Hillslopes below the tree line, which are usually protected from frost action with vegetation cover, are subjected to seasonal periglacial mass-movements when vegetation is removed by, e.g., continuing strong wind and/or human activity. Among various factors which may control the occurrence of such azonal periglacial processes on wind-beaten slopes in temperate low mountains, we took notice of ground-surface material. Observations were carried out on a wind-beaten bare ground which is very locally developed on the peaks near the Goreibitsu pass in northeastern Japan (c. 900 m a.s.l., 37.5°N).
While mean annual air temperature is 7.3 degrees C, air temperature in winter reaches about -10 degrees C on the bare ground where snow-cover is frequently blew off. The area is geologically composed of mid-Miocene tuffaceous sandstone which tends to exfoliate easily from thin laminae. Angular flat stones exfoliated from the laminae are scattered, or accumulated in places, on fine earth which contains pebble- or granule-size stone fragments.
We observed air temperature, ground surface temperature, ground temperature and displacement of surface stones in four winter seasons from 2006-07 to 2009-10. Dislocation of surface stones was observed by transformation of the nine painted lines. Length and dominant slope angle of these lines are 8.3-25.3 m and 6-20 degrees, respectively.
Ground-surface condition is divided into the following two types:C type, which is covered with angular flat stones, and the F type, where fine earth is exposed. We applied the division to the record of surface-stone dislocation in 2007-08 winter.
Records of ground temperature show that freezing and thawing repeated only in the upper part of fine earth at several, not all, points. Temporal observation in a winter showed that needle-ice development was also restricted in a part of the upper fine-earth layer. Mean values of stone dislocation in a winter on C type and F type ground surface were 0.24 m and 0.41 m, respectively. It is obvious that a bigger dislocation occurred on F type ground surface than on C type one.
The above evidence strongly suggests that the difference is the consequence of more intense and frequent occurrence of freezing and thawing on F type ground surface where fine earth layer is almost exposed than on C type one where the layer is continuously overlain with overlapped stones. On both types of ground, freezing and thawing which are the major driving force of surface stone dislocation occur in the upper part of fine earth layer. Differential occurrence of surface stone movement is thus considered the results of differential response of fine earth layer with or without stony cover to freezing temperature. It seems one of the characteristics of periglacial mass-movements on slopes in temperate low mountains.
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