FORMATION PROCESSES OF A TALUS CONE IN THE ASHIO WASTE LAND

Abstract

The Ashio Waste Land, located in the northern part of Ashio Town, Tochigi Prefecture, was originated by a large forest fire in 1891, and has been aggravated under the influence of poisonous gas discharged from a copper refinery nearby.On the steep valley sides in the area are exposed bare rocks, beneath which numerous debris slopes are formed. Processes of the formation of a talus cone, named “Gm Talus Cone”, were examined here. It is developed on a river terrace on the left bank of the Watarase River near the copper refinery, and is composed of slate fragments from a steep slope behind. As shown in Fig. 3, it has a relative height of 54 m, a slope length of 92 m from the apex to the base, and a general inclination of 36°, showing a rectilinear longitudinal profile except for short sections near the apex and the base. On the surface of the Gm Talus Cone, there exist three major types of deposits, indicating a marked distinction in grain size composition as shown in Figs. 6 and 7. These types of deposits are distributed on the surface of the slope in a longitudinally striped pattern (Figs. 4 and 5). The coarse and well-sorted deposit, R type deposit, shows a regular variation in their mean size along the longitudinal profile of the talus slope as shown in Fig. 8. This tendency of the variation is in accord with the relation between the stationary position and grain size of fragments which were let fall experimentally from the apex of the talus cone (Fig. 9). Thus, this type of deposit is considered to be formed through the individual falling down of rock fragments (rock fall). The D type deposit, mainly composed of pebble-sized fragments, forms a small-scale tongue. It was observed on this talus cone that such a tongue was built by a slow (several decimeters per second) flowing down of loose debris, named here dry fragments flow. The poorly sorted deposit which forms a set of small tongue and levee-like elevation, W type deposit, was supposed to be a product of debris flow under the condition of heavy rain. It was frequently observed after heavy rainfalls that the micro-topographies and deposits like this had been formed on the debris cones in the Ashio Waste Land (Fig. 10). Consequently, the followings are concluded on the movement and deposition of rock fragments on the talus cone : among the various sizes of detritus supplied from the steep slope behind to the talus slope through the process of rock fall, a certain size of fragments are distributed in quantity over the wider scope on the mid-slope, and the smaller ones settle near the apex, coarser ones near the base of slope, all showing the tendency to in-crease in grain size toward the downslope direction. The smaller fragments and fine materials which, in the case of the individual falling down, were forced to settle only near the apex migrate downslope by way of such collective processes as dry fragments flow and debris flow. In the next place, small pits were excavated at three points, up-, mid- and down-slope, on the Gm Talus Cone to observe its internal composition. The side walls of these pits are composed of many debris layers which may be divided broadly into two parts, upper and lower horizons (Fig. 11). Layers of the lower horizon contain humus fine materials and many plant fragments as their common feature. As there is no vegetation on the present talus cone and steep slope behind, it is considered that these layers were deposited before or just after the forest fire in 1591.