The Middle Miocene marine strata was found to occur in the northern part of the Hidaka belt, the Kitami-Takinoue region, northern Hokkaido. In this area the Miocene sediments can be divided into three formations, as follows; [table] The following molluscan fossils were found in the basal part of the Kamishiyubetsu Formation; Acila sp., Yoldia haborensis, Serripes notabilis, Mercenaria chitanina, Peronidia cf. t-matsumotoi, Neptunea sp., Natica sp.. This molluscan fauna is correlative with the Chikubetsu fauna (KANNO et al., 1960) in the Haboro coal field, Northern Hokkaido. Foraminifera fossils such as Cyclammina japonica and Cyclammina incisa were also found in the upper mudstone member of the Kamishiyubetsu Formation. Molluscan fossils such as Portlandia hayasakai and Malletia sp. were found in the Oshiraneppu-gawa Formation. These fossils are similar to those found in the Tokomuro Formation in the Kushiro coal field and the Nupinai Formation in the Tokachi region. The molluscan fossils above-mentioned suggests that the Kamishiyubetsu Formation and Oshiraneppu-gawa Formation were accumulated in middle Miocene age.

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Peculiar angular fragments-bearing mudstones, called "Sarashikubi(gibbeted heads) beds" as a field name by KISHU SHIMANTO RESEARCH GROUP (1969), are distributed in the southern coastal area of the Kii Peninsula. The "Sarashikubi beds" belong to the upper formation of the Oligocene to Lower Miocene Muro Group, representing the later stage of the Shimanto geosyncline. Sedimentological study of the beds is important to make clear not only the latest development of the Shimanto geosyncline, but also sedimentation of a certain type of olistostrome bed. The "Sarashikubi beds" (310 m in thickness) are lithologically subdivided into three units, that is, Units A, B and C in ascending order. Unit A is mostly composed of massive angular fragments-bearing mudstones, and intercalates in the middle part a sequence of normal beds, consisting of pebble conglomerate, sandstone and calcareous siltstone. Unit B consists of coarse-grained sandstone, conglomerate and angular fragments-bearing mudstone. Unit C is composed of angular fragments-bearing mudstone bedded in 50 to 300 cm thick. In the present paper, sedimentological properties such as stratification, form, roundness, gravel-size, mineral composition and fabric of gravels and occurrence of matrix were described in detail. Matriceous mudstone is poorly sorted, massive and black siltstone in which no tectonic shear or fissility is observed. "Angular" gravels consisting mainly of sandstone, are classified into three types of occurrence, namely, sharpstone type, segregated type and flowed type. Based on mineralogical composition, these sandstone gavels are derived from the Muro Group itself. Difference of their occurrence reflects the degree of consolidation of source rocks from which their gravels were formed. It is probably thought that the geanticlinal upheaval, grew up at the later stage of the Muro sedimentary history, was collapsed and supplied various types of "angular" gravels. According to the gravel fabric, the upheaval existed to the south of the basin. It is estimated that angular fragments-bearing mudstones of Unit A transported by submarine debris flow under the condition of abundant muddy matrices and low viscosity of the flow, differed from the typical submarine debris flow (MIDDLETON and HAMPTON, 1976). It might be that the ones of Unit C were deposited by immature turbidity current. On the contrary, the rounded pebbly conglomerate, intercalated in the middle part of Unit A, is quite different from angular fragments-bearing mudstones and other conglomerates in the "Sarashikubi beds", in the respects such as composition, roundness, sorting and fabric of gravels. Based on the fabric of gravels, these gravels were supposed to be derived from the northern geanticlinal land, where sandstone glavels of the Muro Group were rounded by fluvial and/or wave action, and then transported by grain flow and/or turbidity current into the sedimentary basin. Muddy flysch facies and abundant slump beds underlying the "Sarashikubi beds" show that the basin was not so shallow. The basin was filled up by deposition of the Unit A, and become shallower. The abundance of shallow-facies trace fossils, trough-type cross lamination and coaly materials, observed in Unit B, supports this interpretation. It is probable that the basin became a little deeper in the stage of the Unit C sedimentation. In the later stage of the Muro Group, geanticlinal upheavals arose here and there in the basin, and the basin became differentiated. The Muro basin became shallower as a whole, though differentiated small basin repeated to become a little deeper or shallower. And then, it was disappeared and uplifted. It is worth to be noted that the "Sarashikubi olistostrome beds" was not formed on trench floor or deep sea terrace, but on the shallower sedimentary basin.

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A lot of geological information by means of ground survey, boring, test pitting and geophysical prospecting for the dam construction is available from the area in the middle course of the river Bonji, a tributary of the river Aka. The anther has made a study about the rock mass controls in geomorphology of the area promoted by this situation. The principle of the study is based on the mass-wasting relating to both geomorphology and rock mass conditions. As a landform reflecting masswasting, a landslide landform, a knick line, valley distribution density and a talus slope including aluvial cone are taken into consideraration. The relations between each landforms and rock mass conditions are summarized as follows; 1) The distribution density of landslide landforms is much higher in the Akagawa formation consisted of siltstone, than the Bonji formation consisted of tuff breccia interbedded by andesite lava. 2) The rock mass at landslide landforms has been loosely fractured to cause gapping fissures, although the appearance is as like as in-situ one. 3) The knick line has been formed by the integration of small scale landslides. This landform is possible to be classified into the following 4 types: Type 1: The upper and lower slope from this line has much difference in their inclination, although both slope merge gradually to each other. The line can be traced as if a plane intersecting a slope, and in fact, it coincides the boundary between Bonji formation and Akakawa formation. Type 2: The upper and lower slope contacts sharply at this line and both slopes have much differences not only in slope inclination but also texture, i,e. the gentle, smooth upper slope and the steep, rough lower slope. The line intersects the contour lines at steep angle as if the lower slope has been conspicuously denuded. The rock mass condition at this type knick line has been deeply weathered. Type 3: The upper and lower slope contacts sharply at this line and the difference in slope inclination is not so much as the other knick lines. The line intersects the contour lines at small angle as if the lower slope has been denuded a little. The rock mass at this line has been slighty weathered. Type 4: The feature of this line is as type 2 knick line, although it is just on the shoulder of the side-cutting slope to the river, and can be traced along countour line. The rock mass at this knick line has been slightly weathered. 4) The rock mass at the slope with dense distribution of valley has been slightly weathered. 5) The dimension of a talus slope is proportional to the productivity of rock fragements at the upper slope judged from the type of knick line.

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Orthoquartzite gravels were found from the conglomerates of the Osaka Group, in the Uji Hills, Kyoto Prefecture. The conglomerates are mostly composed of chert gravels (more than 70%), accompanied with gravels of vein quartz, acidic pyroclastic rock, quartz schist, schale, granite, quartz porphyry and sandstone. Orthoquartzite gravels are rarely found. High rounding of orthoquartzite gravels in comparison with the other gravels suggests that they are reworked gravels.

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