This research was conducted in an area as the wessern end of the Tachikawa terrace which cuts the Musashino upland in the western Kantô Plain. The area adopted is roughly triangular in shape, being 7 kilometers from east to west and 5 form north to south, with Ome City as the apex of the fan topography. The research was executed on the following four items: (A) Observation on minute undulations of the land surface, (B) Mesurement of depths of the Kantô Loam, i. e., so-called Kantô “Loam” or aeolian volcanic ash layers, …… only the Tachikawa “Loam”, the top layer of ash, is found in this area…… and making of an isopleth map of the “loam” depths, based on the data 260 reasearch points. (C) Drawing of land profiles along six selected routes, showing micro-relief features of the gravel layer under the “loam”, and consideration on the relationship between the depths of the “loam” and the minute undulations of the present surface as well as the surface the buried gravel layer. (D) Observation on color and texture of the “loam”.
After examining these items, the environment and the processes of the formation of the “loam” were considered as follows: Although the depths of the “loam”, as reported previously, have been thought to be homogeneous and uniform about 2 meters deep on an average throughout the Tachikawa terrace, smaller depths of the “loam” than 1.5 meters, have been found at some places of the southern half of this area. Besides, one can find different depths of the “loam” layer, not only between on the swells of the undulating surface forming a point-bar-like topography and on the swales forming an old streamchannel-like shape, but within each patched swell or swale. From this fact, it can be said that the relief of the buried gravel layer surface is larger vertically, but has a finer texture holizontally than in the case of the present land surface. Though, to say the least of it, a state of the buried gravel layer surface is more complicate than that of the present surface. As a result of some considerations on color and texture of the “loam”, the miunte undulations of the surface of gravel layers and also these of the present surface, it is clear that the “loam”, at most places in the field, began to deposit on the then existing of the fan deposits. As the studied area, furthermore, being different from other parts of the Tachikawa surface which are quite flat, has a slight relief on its surface, it is considered that the depths of Tachikawa “Loam” layer have undergone various changes through the removal of erosion and redepsition after its original deposition.
Although these characteristics as mentioned above, may not be seen throughout the Tachikawa terrace surface, they may be pointed out as remacable features of the fan-head of the Tachikawa terrace surface.
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