At the northeastern foot of the Abukuma Plateau, broad uplands of Tertiary rocks are distributed along the coast of the Pacific Ocean. These uplands have been reported to be marine terraces according to previous studies. The writer, however, thinks that on the basis of the analysis of topographical maps of this area, most of the uplands are fluviatile surfaces. So the writer investigated the terrace topograhy and terrace deposits, in detail, to explain the forming process and the environment of deposition, and if possible, the Pleistocene crustal movements in this area.
Method of study The distribution and the altitudes of terraces were identified by her field survey, and at the same time, the condition of the surface soil overlying the terrace plains were observed. The observation and analysis of the terrace deposits were made as follows:
(1) The measurement of thickness of the gravel bed at many stations in every terrace.
(2) Sampling and analysis of gravels. For the sampling stations, about 50 outcrops were selected from the 3rd, 4th and 5th terraces and from the present river beds and sea beach in order to compare the present environment of the deposition with that of the past.
Samples were measured in their (a) grain size distribution, (b) rate of gravel lithology and (c) roundness of gravel. (a) Sieving the samples of 20_??_60kg at each station by phi scale, the writer obtained weight percentage, size frequency distribution, cumulative frequency distribution and the phi median value. (b) The gravels of different sizes were classified lithologically into two major groups. The one is the granitic rocks or green metamorphic rocks originated from the Abukuma Plateau, and the other is the chert gravel. Perhaps the latter might have been transported by a sort of longshore current from other regions. (c) Roundness of gravels was measured by the comparison method with Krumbein's chart for size and kind.
Results The following conclusions were reached in this study.
(1) The uplands are classified into five terraces, 160m (1st), 100m (2nd), 100-50m (3rd), 20_??_30m (4th) and 10_??_15m (5th) in height. Since the Pleistocene, intermittent uplifts have been repeated in this area. Evidence of submergence, however, is not yet recognized.
(2) Among these terrace, the 3rd terrace shows the most extensive distribution and is correlated with the middle marine terrace in adjacent regions. So that, it is presume that the land had relatively been stable for a long time in large area during the formation of the 3rd terrace.
(3) There is a red soil layer with 3-5m in thickness covering the terrace deposits of the 1st, 2nd and 3rd terraces. This soil seems to be correlated with the Kanto volcanic ashes from its facies.
(4) Most parts of the 3rd terrace were formed by lateral planation with slight deposition by the rivers which flow eastward from the Abukuma Plateau. That is, they are fluviatile surfaces. Marine terrace is present only in restricted areas. This difference in the origin of the 3rd terraces is distinguished by the facies of granels previously mentioned.
(5) The ancient strand line of the 3rd terrace, wherever it is observed, is about 50m in altitude, so the total amount of uplift is about same throughout this area.
(6) Judging from the position of the present rivers and the 3rd terrace, however, it is considered that there was a southward tilting of very small amount at the beginning of the uplift of the 3rd terrace and accordingly the rivers shifted southward.
(7) The 4th and 5th terraces are the river terraces formed by the rivers which possibly had steeper gradient than those which contributed to the formation of the 3rd terrace.
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