Dry 2: Kobotoke Group Stratum and visiting Mt. Jimba 陣馬山

Whenever I visited forests and mountains around Mt. Takao 高尾山, I felt the place is drier than Tanzawa 丹沢 or Hakone 箱根. Recently, I have encountered a possible explanation for that. The northern Kanagawa Prefecture sits on different geological formations from mountains of western Kanagawa. It would affect features and atmosphere the area gives to the visitors. Let’s visit this week there by going Mt. Jimba (ASL 855m) 陣馬山, in northern Kanagawa.

I’ve met Ephemera strigata there!

The stratum I learned is called Kobotoke Group 小仏層群. Scholars for geology think it was created by the crash of North American and Philippine Plates. They collide beneath Sagami Bay 相模湾, Tanzawa and Hakone. There, tectonic energy tears off upper oceanic crust of Philippine Plate as it slides down beneath the North American Plate. The ripped off crust together with deposited sand and mud in the ocean floor is subject to gigantic pressure from North American Plate and metamorphosed and pasted on the sole of North American Plate. So, Kobotoke Group is made of multilayered stratum of sandstone, mudstone, chert, basalt and gravel rock accumulated in the bottom of Pacific Ocean. By humongous force of compression the stratum is broken up in a complicated way here and there and pushed up slanting with the edge of North American Plate. Researchers consider the basement layer in the Southern Kanto Region 関東地方 is being created in this way since 145 million years ago. Through this stratum, volcanos like Hakone burst out its magma. Or, former volcanic island of Izu 伊豆 in the Pacific Ocean is moving on this stratum of Philippines Plate and began poking out from the sea bed some 60 million years ago. When there was no volcanic opening or island, the basement layer appears on the ground. Kobotoke Group is one of such areas where we can see the foundation layer of Kanto Region.

Sanogawa River 佐野川 bed on the foot of Mt. Jimba.
 Yeah, they are gravelly, but
 don’t you think the size of stones are definitely smaller than …

This in Yadoriki Water Source Forest やどりき水源林.
 This photo is of rocks far younger than the above.

Mt. Jimba is one of the easiest points to “experience” Kobotoke Group. This is uber popular hiking destination from downtown Tokyo, probably next to Mt. Takao. To take the easiest hiking route, we first go Fujino Station 藤野 of JR Chuo Line 中央線. It’s a simple train ride for about 1.5 hour from Tokyo Station. Many Tokyoites, including school excursions, come to hike here. From Fujino Station, we ride Kanacyu bus service #NO-08 to Wada 和田 terminal stop (time table is here). Standard hiking course you find in a tourism guide tells you to get off the bus at Entrance to Mt. Jimba Hiking Course 陣馬登山口 Stop, and start walking Ichino’o Ridge 一ノ尾尾根. This course is very simple, or monotone, going up, which may be convenient for trail runners, but a bit boring. This week, I tell you a hiking course different from the regular. I think mine is more checkered, and interesting to walk. Besides, the number of hikers taking this route is not that large as the standard route which frequently receives a party of more than 100 heads, like of lively kids. In today’s course, we can enjoy the soil of Kobotoke Group beneath our shoes in a more relaxed way. So, please keep on riding the bus till the terminal stop Wada. It takes about half an hour from Fujino Station.

JR Fujino Station

This is the bus stop to Wada.
 The photo was taken immediately after
 the departure of 2 temporary large services, completely packed.
 You see?

Wada Bus Stop.
 The café is open weekends.
 They have public toilet here.
 For today, there are two more toilet chances,
 but all of them, including this one,
 are always congested + no western style facility.
 FYI, Fujino Station has western style toilets.

For today’s itinerary, we get down these steps to …

this direction.

Wada Stop is in Sanogawa 佐野川 community. They have several traditional rural houses that themselves are worth a visit. You’ll find they do not have rice paddies to speak of. That’s something for a traditional village in Japan. It’s because Kobotoke Group is so porous that rains sink into the ground very deep without hanging around. i.e. The area is not suitable for rice cultivation. The traditional meal in this community has been based not on rice but wheat, like Udon noodles. If you are a student of geology, please walk for 1.5km from Wada Bus Stop along a small stream called Sanogawa River to the direction of Wada Pass 和田峠. You find points like

North latitude 35 degrees 39 minutes 35 seconds and East longitude 139 degrees 9 minutes 28 seconds,

or

North latitude 35 degrees 39 minutes 9 seconds and East longitude 139 degrees 8 minutes 54 seconds.

Those are large slopes where Kobotoke Group comes out above the ground along a paved road. Though, I prefer walking on the stratum. We can feel the ground directly, can’t we? So let’s turn right around 300m point after the bus stop, to the peak of Mt. Jimba.

The second toilet spot for today.

In mid-April, Sanogawa community is floating in flowers.
 The two structures on the left of this photo are traditional warehouses.

Tea farm in Sanogawa.
 The first flash green teas are available in late May
 at a souvenir shop in Fujino Station.

For North latitude 35 degrees, please go straight here.
 We turn to the right on this photo.

 

But not this paved one on the left, but …
 Could you see a stone monument on the right edge of this photo?
 That is an entrance for traditional
 commuter road to Mt. Jimba.
 An excuse: It’s almost next to a private structure,
 and I hesitated to take a direct picture.

It seems to me this hiking course was a commuter road till 40 or 50 years ago for villagers of Wada. Mt. Jimba is a part of an area spreading from Mt. Takao to Okutama 奥多摩 and Chichibu 秩父 mountains whose western side is Kofu Basin 甲府盆地. Due to its logistical importance for the defense of Edo, aka downtown Tokyo, this part of Japan has been owned by national or regional government for more than 400 years. People of Sanogawa community have had limited title in terms of ownership of the forests. Forestry has not been developed as business here. Instead, they received permissions to use resources from forests, and the hiking course shows us the traces how people used their backyard. Unlike Tanzawa’s Yadoriki Water Source Forest which was once a property of one of the largest forestry companies in Japan, the place is not entirely covered by afforested trees. Instead, large Quercus actissima and Quercus serrata are standing in the other side of afforested area, with tell-tale signs of regeneration by coppicing. The invasion of evergreens, like oaks and Japanese laurel, is very limited. Why?

The fallen leaves and bushy small trees were once useful for firewood and tinder. Japanese forest people also coppiced suitable trees regularly for baking charcoal, and rearing shiitake mushrooms. When people used forest in this way, the climate for Kanagawa and Tokyo would provide forests with lots of coppiced trees but fewer evergreens, as we find in today’s hiking course. In contrast, Yokohama’s Niiharu Citizen Forest 新治市民の森 has large Quercues actissima and Quercus serrata AND lots of oaks and laurels. We know landlords for Niiharu stopped using the forest around 1950. Clear forest floor around Mt. Jimba indicates villagers of Sanogawa community managed forest floor frequently, and probably until recently. Actually, in this hiking, we can find a ruin of kiln for charcoal baking, surrounded by coppiced trees. Measuring the size of Quercues in this mountain, I guess people shuttled to the kiln and cut trees around it till the 1960s.

We can see how stratum of Kobotoke Group was used in the remnant of the kiln. The piled up rocks are all flat like a sheet. It would have been very convenient to build a structure with such rocks. Granted they are old, but the material look porous even for my amateur eyes. From the point of the kiln, the road of the hiking course becomes more and more dry and gravelly. Yeah, in Tanzawa we can meet lots of scree-covered slopes. But they are rockier. Here, the soil beneath our shoes is also rocky but finer. This is a parched mountain. We frequently meet with parts of road made of slanted stratum. After about one hour climb from the entrance of the hiking course, we arrive at a crossing with Ichino’o Ridge. From here to the peak of Mt. Jimba, it’s a main street of the hiking course. The roots of trees show up bony on the ground. It’s a sign of large volume of visitors … After half an hour from the crossing, we reach to the peak of Mt. Jimba.

This hole was once a kiln for charcoal baking.
 You see? The ground of the area is clean.
 No evergreens found here,
 and deciduous trees grow in folk.
 It’s the sign of coppicing.
 And the trees are not large.
 They are not old as we find in Niiharu.
 When forest floor in Kanto Region
 can receive lots of sun shine during winter,
 shade-loving evergreens have disadvantage to grow.

Kobotoke Group stratum we can see up close.

Desiccated …

The southern slope of Ichino’o Ridge is afforested area.

Ichino’o Ridge

It’s amazing to see violets open their flowers
 in such a stumped hiking road.

We are entering the peak of Mt. Jimba.
 The place has 3 mountain huts/café and a toilet.

Literal translation of Jimba is “a stable for war horses.” Archeologists found the peak of Mt. Jimba has been a meadow for centuries. Villagers from the foot of the mountain used the place to harvest thatches for their houses. Legend says when Warlord Takeda Shingen 武田信玄, the protagonist of Kurosawa’s “Ran,” attacked the territory of his neighbor warlord, Hojo Clan 後北条氏 of Odawara 小田原, he came to the peak of Mt. Jimba from Kofu 甲府, and set up his main position. We can understand the logic of his choice. The view from the peak of Mt. Jimba is wide. Weather permitting, we can figure out Tokyo Skytree over there, and Enoshima Island 江の島 to the south. To the west is Mt. Fuji and the northern mountains are of Okutama and Chichibu. It’s the best location to plan your next move to attack Tokyo. Interestingly, the other sides of slopes for the mountains in the north of Mt. Jimba has stratum formulated during Jurassic Period, i.e. one period older than Kobotoke Group. I recalled the peak of Tanzawa Mountains spreading to the south of Mt. Jimba is made of stratum one period younger than Kobotoke Group … Kobotoke Group can be found in a very narrow area. It’s really a stripped crust sticking to the large land mass … woooooow …

To the peak of Mt. Jimba

The peak. Many people hike here these days.
 A former meadow is now like a picnic lawn.

At the top there is this indicator showing
 the names of mountains and landmarks
 we can see from Mt. Jimba.

It was supposed to be able to admire Mt. Fuji over there …

The other side of the mountain we can see here,
 Mt. Shoto (ASL 990.3m) 生藤山 is of Jurassic Period.

From the peak of Mt. Jimba, we can descend to (1) Sanogawa community again, (2) straight down to Mt. Jimba Hiking Course Stop via Ichino’o Ridge, or (3) down north to Jimba Kogen-shita 陣馬高原下 Bus Stop in Tokyo. All takes about 2 hours to the bus stop. Whichever direction you take, the bus service is quite limited to JR Fujino Station (; time table from Wada Stop is here) and JR Takao Station 高尾 (; time table from Jimba Kogen-shita is here) so that we should measure the length of picnic at the top of Mt. Jimba. To Sanogawa community, we can either take the same route at the first crossing we took for climbing, or turn right at the second crossing with Ichino’o Ridge. In this route, we reach to Sanogawa community via front yards of local houses. Before arriving to the paved way from Wada Stop, we once again see the forests of the village on Kobotoke Group. The forests near the houses are of deciduous broad-leaved trees and higher elevation is sometimes covered by afforested conifers. In spring, the contrast of continuing hues of green on the slopes, with dotting white-pink colors of cherry blossoms, is like an Impressionist painting … I just wondered the scenery might be thanks to dry Kobotoke Group. If the soil could keep water more, this area should have had larger population with more rice paddies, and more traces of human activities, aka development. It did not happen. The soil beneath our shoes today continues to be gravelly and powdery. Hmmmmmm ...

The second crossing on Ichino’o Ridge.
 The road on the left of this photo is Ichino’o Ridge.
 To Sanogawa Community we take the narrower road of the right.

The road is of fine clay …

Sanogawa Community in spring …

Do you know where people say the best place for cherry blossoms is in Tokyo? It’s along the ridge way from Mt. Takao to Mt. Jimba. They say in every late April, old trees of Cerasus jamasakura decorate the ridge of deep mountains quietly but gorgeously. On a map from Mt. Takao to Mt. Jimba, it’s about 5.5 hours … Let me see if I can try this itinerary next spring. The route is entirely on Kobotoke Group. The hiking route must be dry, gravelly and powdery.

If you find an environmental issues in Kanagawa Prefecture, please make a contact with Kanagawa Natural Environment Conservation Center 神奈川県自然環境保全センター

657 Nanasawa, Atsugi City, 243-0121 〒243－0121　厚木市七沢657

Phone: 046-248-0323

You can send an enquiry to them by clicking the bottom line of their homepage at http://www.pref.kanagawa.jp/div/1644/