山形県向町前森原扇状地について
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Prof. T. Tomita (1951) referred to the Maemorihara plain when he discussed on the structure of alluvial fans in Taiwan (Formosa). He stated "there may be eroded fans even in the Main Island of Japan, such as those in arid regions. Probably it in hard to confirm the possibility of the formation of eroded fans in the Main Island of Japan simply because of the deficiency of finding the outcrops showing the structure of alluvial fans", and treated the Maemorihara Phain as an exsample which has the possibility of such structure. Although the present writer does not necessarily agree with his opinion, the writer studied Maemorihara Plain recently to analyse the structure of this tongue-shaped plain, and concluded a, follows;<BR>1. Maemorihara Plain is composed of two portions with different levels, the upper and the lower Figs. 1 & 3).<BR>2. The heights of the basements of deposits that constitute the upper and the lower levels are approximately same, while the thicknesses of the deposits are different, resulting in the diffecence of the levels Fig. 4).<BR>3. The deposits that constitute the upper level arc sorted well and those of the lower level are sorted less. This is caused by the epicyclic changes in the development of the stream system.<BR>4. The upper and the lower levels were built as terraces of erosion in the present shape of an elongated fan, that is in the tongue shape.<BR>5. Factors that contibuted to the formation of such a shape are the distribution of loose Pleistocene sediments and the shifting of the courses of the stream loaded with a large amount of breccia produced from the granitic mountain.<BR>The writer has observed several fans which suggested him of the structure such as this, and thinks that not a few of the fans in Japan may be of the erosional origin rather than accumulated one.<BR>Fig. 1 : Map showing the topography and geology in the vicinity of Mukaimachi, Yamagata Prefecture.<BR>Mu : Mukai-machi, Ma : Maemorihara<BR>1-4 : Locality numbers of cross-sections shown in Fig 2.<BR>Fig. 2 : Cross sections showing the relations between each plain terraces. The sites of each section are shown in Fig. 1.<BR>Fig. 3 : View of the mountain, hill, and two terraces it Locality 4, sketched from the opposite bank.<BR>A·B : See Fig. 4.<BR>Fig. 4 : Geological columns showing the structures of the lower Maemorilhara plain (B) and that of the upper one (A) respectively, at Locality 4.
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