Geomorphological Development of the Western Hidaka, Hokkaido

著者 OUCHI Yasushi 雑誌名 The science reports of the Tohoku University. 7th series, Geography 巻 24 号 2 ページ 165-182 発行年 1974-12 URL http://hdl.handle.net/10097/44985 Geomorphological Development of the Western Hidaka, Hokkaido

Yasushi, OUCHI

Introduction Across the western fringe of the Hidaka mountains, such rivers as the Saru and the Mukawa rising in the mountains, flow into the Pacific Ocean. In this area, the geomorphic surfaces i.e. coastal terraces, river terraces, gentle slopes, and fans are well developed. These geomorphic surfaces have been studied on various view point of geomorphology. Previous studies would be divided in two categories, i.e. those oriented to crustal movements clarified by the correlation of terrace surfaces, and those oriented to geomorphological development related to glacial stages or periglacial phenomena. As the former, Hanai (1934) and Ogasawara (1951) reported about the classification of coastal terraces and the crustal movements around the Cape Erimo. Sakaguchi (1959) reported the crustal movements throughout the Hidaka Coast, and the ages of terrace formation. As the latter, comprehensive studies were done by Imai (1960, 1961) and Torii (1963, 1968). Torii mentioned on the buried topography too. Since then, however, few study has followed in this area. Examining previous reports, the writer thinks that some problems remain to be solved. There are some different opinions on classification, distribution, correlation and chronology of terrace surfaces. Since Sakaguchi (1953) proposed the chronology in and around the Hidaka mountains, there is no apparent progress on chronology of geomorphic surfaces, i.e. the chronology is not yet established, and there is enough room to make more precise chronology based on glacial eustasy and tephro-stratigraphy. The present writer tried to reconsider the chronology and development of geomorphic surfaces in the Hidaka district. The study area was selected between the Mukawa river and the Shizunai river, where river terraces and coastal terraces are wide and remarkable. For the study, precise interpretation of airphotos and large-scale maps, levelling of the surfaces by an aneroid barometer and observation of the surface materials and the pumice beds are carried out. Radiocarbon dating and pollen analysis of peat layers are added for the chronology. The landforms of the Hidaka district including the study area, are the reflec- tion of geological arrangement and structure. The mountains are composed of the Pre-Tertiary sedimentary, igneous and metamorphic rocks and the fringing 1 6 6 Y. OUCHI _// 0 -,,,94// 1 \V --1--/ / i ', IN t Y ,,,, •,-_7)-2 .---,...) , Q, N94. r 4.- /., ^ I, _ ,rsios-7-4114, r--- E.( _i) ,.Ve'-L.,-1!;))01.4r1-ar a ,,, ,rs -OKACill R°13'HR° '' 1,--„.. , 44al 4,HIDAKAtriCiLid.-Ni8,.;;,l's it)__.,,, ,,17.„,., it 4 ,..,l„. f 1u ' 0 \q '4P)r3f-F--tak?..e,.-,..„1-w)\-,-,,,_) V ( a( \ HOBETSUU 0.1,,, , dk(v.07.31 . lArgirk.,-•k.,,.._,.--'---,;.-"*.... ,,...... „, s',..,.....,•••_.) 7 .Q.:,j, 3

--'- --' f% irreApok - 4sil .v.. ' ._,:t FMKMUKAWAAlj,,i,it3,s,,..,'''' R."K • .,."Cl,1F,A*tr'ee/ - „(,,),,,,,414,v_ip,,,, 0 ATSUBETSUR1,..,,,,,..,vti.Ail 4*/fi'S'.',,,.1)'-‘ 411KATA Adea4,04:',.-v‘..41.'"--••-..,',--..„ ,,,,,, ,,_,-i.AP- NIIKAPPU RM Ir.,(\.(\ AL-11.-,wi--At aIZZUUNNA1R ' u*11ir °Al - k.-.4,;1C ' AS HIROO 444 • ',1-1 4‘ vo tO ^ AL/ , ---) ) .., ..:1--) ..„ URAKAWA A,pliti,--,A\ks \------ditt. A C. ‘ , ____,•-?,7 0 20 ,m cri, look m Contour interval: 100 m CAPE ERIMO

Fig. 1 Restored map of the southern part of Central Hokkaido eliminating the valleys less than 2 km wide

hills or lowlands are composed of Neogene sedimentary rocks (Hashimoto et al. 1958). The geological arrangement is generally accordant with the direction of Hidaka mountains, which is considered to be a sort of tilted block with steep slope on the east side (Hanai 1934), where many fans widely develop on the mountain fronts. On the contrary, on the west side, rivers flow in accordance with the mountain slopes, and such small basins as the Sofu (Hidaka) and the Shimukappu are formed at the patch areas of Tertiary. From the restored map (Fig. 1), discontinuity of inclination is noticed along

\ Geomorphology of the Western H idaka, Hokkaido 161

III SURFACEI ' TOMO. SURFACEn Ili SURFACE 11 SURFACE IV SURFACE V SURFACEVI SURFACEVII E SURFACEVM rkg GENTLESLOPE 10 FAN a HILLS&MOUNTAINS

S4IMUKAPPL1

HI0.4NA 21

IM/ANAI

Sec. 3

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FURENAIE

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Fig. 2 Geomorphological map of the study area (wegtern)

3 168 V. oucHi

PACIFICOCEAN Fig, 2 Geomorphological map of the study area (eastern) the altitude of ca. 500 m, that is generally accordant with the geological boundary between the mountains and the fringing area. The area about 250 m in height is hills here. The geomorphic surfaces are limited out of the area. In the mountains, however, small river terraces are scattered in the valleys or basins along the Mukawa and the Saru (Fig. 2). The basins such as the Sofu and the Shimukappu continue to gorges at dykes of metamorphic rocks (mainly serpentine) which are conspicuous knickpoints in long-profile (Fig. 3, 4). River terraces are separated at gorges and not continuing throughout the whole stream. Around the Niikappu and the Shizunai rivers, the mountains are closer to coast, and geomorphic surfaces are distributed only on the coast. Besides the terraces, gentle slopes and fans are seen behind the terrace, some of which overlap the terraces and others are cut by them. In the upstream areas of the Mukawa, gentle slopes spread from the mountain fronts, but not continue to the terrace surfaces. Gentle slopes seem to be related to the formation of terrace.

Description of Geomorphic Surfaces Geomorphic surfaces can be classified into eight classes of terrace surfaces, gentle slopes and fans (Fig. 2 P,-,6). m

400

Fig, 3 Longitudinal profile of the river floor and Terrace Surfaces 300 along the Mukawa ,-v

200 10"14°.

IV VI

1 00 .....- iu 0 iv

4 20 50 60 70 km a. 0 4 4 a. LLL 40 O 0 z 1.1 03 4 0 z 0 4 a

tn a rn m a VI 400 ^-•x. Ca, Fig. 4 Longitudinal profile of the river floor and Terrace Surfaces along 300 the Saru IVV" 0 V -

200 AR 0 FL)..

IV 100 • .....••"'"

20 30 2 0-4 50 60 10 cc 4 z :7t 4 km 0 z0 te) 4:c LU LL cc z 4 0 cc 0 ac co 1-3 0

0 5 ' 170 V. OUCHI

m I

200

100 •

NIKAPP° VII ------

a. O Et- N c, z

Fig. 5 Longitudinal profile of the river floor and Terrace Surfaces along t11.2. Ni ikappu (The scale of this figure is the same with that of Fig. 6)

200 m

•••••^^••

100 ."""

SHIZUNAI R.

10 0 km z z z UJ 0 0 X I tf)

Fig. 6 Longitudinal profile of the river floor and Terrace Surfaces along the Shizunai

Surface I: The highest terrace surfaces remain as small patches at 170 m a.s.l. on the left bank of the Saru river, while they are distributed at 160-230 m a.s.l. along the Niikappu and the Shizunai rivers. Gravel is not seen on the surface, but silt or clay 0.5 ti 1 m in thickness. This is probably a marine terrace saved from lateral erosion during the next lower surface formation. Imai (1961) considered this surface as an erosional surface, and recognized another higher surface, which the writer could not identify. It is probably a hill-top surface.

Geomorphology of the Western Hidaka, Hokkaido 171

Surface II: The next lower surfaces are widely distributed on the coast 120— 130 m a.s.l. between the Mukawa and the Kenomai, and 150•175 m a.s.l• on both sides of the Shizunai. Terrace gravel is scarcely seen on the surface, but silt or clay directly on the baserock. The surface is fairly dissected, and its flatness is lost. Surface III: Fringing the Surface II, the next lower surfaces are widely distributed on both sides of the Saru and 80-100 m a.s.l. on the left side of the Shizunai, and are fairly dissected like the Surface II. In the former area, the terrace scarp faces the shoreline and in the latter, it faces the present river channel suggesting the river eroded the surface. Terrace deposits generally seen near the terrace scarp (Loc. 13) consist of alternation of flat gravel and sand. They change inward only of silt or clay. Near Toyohata on the left side of the Shizunai, most of the surface is covered with slope deposits. Surface IV: The next surfaces are distributed especially in the west coast, where the terrace scarp directly faces to the shoreline. The typical surface is seen near Monbetsu. The altitudes of the surfaces are 50-70 m a.s.l., a little higher near Monbetsu-Atsuga and lower near Mukawa. The terrace deposits (Loc. 12, Fig. 7) consist of well-sorted flat gravel (pebble-cobble) layer 2 m in thickness and dark reddish in color indicating progress of red-weathering. Although the surface is fairly dissected by streamlets, it keeps pretty flatness. The terrace scarp of Surface II is too obscure to follow, i.e. the shoreline at the formation of Surface III is not clearly recognized, but it can be vaguely identified,

4 m •; .:1• X.'"'X'3011.• J'etr.tez,u, sh.t - rr—H - — -r „ - —Si _

• *4.1, . 77.-Sat• r-- grne''t.A#e4,4);ibi-L";#;;f141.1.1401'11./.7P7h.MdM 54101/2hi,FdPit,i.fy,MV•tieWMI,Aijilkitfili --Sam. t 'lk '44;.;10.**:*4.10.*1:'14`1 , ca. filig:••yletii.ki•iguleei,ati —p

JO, mn.• ...... • •; • .0!.?• • • • • • • fl.111•Oblirli Sac c13; • • • • , —7— cr• .• 0 .."Z5 'CZ"• CS. C? c=3.Dcz,"E'•*?=^*:"..c, cz, • - 4:flige.7-1**Pok..Wri.x.....•.:•^=. 5;t C •••t••,'-•••••==...• ".•.,1=7cm.c,,•••• • • • •••••••11c).•• o • • • c:P • • c, . • • Sal 4 •C:;." CD. ^c=". • 1=>•.r.•••• • .43 k•—•••• I Ltt' ---;*t • ?^. AL- B L

Fig. 7 Cutting at Loc. 12 H humus soil Si: silt Saf: fine sand Sam: medium sand Sac coarse sand P: pebble C: cobble Sal: sand lens B:- bedrock

172 Y. OUCHI

m 200 I— I —II I —-----: --- - _------—III

--- -- 100 --_— ____.------—

IV------__ _

cn I— < a. a _ 0 0 )- 0 (f)

Fig. 8 Altitudinal distribution of Terrace Surfaces along the coast neglecting the overlying deposits on the Surface (Fig. 8). Surface I-IV are generally marine in origin, but some surfaces along the rivers are probably fluvial in origin, such as small patches in the midstreams of the Mukawa and the Saru. Further lower surfaces are Surface V, Surface VI and Surface VII in descending order, which are almost distributed along the river. Surface V : The surfaces are distributed from the midstream of rivers. Along the Mukawa, they are distributed on the left side. Their altitude decreases monotonously downstream, and it is relatively low in Mukawa and Hanaoka area (Fig. 3). The surfaces are deeply dissected by streamlets and undulated partially. At the Loc. 1, the terrace gravel 6 m in thickness lies on the bed rock composed of tuffaceous rock, and in the midstream it is less than 4 m in thickness. It is composed of metamorphic rocks in majority , and sandstone, tuffaceous rocks, conglomerate in minority. It is darkbrown in color, due to weathering, and the matrix is solidified. Along the Saru, the surfaces are alternately distributed on both sides from Nioi suggesting the former meandering. The altitude decreases monotonously downstream. The surfaces are gentle at coast and along the river (Fig. 4). The thickness of terrace gravel becomes thin upstream (Loc. 14 18). Deposits and dissection are similar to those along the Mukawa. Along the Niikappu and the Shizunai rivers, the wide surface is seen from Misono to Mena, and straightly extends to the shore with N-S direction. This is accordant with Surfaces II, III. The gradient of surface is steeper than at the Mukawa or the Saru. The surface shows some displacements (Fig. 5, 6). The Geomorphologyof the Western Hidaka, Hokkaido 173 thickness of gravel is 5 —6 m at Loc. 23, 27 and 9 m at Loc. 25, 28. Gravel is composed mostly of igneous rock and is reddish brown in color with matrix. Surface VI: They are well developed in the midstream areas and the distribution is uneven in the upstream compared with Surface V. At Iwachishi , Iwanai, Hidaka, and Shimukappu, this surface is the most prevailing terrace surface, which is deeply undercut by valleys') and dissected but keeps flatness. In the midstream areas, gravel beds are 3 —6 m in thickness (Loc. 8, 9 and Loc. 21). Surface VII: The surfaces are generally accompanied with Surface VI, however, their distribution is less even upstream than that of Surface VI. For instances, along the Mukawa and the Saru rivers, the distribution is limited downstream at Izumi and Nioi respectively. The surface is little dissected and keeps flatness. The terrace gravel is thicker midstream (4 —6 m: Loc. 6, 4, 19, 20) than upstream (2,...,3 m : Loc. 7, 10), and its facies is as fresh as Surface VI. Surface VIII: Fluvial lowland with some terraced alluvial surface. Gentle slopes: Behind the above surfaces, the gentle slopes are seen here and there. Some are covering terraces, and others are cut by them. They develop also in the mountain area as typically seen near Tomamu (Fig. 2). It is possible to classify these slopes into the formational order based on their relation to terrace surfaces and overlying deposits. The present writer introduces some cases of their landforms and deposits seen in the field. Near Biratori, a gentle slope covers the rear part of Surface V (Fig. 9-1). The slope deposits are composed of tuffaceous breccia over 2 m in thickness at the foot slope. The gentle slope developing between Surface II and Surface V has a steep scarp at the slope end. Maybe this means that the slope surface was eroded by Surface V. Near Izumi, on the left bank of the Niikappu river, a gentle slope extending from the Surface IV is cut by the Surface V at the slope end with a scarp not very steep, the shoulder of which is round and obscure (Fig. 9-2). Gentle slope extending from the mountain side covers the Surface VI near Iwanai (Fig. 9-3). Hidaka and Iwachishi, respectively. Wide gentle slopes extend from the mountain side near Tomamu at the most upper stream of the Mukawa and some of them are terraced at their ends facing to the fluvial lowland. These wide gentle slopes are probably a sort of hill slopes, consisting of some slopes. A small gentle slope with a cutting at the end is observed (Loc. 11 and Fig. 9-4). Gravel bed with clay matrix imbedded by a few

l) The relative heights above the river floors are less than in the mid- or downstream areas (Fig. 3, 4). 174 Y. OUCHI

I 4 rimLoc11 s pfa

470- . peat

G. S. (gentle slope) —100 460 -1 z V 450 — bedrock :ta? VI —50 440 — ----30m a.sl.

1.5km NNW SSE WNW ESE 2 3 IV 220 140— G.S, L, 120— V G.S.2oc s'-.VI-180 100— VI G. S. S1,4-180 -160

SWNE80- 2km NW 750m SE-160 •••••••pumice( En )=rapumicelSpfa) • • • • angulargravelround gravel Fig. 9 Cross sections showing the relation between Terrace Surfaces and gentle slopes 1: Sec. 1 in Fig. 2 2: Sec. 2 in Fig. 2 3: Sec. 3 in Fig. 2 4: gentle slopeat Loc. 11

veneers of peat is overlying the silicic bedrock of Tertiary.Gravel material is the same with the bedrock. Bedrock is exposed without gravel in some places on the slope side. Fan: Some cover the fluvial lowland, and some develop behind the terrace surfaces. They are clearly distinguished from the gentle slopes which has neither catchment area nor thick deposit. The overlying pumice fall deposits are described later.

Discussion (1) Correlation of the Geomorphic Surfaces Special attention was paid for the correlation of the geomorphic surfaces, especially when they are separately distributed. To correlate the surfaces among some river basins, the Surface IV was useful as a key surface. The identification of pumice fall deposits was mainly used for the surfaces below the Surface V, and the altitudinal distribution (Fig. 8) for the surfaces above Surface V. For the Geomorphology of the Western Hidaka, Hokkaido 175

VII IV V VI m Loc. 12 2 5 14 18 22 23 25 28 3 21 26 20 24 0 Et= a • 4 „., b b c b d, d V b 1 • • C 747, • • V V V v d VV V d VV ^- .\ 0" ob VV Oo ii • 0 V W VV • o ,318 e • • 2 d . O0 0• Vv V V . 0 OQ %• 0 00 0 ^ O n• v yy 12:6 7e5 tS'• 3 000 c 00 :0 10: d 00. 42.1 7771 .,„ 0• ..• 0 , °o 4 0 4 Vvi 00Q - - 0 g. C vv po 00 0 1 oO 0 0 V 0 v v 13 5 V 0°1 00, 0 O. 1.3o. cr. 6 El humussoil ;9, a46 7 8 9 o.aP •771.relictpumice&lapillisoilESpumiceffilsand Eiclay En gravel

8 pumicelloamy)[]silt "ek; bed rock

Fig. 10 Relation between Terrace Surfaces and the pumice fall deposits a: T h: En-a c: En-b d. Spfai e: Spfa2

correlation along a river, surface continuity (Fig. 3, 4, 5, 6) was also useful. Lots of pumice fall deposits aree supplied from Mt. Shikotsu, Mt. Eniwa and Mt. Taru- mae. They were chronologicallylogically classified by Yamada et al. (1963) and Naming Commitee (1972). The pumicemice fall deposits from Mt. Shikotsu (Spfa) identified in Pleistocene are classified into two, Spfa, and Spf a, (Katsui et al. 1960). The lower part of Spfai is dated 32200 {3ligg yrs B.P. (Ishikari Group 1965). Ogasawara (1941) reported the pumice fall deposits as "Erimo sand", which were later remarked as key beds for correlation (Sakaguchi 1953, Kaizuka 1956, and Imai 1960). Besides Spfa, the pumice fall deposits from Mt. Eniwa (En) and from Mt. Tarumae (T) are reported (Yamada et al. 1963, Naming Commitee 1972) in which some datings of En are reported from late Wiirm to early Holocene.2) T belongs to Holocene, and some of them fell before several decades. The writer examined the relation between the gemorphic surfaces and these pumice fall deposits, based on some examples of cuttings shown in Fig. 11. Spfa is overlying Surface V 5 m in thickness at Loc. 14 near Tomikawa, which is composed of two layers, the white lower 1 m in thickness contains pumice 21 For instances, the younger En-a is dated 8520±160 yrs B.P. (GaK-1074),the older En-a 23500±800 yrs B.P. (GaK-3260),and En-b 21900±700 yrs B.P. (GaK-3263) (Naming Commitee 1972).

0 o ° 176 Y. OUCHI

Loc.11 (,) 0 vv (r) (i) u UJ O O cc a, O Lu z 1.1J AP .9- O O U I- 0 U co if) dl 1 2 411"'34-0

4 11J (-) •4C 14.1 1.1.1 LAI U m ",L O I 5% 1.- (,) U CLJ O 0: DATING 0. O O U O O 1%;) 0 > 5% Ct z 0 6 I NAP O L 0—4 I-EL U Ct 100% 1 2 3

Fig. 11 Pollen diagram of peat layers from the cutting at Loc. 11

grain, glass grain and a little black lapilli, and the yellowish brown upper 2 m in thickness contains same kinds of grain. These pumice grains consist of crystallized glassy material, and appear like slender sawdust. The lower corresponds to Spfa2, the upper to Spfai, and dark brown silt 20 cm in thickness is imbedded between them. Above Spfa, weathered brown pumice 60 cm in thickness (En-a) is seen, the top of which changes gradually to black relict soil. T covering En-a is composed of coarse pumiceous sand. Spfa, 2 m in thickness, Spfa2 1.3 m and silt or clay 2 m in the descending order are overlying Surface IV at Loc. 12. The top of silt/clay contains fine lapilli and pumice grains which are not apparent whether to be in series with Spfa or to be independent older one. Spfa covering the Surface V reduces the thickness to 2 m at the Loc. 28 near Mena. Reddish brown pumice overlies Surface VI and Surface VII at Loc. 3, 21, 20, 10. This pumice grain is less than 1 cm in diameter and strongly weathered, but it is more coarse than that of En-a, therefore the pumice seems to be En-b. Spfa overlies Surface V and higher Surfaces, and En overlies Surface VII and higher Surfaces (Fig. 10). Concerning the pumice fall deposits on the gentle slopes, there are some cases mentioned above (Fig. 9). Most gentle slopes behind Surface V are covered with

O Geomorphologyof the Western Hidaka, Hokkaido 177

Spfa, but some are covered with En. Gentle slopes behind Surface VI and VII are generally covered with En. T overlies all geomorphic surfaces including fluvial lowland. Thus, the writer's correlation is shown in Fig. 2, and in Fig. 3, 4, 5, 6 and Fig. 8.

(2) Deformation of the Surfaces The geomorphic surfaces of the western Hidaka must have been influenced through their development by crustal movement, climatic change, glacial eustasy, and rock control. Sakaguchi (1959) classified the terrace surfaces into C, T, K etc., and he considered that C terrace surface corresponds to the Shimosueyoshi age (R/W) and T terrace surface to the Wiirm glacial. About the crustal movement he described that the uplifting was remarkable from Niikappu to Shizunai during the formation of C terrace, and it is remarkable near Monbetsu during the formation of T terrace.3) Imai (1961) also classified the geomorphic surfaces, namely, Level-I, Level-II, Level-III, Level-IV, Level-V, and others. And he considered the geomorphological development from eustasy, accepting crustal movement mentioned by Sakaguchi (1959). He concluded that Level-IV corresponds to the Poroshiri glacial proposed by Minato et al. (1954), and Level-V to the Tottabetsu glacial, and further, the former to the Musashino Surface and the latter to the Tachikawa Surface. To avoid confusion, the relation of geomorphic surfaces between the former researchers and the writer is shown in Table 1. Difference in interpretation of terrace surfaces among them is noticed, and the ages of surfaces are not in concord between Sakaguchi and Imai. As shown in Fig. 8, it is true that Surface I, II and III have relatively higher level between Niikappu and Shizunai, but Surface IV no height difference along the coast, rather it has somehow relatively higher level near Kiyohata. Hence, the writer considers such crustal movement as follows. The altitudinal distribution of Surface IV different from that mentioned by Sakaguchi (1959) is probably due to the difference in classification between Saka- guchi and the writer. Along the Niikappu and the Saru, Sakaguchi, may have considered that Surface HI and Surface IV were close in ages of their formation.

T terrace is riverine accordinghis classification. He discussedthe crustal movement based on T terrace surface as an indicator of a sea level at the time like the case of coastal terrace. But it is not clear which point on the surface was levelled. Even if it was selectedclose to the present shore line, the measuredlevel does not alwaysmean the sea level at the time, becausethe so-calledT shaped phenomenaof the terrace are hardly seen here. 178 Y. OUCHI

Table 1 Relationship between the former researchers and the writer

Ouchi (1974) Imai (1961) Sakaguchi (1959)

VII Surface VI Surface Post Erirno-sand Surface K Terrace Surface V Surface Level-V T Terrace Surface IV Surface C Terrace Surface III Surface Level-IV II Surface Level-III I Surface Level-II Level-I

In any case, the uplifting seems to have been remarkable along the coast between Niikappu and Shizunai during the formation of Surfaces I, II and III. Along the Niikappu and the Shizunai rivers, the gradients of Surfaces become steeper, and the relative heights above the present river floors increase toward their upstreams. Fault lines oblique or parallel to the shoreline are reported e.g. Seppu fault line (Matsuno et al. 1958), and one of them passes from Meiwa to Loc. 28 (north of Mena). The Surface V along the Shizunai river increases the height abnormally upstream just from this Loc. 28. Surfaces I, II, IV and V on the left bank of the Niikappu are distributed from Meiwa to Izumi suggesting the former river channel shifted southwestward and Surfaces I, II and IV have very high levels (Fig. 5). This is probably due to the past uplifting remarkable in the upstream accompanied with the mountain building. The uplifting probably con- tinued until the formation of Surface III, according to the altitudinal distribution of Surfaces I, II and HI (Fig. 5, 8). On the other hand, along the Mukawa and the Saru rivers, the gradients of Surfaces are gentle near the coast and increase monotonously up to the midstreams (Fig. 3, 4), and especially along the downstream of the Mukawa, Surface V is situated at relatively low level.4) As concerns coastal terraces, Surface III and IV relatively reduce their height in the west of Tomikawa (Fig. 8), where, the crustal movement was less marked compared to the eastern coast. Thus, near the coast, the Surfaces, especially higher ones (I, II, III) were uplifted in east, i.e. heavier uplift, closer to mountains. This tendancy is explained by the influence of Hidaka orogenesis. The down- or midstream areas of the Mukawa and the Sam are located in the most remote area from the Hidaka mountains. Thus, the coastal area along the Mukawa and the Saru can be considered the most stable area, where the geomorphic 'surfaces suffered the least crustal movement. 4^ Torii (1963)presumed a NW-SE fault line whichpasses through Kasuga, and he (1968) considered that the area southwest of the fault line was subsiding. Geomorphology of the Western Hidaka, Hokkaido 179

(3) Geomorphological Development Therefore, the relation between the Surfaces and the sea level oscillation can be elucidated here. Relative heights of Surfaces V, VI and VII above the present river floor are larger here than in the upstream areas from the great knickpoints (Fig. 3, 4) which are the lithological boundary between lower Neogene and upper Pre-Tertiary and the differential erosion. The thickness of terrace deposits, the larger the lower, would accord with the above. Surfaces V, VI and VII submerge the fluvial lowland smoothly downstream with oblique intersection (Fig. 3, 4). Such phenomena are seen along the Niikappu and the Shizunai (Fig. 5, 6). Along the Shizunai, the Surfaces are steeper in gradient. Therefore, Surfaces V, VI and VII are concluded to have been formed at the dropping sea level, to say, in the Wiirm glacial. However, IV seems to be formed in the Riss/Wurm interglacial as considered by Sakaguchi (1959), because the Surface is the lowest coastal terrace with marine deposits thereon. In the downstream area of the Mukawa, a buried terrace and a buried valley were reported by Torii (1963), and Surface VI extends to the buried terrace, and Surface VII to the buried valley. The buried terrace was formed at the interstade before the Wiirm Maximum (Torii 1968). Otherwise, the formation of Surfaces V, VI and VII are limited to some ages from the ages of pumice fall deposits, i.e. the formation of Surface V was before ca. 32000 yrs B.P. at youngest, and Surface VI and VII were formed between ca. 32000 yrs B.P. and ca. 8000 yrs B.P. at widest. From the surface classification, the sea level oscillation curve5) in Wiirm glacial proposed by Minato (1966, 1974) and pumice fall deposits mentioned above, the ages of Surfaces V, VI and VII are more accurately considered as follows. The ages of Surfaces VI and VII gained from the pumice fall deposits range from the end of Gottweigel interstade to Wiirm Maximum, i.e. Surface VI adequate- ly corresponds to the end of GOttweigel interstade, and Surface VII, probably, to near Wiirm Maximum. The age of the buried terrace mentioned by Torii (1963) may correspond to the former as mentioned above. Surface V was formed probably in Early Wurm (Altwiirm), because the surface is undulated and the deposits are weathered. Thus the formational process of Surfaces IV-VII can be explained based on the sea level oscillation. Surface IV was formed by the transgression in Riss/Wiirm interglacial, then the valleys began to be cut with the regression to Early Wiirm. Thereafter, Surface V was formed at staying of sea level in Early Wiirm. 5) The writer prefered the sea level oscillationcurve proposed by Minato (1966, 1974) based on the Japanese data to Fairbridge's (1961)and others'. 180 Y. OUCHI

Surface VI was formed in Gottweigel interstade following Early Wiirm, strictly at the end of the interstade just before the next regression (Vorrtickung phase of the Main Wtirm), because Surface VI is not overlain by Spfa. Meanwhile, deep valleys began to be cut with the regression. Then, Surface VII was formed with some duration in Warm Maximum, when the sea level was lowest. At last, the sea level gradually rose up to the Holocene level and Surfaces VI and VII were submerged beneath alluvium in the downstream areas. On the other hand, the terraces were not well developed in the upstream areas of the Mukawa or the Sam, because the bedrocks were resistant against erosion. Surface V must have developed with some extent, however, it almost disappeared by erosion (mainly by incised meandering) until the formations of Surface VI and VII, which developed on a local base level decided by intruded dykes. From the overlying pumice fall deposits, these Surfaces developed surely keeping corre- spondence with those in the down- or midstream areas. Concerning Surfaces I, II and III, their formation and development can not be fully clarified in relation to the glacial stages, because the Surfaces suffered crustal movement and erosion and kept hardly deposits except Surface III. May- be most of them are marine in origin. They are quite distinguished in dissection and relative height with Surface IV. They are expressed like gentle slopes, and their formation was probably in the processes of succeeding regressions after an interglacial. From these facts, they may be correlated to the Tama surface in Kanto district. Most of gentle slopes behind the terrace surfaces were formed succeeding Surface VII and preceeding Surface VII, i.e. their formations seem to have been also in some duration of Wiirm Maximum and to be due to such periglacial action as solifluction at the terrace scarp or the steep mountain slope. Some gentle slopes were formed after Surface V and before Surface V. Concerning the gentle slopes in the mountainous land, it is difficult to know their ages in relation to the Surfaces. In order to know the age of the slope, a piece of wood contained in the lowest peat layer at Loc. 11 was dated 38100-±Mg yrs B.P. (N-1729) by the radiocarbon dating. This date is of early Gottweigel interstade. Pollen analysis of the same peat layers (Fig. 11) shows that, as arboreal pollen, Picea indicating cold climate is in majority, but Abies and Pinus indicating warm climate are intermingled, and as herbs, Cyperaceae and spores are conspicuous. Therefore, the climate was colder than the present, but it was not so cold as Tundra climate. Consequently, the formation of the gentle slopes was also in the ages from early Grittweigel interstade to Early Warm in addition to those in Wtirm Maximum. And the periglacial action like above seems to have been also active in the former ages, some of which are considered to have been more warm in climate than in Geomorphology of the Western Hidaka, Hokkaido 181

Table 2 Geomorphological development of the study area

Sea Level Geomorphic Incidence Pumice Fall Minato (1966) Naming Commitee Fl 8 Terrace Surface Gentle Slope & Fan (1972) etc.

T a) -0 Fan (5 10000 B.P. VM En•a

I I En-b C I" G.S. a

29000 B.P.

vl Spfaz, Spfai 43

3100' (32100±4700 B.P 4.J•0 G.S. .24 (38100-±rogB.P 44000 B.P.

Iv I G.S. a 60000 B.P.

ca. 70000 B.P.

111

II 1 CI) I I-4

Wurm Maximum from the instance mentioned above. The geomorphological development in the study area is summarized in Table 2.

The writer is grateful to Prof. K. Nishimura of Tohoku University for his continuous and valuable guidance. Acknowledgement also goes to Dr. K. Hibino of the Institute of Biology, Tohoku University for his guidance of palynology. Thanks are also due to Dr.

I 182 Y. OUCHI

T. Nakata, Mr. N. Chida and Mr. K. Yasuda of the Institute of Geography, Tohoku Univer- sity for their useful advices. The writer thanks to the administrative staffs of Mukawa and Saru, Bureau of Deve- lopment and Construction, who were kindly enough to give the writer large-scale toposheets. And the writer would like to thank Mr. T. Imaizumi who helped the writer in the field investigation.

References (* in Japanese ** in Japanese with English abstract) Fairbridge, R.W. (1961): Eustatic Changes in Sea Level. "Physics and Chemistry" 4 London 99-185 Hanai, S. (1934) : On the Geomorphology around the Cape Erimo.* Geogr. Rev. Japan 10 720 Hashimoto, W. and Ishikawa, T. (1958): Geological Map of Hokkaido 1f200000.* (4) Southern Part of Central Hokkaido with explanation Institute of Underground Resources Survey of Hokkaido Imai, T. (1960): Erimo Volcanic Sand in Hokkaido.** Ann. Tohoku Geogr. Ass. 12 (2) 41-44 (1961): Geomorphological Development of Hidaka Coast.** ibid 13 (2) 57-65 Ishikari Group (1965): The 14C Date of the Shikotsu Pumice Fall Deposits in Ishikari Plain.* Earth Science 81 12 Kaizuka, S. (1956): Some Notes on the Geomorphology in Tokachi Plain Especially on the Lower Terraces and the Volcanic Ashes — * Geogr. Rev. Japan 43 (2) 232- 239 Katsui, Y. and Murase, T. (1960) : Some Considerations on the Activity of the Shikotsu Volcano.** Jour. Geol. Soc. Japan 66 631-638 Matsuno, H. and Yamaguchi, S. (1958): Geological Map 1/50000* "Shizunai" with explanatin. Hokkaido Kaihatsu-cho (Development Board of Hokkaido) Minato, M. and Hashimoto, S. (1934): Poroshiri Glacial Stage. Tottabetsu Glacial Stage. Poroshiri-Tottabetsu Interglacial Stage.* Jour. Geol. Soc. Japan 61 709 Minato, M. (1966): The Final Stage of Land Bridges in the Japanese Island.** Earth Science 85-86 2-11 (1974): "Quaternary System of Japan"* Tsukiji Shokan ps. 167 Naming Commitee for Volcanic Ashes of Hokkaido (1972): "The Distribution Map of Volcanic Ashes of Hokkaido" with tables of correlation and "C dates and references Ogasawara, Y. (1941) : Coastal Terraces around the Cape Erimo.* Jour. Geol. Soc. Japan 48 181-182 Sakaguchi, Y. (1953): Some Notes on the Geomorphology around the Hidaka Range — mainly on the Chronology in the Quaternary Period —* Geogr. Rev. Japan 48 563-570 (1959): The Crustal Movement of Hokkaido in the latest geologic Age.** ibid 32 (8) 401-431 Torii, E. (1963): On the Groundwater of the Mukawa River Valley with Relation to its Buried Topography.** Jour. Tokyo Geogr. Soc. 72 (3) 115-125 (1968): The Buried Topography along the Southern Coast of Hokkaido** Geogr. Rev. Japan 41 (2) 63-80 Yamada, S., Katsui, Y. and Kondo, Y. (1963): Distribution and Chronology of the Quaternary Pyroclastic Deposits in Hokkaido.** Quaternary Research 3 (1)–(2) 80-87