PIEDMONT GENTLE SLOPES IN THE KANTÔ DISTRICT

At the foot of Mt. Ôgi and Tsukuba in the Kantô District (Fig. 1), conspicuous development of gentle slopes is known, which have been vaguely considered as pediments, or pediment-like slopes. Gentle slopes are located at the foot of a fault scarp (Mt. Ôgi), at the foot of an old volcano (Mt. Myogi) or at the foot of a granite mountain (Mt. Tsukuba). It is the purpose of this study to assure if they are pediments or not. The geology of Mt. Ôgi consists from north to south of Kobotoke group (Cretaceous), Katsuragawa formation (mid-Miocene) and Misaka group (Oligocene). Ôgiyama fault runs between the former two. On the Katsuragawa formation gentle slopes develop with a longitudinal range of about 100 m. Trans-verse profile is almost level at the upper limit of the gentle slopes, and undulated at the lower limit. Tops of undulation consist of harder Misaka group. The escarpment behind the gentle slopes coincides with the Ôgiyama fault scarp and this fact means that the gentle slopes were formed not by pedimentation but by rain wash and soil creep on the softer Katsuragawa formation during the course of slow elevation. Age the formation of the gentle slopes is concluded to be lower to uppermost Pleistocene. To the northeast of Mt. Myôgi, gentle slopes develop on the Miocene bed and the escarpment behind the gentle slopes is the flank of an old volcano. Longitudinal range of the gentle slopes is about 300_??_1000m and the inclination is 5° _??_8°. Both the longitudinal and transverse profiles are undulated. There are talus deposits dropped from the escarpment. Formation of the gentle slopes occurred in middle Pleistocene. Around Mt. Tsukuba gentle slopes are found in the coarse crystallized granite area, which is sometimes covered with fine crystallized granite or hard rock. Longitudinal range of the gentle slopes is 500_??_1500m and the inclination is 7°_??_12° . The gentle slopes are veneered by strata of cobbles, boulders and coarse sands, but in these strata bedding and sorting are not remarkable. The gentle slopes were formed by the parallel retreat of escarpment due to weathering, gravitation and rainfall, and modified by sheet wash, The age of the formation of the gentle slopes is middle Pleistcene. Agreement on the origin of pediment is not yet obtained, and on the features of pediment there are many kinds of decription. For examples, the longitudinal profile of pediment is convex or concave; the escarpment end is straight or zigzag; and the transverse profile of escarpment end i s level, convex, concave or undulated. Picking up the common facts of all these pediments, the definition of pediments might be given as follows: the pediment is an erosional gentle slope with knickpoint at the foot of the escarpment behind, truncating the rock formations of the mountain ranges and developing continuously during the planation of mountain ranges. Thus, it is concluded that the gentle slopes around Mt. Tsukuba are genuine pediments and those of Mt. Ôgi and Mt. Myôgi can not be identified as pediments.