involutions are observed on the boundary between volcanic ash and frost shattered rock fragment layers to the northeast of Mt. Takamori (1220m) in the central Kitakami Mountains. ...According to the radiocarbon ages (Endo 1977, Inoue 1978) of Koiwai Pumice between the two layers, the involutions were formed after 13, 000- 16, 000 y. B. P. under cold climate.
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