MIURA  Osamu

SUGAWARA  Kei

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MIURA, Osamu

SUGAWARA, Kei

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Landforms on the Ridge of Mt. Goyo, southernKitakami Mountains著者 MIURA  Osamu, SUGAWARA  Kei雑誌名 The science reports of the Tohoku University.7th series, Geography巻 20号 2ページ 237-244発行年 1971-03URL http://hdl.handle.net/10097/44938Landforms on Osamuthe Ridge of Mt.  Goyii, Kitakami Mountains MIURA and  Kel SUGAWARAsouthern    In the southern Kitakami Mountains, pre-tertiary granitic  — rocks are distributed widely. Mt.  GoyO is one of these  grani-tic rock areas consisting of z granitic rocks called  GoyOzan ( granitemass (Onuki 1969).'`'./ er Mt.  Goy& about 12 km north --, of Ofunato city, Iwate  pre-  :---- fecture, rises like a monadnock above the surrounding summit surface of uplifted peneplain                            ((about 700-800 ma.s.1.). The top is a relatively broad ridge line ranging about 3 km  from northeast to  southw.  st  (Fig. 1).  F There are  small hills and peaks on the ridge line, the relative heights of  which are small and little  1341.3  m  above sea level and others so small that a,n individual summit  si and peaks  can be distinguished  wit] such  landforms as tors, block  stream It is the  purpose of this paper to  p mountain  ridges by means of the  ob.    The  eastern half of the ridge  sur to WSW, but the western half is  n slopes of the ridge are asymmetric, Block fields  and block streams are  SI .0,30 ,,,/,____/_-- 000,, q  f  cp  .,?y t  2   •)          .f3 t6  '  "-"°°            ,,I.(A      1,5',c'c,   '-„        ,,,,,„—i, ...1'',  e N  .„..,_  0  25,  5,,,,,-,0  m0.- 5  .090  0crA:               r -  oTuno 11;1;21,. •  Tye'          Kamae i    Myt.Cool+       A 1-160-F   Nyikami           runnto N Anwc..?..,3ti- -4- 00e NeW  .  lift t  Kescrinuam °  IP  'in ;    Pi . .  ig. 1 Location Map Block scattering in closed                              broken line Bare rock masses and block 111 ls  s                              concentrated belts at heavy lines li , t  r l tive  i  r  ll  little varied. The elevation of the highest peak is  ,Te  l  rs are about 1310 m. The relief of the ridge is n ividual it  urface is considered about the level. Nine hills    i tinguished  ithin the area of our investigation. There are  ts  , k a s and block fields around the hills and peaks.  )o   er   oint out the processes of the denudation of  ;es by eans of the  observation of these landforms.  r lf   e face is relatively broad and smooth from ENE :  tern lf  arrower and steeper from NE to SW. The e  etric, NW slope is gentler and SW one is steeper.  k a s   seen on the gentle part of slopes in general.238 0.  MIURA and K. SUGAWARAThe Western Half    At the western-end appears a hill (location 1), but there are no bare rock masses as mentioned below. On all slopes many blocks are scattered forming block fields. Two conspicuous block-concentrated belts are recognized, one stretches N 70°W from the summit of this hill to the NW faced slope and the other is on the E faced mid-slope with similar feature. The former is larger and rather distinct and its long-profile is shown in Fig. 2. The upper part of this has a width of 5-10 m and convex cross section, the lower part shows concave one and broadens gradually downward. It is assumed that the concave cross section  indicates the initial stage of valley head formation caused by the washing away of  eathered fine materials by run off. Though these block-concentrated belts are  sii ilar to the block streams in appearance, it is not likely that they have been  trai.sported a long distance or to have been accumulated at present locations. The bare rock masses collapsed in situ, and blocks were produced. That is, the block-concentrated belts are the remains of bare rock masses in the past.    As for the size of blocks, many are 1.5-2 m in diameter and the maximum is over 4.5 m, and the thickness of blocks is not over 1 m depending on the nature of joints in the bare rock mass. Many blocks are angular but some are subangular whose edges cut round. Although the base of piling blocks can not be found, it seems to reach to a considerable depth. A pile of large blocks with many small rock fragments tilling the gaps, attains a depth of about 5 m. On the hill of location 1, the land surface beside the block-concentrated belts is wholly covered with shrubs less than 1 m in height. However, soil develops poorly, and only thin humus and regolith are found on the blocks at many places.    At locations 2,3 and 4, the bare rock mass lying on the lower gentle slope has collapsed already and a block-concentrated belt as those of location 1 is formed (Fig. 2). The bare rock mass with the width 4-4.5 m at the top rises distinctly 5.5-6 m in height above the surrounding land surface and its length is measured -  L7-1     -a_ Loc2    socantrated WitBare  rock  luta —T.-7N  NF 7w  N  Sew 45" se'w 1.31,Orta56  1290  230 Loc.1177   kto Block  concentrated belt CN  7inti  N  26.7' N60'.4 to 2 Long profiles I and N6S14 103  so at locations EALLIDILL115'40 NJ 70.W  41.  ;  20 .1010  1300  .124C  1230  1270  1260Fig. 2           Landforms on the Ridge of Mt.  Goy& southern  Kitakami Mountains 239 about 30 m. The rock mass is cut vertically (about  80°) at the joints as round slice, horizontal joints are found at the upper part. The thickness of the slice is limited 30-70 cm in size. In a ward, the rock mass has started to collapse and to produce the blocks, while the cracks along the joints are considerably widened. The top is especially loose and some blocks are stripped off.    At location 3 the bare rock mass is similar to those above-mentioned but there is no distinct block-concentrated belt. The rock mass protrudes about 5 m high and its top is 4-7 m and 40 m in width and length respectively.    At location 4, there is a little protruding rock mass which seems to be in an early stage of exhumation process from the  sub-surface, and the bare head of the rock mass 3 x 30 m in dimension appears only at the top. Its NW slope is covered with dence vegetation and separate blocks are not apparent. At the opposite side vertical fresh rock wall is exposed 3 m high and with E-W orientation, being the upper part of the rock mass. This exposed rock is cut by  joint systems as mentioned above. However, it is remarkable that the cracks along the joints are not developed yet and the exposed rock surface appears relatively fresh. The Eastern Half    There is no distinct bare rock mass protrusion on the eastern half of the ridge which is relatively broad and gentle, but blocks are scattered here and there. They are rather small and round, and are more sparse than those of the western half. A small head of rock mass extends E-W in direction to the east of the top summit of Mt.  Goy() (location 5), where bare rock surface with width of about 3 m and the length of about 10 m is exposed but the exhumation of rock mass does not occur. At location 6, there is a rather small rock mass protrusion about 3-4 m high standing on the shoulder between the gentle ridge slope and the steep mountain flank like a stack or a tower. According to air-photos, there are no conspicuous rock masses or block fields such as those of the western half beyond this location, but there are some stack-like bare rock mass protrusions, covered with the forest dominated by Tsuga diversifolia Max., while, the vegetation to the west of the location is consisted of shurbs or grasses. Surface Materials    Soil formation is very poor in block-concentrated belts and in areas of rock masses, where a little fine weathered granitic and humic materials are sticked upon the bare block surfaces with the plant root system. On the contrary, soil develops well on the cols among peaks of the western half and on the broad ridge slope. Six trenches are digged for soil profile observation at the col between small peaks or on the gentle slope. These trench sites are not so inclined (2°-8°) and covered with240 0. MIURA and K.  SUGAWAR  Ashrubs and grasses except the trench 5. Profiles of surface materials are shown in Fig. 3.  All the lowest horizons consist of weathered granitic sandy materials including many rock fragments and mica flakes. At trench 4 and 5 in the eastern half, weathered core stones like boulder are excavated from the same 50 [Western  -half]  trench-1  t--2 I N Ili I Kati  V • q     • . C-ZIWZig 100cw. t - 3 o  °    [Eastern-611  t-4  t-5   If!                  4Legend                   •T •••••,,•,      .44:11A ...”.4.).  ,ry  EFig. 3 Columnar profiles of surface materials       A: Litter B: Surface soil C: Fresh rock fragments D: Core        stone E: Sandy materialshorizons. It is uncertain whether such core stones are originated from the weathered bedrock or from the separate blocks, for the trenches were not large enough to decide the origin. It is noticeable that relatively fresh rock fragments originated from blocks or core stones in lower horizons are accumulated closely under the surface humus horizon in all profiles. They are fresh and platy (5-10 cm in diameter), and are deposited horizontally. This peculiar depositional facies is distinct at trenches 4,5 and 6. Thus, there are two materials which indicate different weathering processes, one is core stone which suggests the advance of sub-surface weathering, and the other is fresh rock fragments . The horizontal arrangement of rock fragments is probably caused by the movement of materials near the surface. But, it is not obvious whether this movement is directly related to the process of rock mass exhumation during the latest time or not . Denudation Process    The landforms such as bare rock masses, scattered block fields and block-concentrated belts seem to correspond to the phases of denudation processes of the mountain ridge.    Partial weathering of granite mass of bedrock proceeds in the first phase , and unweathered rock mass and weathered materials are differentiated . Judging from the almost similar directions of rock masses or the block-concentrated belts Landforms on the Ridge of Mt.  Goy& southern  Kitakami Mountains 241 and their long extension, the pattern of sub-surface weathering seems to be directly influenced by the structure of bedrock. But  actual process in this phase is not clear with the collected field data.    The second phase is a dynamic process, for instance, rock mass  exhumation and protrusion, rock mass collapsing and block field formation. The important process in this phase is the removing of weathered materials. Location 4 is in the early stage of the protrusion, location 3 is in the full stage and location 2 has already been in the collapse stage. The block field of location  1 is in the far advanced stage of this phase. The block-concentrated belts are the disintegrated rock masses in origin. While these phases, the land surface is lowered, that is, the denudation process.    The chemical weathering is dominant process in the third phase, when, subaerial weathering and removing of materials in short range are rather dominant than the lowering of land surface. The eastern half of the ridge seems to be in this phase.    Following problems are still  unsolved; 1) whether differential weathering actually took place in the past or not. 2) whether each process such as weathered materials elimination or rock mass protrusion, is periodical one or not. 3) whether such landform processes are correlated with the climatic changes during late Pleistocene or not.    The writers would like to acknowledge the continuing guidance of Prof. K. Nishimura and wish to thank Mr. H. Makita Institute of Geography, Tohoku Univ., for his advice in field.                                       Reference  Onuki, Y.  (1969): Geology of the Kitakami Massif, Northeast Japan Tohoku Univ., Inst.           Geol. Pal., Contr. 69,  239  ps, t•JPhoto 1A  view ofwesternhalf fromeastern half O --ch*  C.)Photo 2 Ablock-concentrated at location 2belt  on Efaced mid-slope  (A)at location 1. Barerock mass  (13) andcollapsed rock I'll  a.Landforms on the Ridge of Mt.  Gov& southern  Kitakami  Mountains 243Photo 3 Vertical  rock  wall of a rock  mass at location 4Photo 4 Protrusions  and  collapses  of rock mass

Landforms on the Ridge of Mt. Goyo, southern Kitakami Mountains

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Landforms on the Ridge of Mt. Goyo, southern Kitakami Mountains

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