地理学評論
Online ISSN : 2185-1719
Print ISSN : 0016-7444
ISSN-L : 0016-7444
牧ノ原礫層に関する2, 3の問題
井口 正男
著者情報
ジャーナル フリー

1954 年 27 巻 5 号 p. 184-192

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The Makinohara upland capped with Makinohara Gravel Beds is located in the southwestern part of the lower course of the Oi River, Shizuoka prefecture. These gravel beds, spreading over 30km. north-south, and about 10km. eastwest, have a thickness of 30m to 40m, In the beds the gravel size varies vertically. In this investigation the writer has attempted to adopt the latest methods of research on gavels in his study of the southern part of the Makinohara Gravel Beds (Fig. 1).
(1) The Zone of Large Gravels. In each station the gravel bed was divided vertically into several horizontal units of homogeneous gravel size; from earli unit one sample was collected for mechanical analysis. The amount of the sample varied between about 10 and 30kg. according to the coarseness of the material. The samples were sieved into “Udden” grades, and then their median diameters were determm-mined. Horizontal units of especially large median diameter were found in every station. To this horizon, the writer gives the name “Larger Gavel Zone” (Fig. 2.). And he regards this horizon as representing a simultaneous river bed of former titre (Fig. 3 and Table). The Larger Gravel Zone from the stations numbered 10 to 17 (Fig. 1) and the gravels of the lower course of the present Oi River lie at almost the sat-ne gradient. However, the decreasing rate of gravel size in relation to distance of the former is 8 times larger than that of the latter (Fig. 3).
(2) Former Coast Line. A projected profile of the southern part of the akinohara upland shows an escarpment less than 10m. in height, at a position about 600m. north from station no. 18 (Fig. 4). Three sets of sandstone gravels were collected at random-n from exposures of the Larger Gravel Zone at each of the stations on both sides of the escarpment. The first set is of a size of 32_??_16mm., the second set is of 16_??_8mm., and the third one 8_??_4mm., each set consisted of 100_??_150 pebbles. For the first and the second sets, the statical parameters of roundness of each pebble were visually determined by comparison with the “Pebble Images for Visual Roundness” shown by W. C. Krumbein. As to the third set, the pebbles were photographed in the laboratory, then. the parameters of roundness were estlinated by the method used by Wadell. The pebbles in set show a remarkable roundness at th south side of escarpment, compared to their roundness at the north side of escarpment (Fig. 4 and Table). The distribution type of size of the gravels contained in the Larger Gravel Zone shows a remarkable difference between the southern and the nouthern parts of the escarpment. The distribution type of size of gravel in the latter resembles that of gravels of the present Oi River bed (Fig. 5).
Based upon the research the writer presumes that the former ccact line in the period when the Larger Gravel Zone was being deposited was located as shown in Fig. 1.

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