Subsurface Geology of the Musashino Terrace

Abstract

The outline of this paper is as following.1. The stratigraphical sequences of the type locality (Neighbouring north-eastern area of the Musashino terrace) are as follows : Kanto volcanic ash bed 78 metersMusashino gravel bed 28 metersTokyo groupUpper Tokyo formation 10± metersMiddle Tokyo formation 30± metersLower Tokyo formation 180± metersMiura group2. The basic ground, i.e. Miura group, is of undulating structure and dips toward the northeast with about 1° as a whole.3. On the Miura group, there are found underground valleys which seem to be old river-roads, and which go straight from west to north-east.4. The Lower Tokyo formation is distributed meeting with this basic ground and is developing its formation progressively from west or south-west to north-east.5. On the surface of the Lower Tokyo formation, there are some buried valleys and the Middle Tokyo formation is distributed covering these valleys. The direction of these valleys is analogous to that of the valleys found on the surface of the Miura group. 6. And these two type valleys' features conform with the geological structure of the Miura group.7. It is possible to consider that this fact is related with a series of tectonic movements in the Kanto basin. These movements are well known as the Kanto tectonic movement.8. Judging from the direction of buried valleys and the conditions of development of the geological groups which cover the said valley, we guess that all the water system of this district was situated at least at the north-east or at the east-northeast, before sedimentation of the Musashino gravel bed.Therefore, as to the distribution of the Tachikawa terrace and the river-road of the River Tama, we guess that they had been formed after the formation of the Musashino terrace, by the movements which changed these old river-roads, this is to say, by the inclination of the basic ground which moved relatively ascending the north-western part of this district to the south-eastern part.