The authors thought that a breakdown of a slope is basically attributable to the augumentation of the slope's instability potential, and that the time-variation of causative characters related to geological and topographical conditions must be taken into account in the discussion of the mechanism of landslide development. The Chichibu basin was chosen as the area of the present study, and the mechanism of landslide development was considered mainly from the geological stand-point. The major object was the landslides caused by the action of weathering. Bedrocks The physical properties of rocks are correlated to one another, and it has become clear that they can be represented by the uniaxial compressive strength and the dry shrinkrage ratio. Among the Tertiary rocks of the Chichibu Basin, mudstone shows that its physical properties vary with geological horizons, and the ralation between its physical properties and the geological horizons is represented by two cycles which correspond to the Oganomachi group and the Chichibumachi group respectively. Weathering zone The weatherability of fresh rocks can be expressed by the uniaxial compressive strength and the dry shrinkage ratio. A weathering zone is easily developed in a area where the basement is composed of rocks with small uniaxial compressive, strength and large dry shrinkage ratio. A weathering zone can be divided into four subzones, I-IV, from the ground surface to the fresh bedrock down below. The physical properties of each subzone change according to the degree of weathering, but their changes from one sabzone to another are gradational. Moreover, it has become clear that the quantity of cracks in the weathering zones of the study area is far from negligible. Development of weathering landslides On the basis of the factors which augument the slope's instability potential with the lapse of time, the authors classified the landslides into three groups, namely, the weathering landslide, the stream erosion landslide, and the head erosion landslide. In the present paper the authors focussed the discussion on the mechanism of development of the weathering landslide. In order to infer the state of development of the weathering zone before the breakdown of the slope, the authors carried out seismic prospecting along the margin of the destroyed slope. There are two types of landslides, one occurs mostly in the first velocity layer with less than 0.4 km/s, and the other extends down to the second velocity layer with 0.4-0.8 km/s. It is evident that the character related to the cracks of the weathering zone and to the softening of rock fragments is contributing to the development of landslides. The depth of the weathering zone has its limit in accordance with the slope's gradient. The depth of the weathering zone increases with time, and in this process the part that overstepped the critical stability would be broken down.
The authors discuss about the distribution of the recent patterned ground in the Khumbu region and Hidden Valley of the Nepal Himalayas (Fig. 1) by their investigation carried out as a part of “Glaciological Expedition to Nepal, 1973 and 1974”. Beeing influenced by each climatic and geomorphic conditions, the distribution and development of patterned ground in these two regions show the different features. The holizontal and vertical distribution of patterned ground in the Khumbu region are shown in Fig. 2 and Fig. 3 respectively. Vegetated patterned ground develops in large area up to 5500 m because dense winter freezing acts over the vegetated ground fed by monsoon rain and high temperature in summer. Earth hummocks appear just higher part of the forest line (4200 m) and occupy the alluvial plain around Periche and moraine slopes around Dzonglha Karka (Fig. 3). Vegetated lobes are formed by solifluction on flanks of moraines and valley slopes around 5000 m typically, but turf banked terraces and ploughing blocks are also formed on vegetated slope above the forest line. Frost cracking occures in winter in various parts of the study area. On alpine mat vegetation of flat surface of moraines, frost cracks develop into non-sorted polygons, 0.8-1 m in diameter (Fig. 6). Sorted patterned ground is rare in the Khumbu region and is found only at the bare ground in front of glacier turminus and at the bottom of pond which are dried up in autumn. Neither vegetated nor sorted patterned ground is observed above the upper limit of continuous vegetation cover where mountain slopes are occupied by glaciers, steep rock walls, talus and block slopes. The characteristic of patterned ground in the Khumbu region is found to be common to the south slope of the Eastern Nepal Himalayas. Since most part of Hidden Valley is situated above the limit of vegetation cover, sorted patterned ground distributes in large area, but vegetated patterned ground, such as earth hummocks and turf banked terraces, is found along the small atreams drained from glaciers (Fig. 10 and Fig. 14). Sorted polygons (both large and small scale) occure on the flat surface of the valley's bottom and top of the crest, but sorted stripes (Fig. 12) and sorted lobes cover most of slopes. It was obserbed that the sorted stripes developed even in the steep slope over 30 gradient and the period of survey (Fig. 13). Striated ground, reported by SCHUBERT (1973) in the Venezuela Andes, was found in many places in Hidden Valley, and its direction coincides with the wind direction in the morning and at night. The climatic and topographic conditions in Hidden Valley are adequate to forming the sorted patterned ground in such large areas. Though there is not so much difference in mean monthly air temperature between in the Khumbu region and in Hidden Valley, the freeze-thaw cycle is much more in Hidden Valley than in the Khumbu region (Table 1), because Hidden Valley, situated north of main axis of the Himalayas, has less cloudy days in the monsoon period. The geomorphic feature in Hidden Valley which consists of gentle and non-vegetated slopes is also suitable for developing sorted patterned ground up to the snow line. It seems that the characteristic of patterned ground in Hidden Valley mentioned above represents Tibetan slopes of the Himalayas. The distribution of patterned ground on both Nepalese and Tibetain slopes of the Himalayas are found to be different from the ones of Japan Alps and European high Mountains.