Crash of Titans in our backyard: Looking stones in Togawa Park 秦野戸川公園

 

Let’s start our Tanzawa stone hunting from the backyard of suburban Kanagawa. This summer, we did a geological hunting session in the bank of Mizunashi River 水無川 for Tokawa Park 戸川公園. The place is popular with families in Hadano City 秦野市. When weather is right, the river does not have much flow, or even no water. Its reason is exactly from Plate Tectonics. Hadano City sits on Hadano Basin that is created by earth and sand accumulated from collapsing strata by crustal shortening. Coral seabeds and islands on Philippines Plate crinkled when the plate crushed with the Eurasian Plate. Inevitably lots of earth broke down along the active faults. Such sediment from the crash of Titans came down along the rivers of old Tanzawa and piled up in the area now called Hadano Basin. Then, volcanic ashes from Mt. Fuji and Hakone Mountains heaped on the crumbled soil. The result is a Basin on lots of active faults and more than 150m thick strata made of gravel, sand and volcanic ashes. The structure lets the water from Tanzawa mountains become groundwater, not river. The Hadano City has lots of famous springs here and there, but the visible waterflow on the ground is limited. Mizunashi, = “No Water” in Japanese, River does not have much regular flow, unless the underground water system reaches its limit due to torrential rain.

Tokawa Park in summer has
lots of families who enjoy breeze from the “water.”

Not much water for Mizunashi River

The Park itself is on the line of contact between Hadano Basin and “Oyama Subgroup 大山亜層群” for Tanzawa Group of geological feature 丹沢層群. We can regard the place as the point where the wrinkles of Philippine Plate start at the aboveground level. This birth story of Hadano tells Tokawa Park is a good place for collecting many kinds of rocks tumbling down from Tanzawa Mountains especially from the surface of shortened crust. In addition, the water systems of steep Tanzawa slopes carry rocks not only from Oyama Subgroup but also from a bit far Togatake and Susugaya Subgroups.

Mt. Oyama over there from Tokawa Park.
Rolling stones come this far from there, baby.

The sample map I made from the pebbles collected
from Tokawa Park and Yadoriki Stream.
The black arrow shows the point we did stone hunting.
The blue area is Oyama Subgroup,
and light green area is Togatake Subgroup.
The point itself is on the white area for Hadano Basin.

The birth order of these geolotical groups is, the oldest Togatake 塔ヶ岳亜層群, the middle Oyama, then the youngest Susugaya 煤ヶ谷亜層群 Subgroups. Togatake and Oyama Subgroup areas have rocks evolved from volcanic magma, mainly Andesites, that cooled rapidly near the surface. They were originally from very active underwater volcanoes so that the chemical reaction with the sea water lets them be colored green. In large rocks procured from the area we can often observe chunks of volcanic bomb trapped and fixed in the volcanic ashes by tectonic pressure. Some good and polished example is situated artfully in the slope of Tokawa Park when we come down from the visitor center to the river. Some are good for using as benches. 😉 Actually, Togatake Subgroup has a strong connection with Yadoriki area 寄. I tell you about it when we look the pebbles in Yadoriki Stream.

Mr. Kiyoshi Sudoh
of Kanagawa Earth Science Association
was our instructor for stone hunting.
This rock has volcanic bombs inside, like

Bomb trapped, or so called tuff breccia, of Tanzawa.
By the way, the rock itself looks perfect
 to have lunch on it, don’t you think?

The dry riverbed of Mizunashi River in Tokawa Park is wide. We walk gravelly fields for some time to reach to the water. Though, for stone hunters of Tanzawa’s tectonic journey it’s a fairly convenient space. We stroll there, even without rainboots, and pick up interesting pebbles. Each doubtlessly has marks of Crash of Titans. The stones here were formulated some 15 to 17 million years ago 240km offshore from the point as undersea volcanic tuff (and occasionally basalt). i.e. The rocks here were the memory of active volcanoes older than Mt. Fuji (; the last eruption was in 1707) or Hakone Mountains (; it’s still erupting). They were carried by the Philippines Plate and arrived here about 5 million years ago. Larger and thicker Eurasian Plate with heavy basalt and peridotite did not yield easily to the pressure from the Philippines Plate. Before came here, the uplift from the bottom of the ocean started to wrinkle at the point of Crush of Titans. Around Tokawa Park area the crustal shortening become ferocious on shore and many fault lines created crumbling rocks. Oh, yes. It’s in present tense. Philippines Plate continues advancing 10cm per year. Do the math. 1 million years produce steady and never- conceding 100km progress ... In the process, many kinds of chemical reactions occurred for tuff and basalt. These straight volcanic products turned into another kind of rocks with different textures and colors. The pebbles we find here underwent such transformations in the relatively shallow part of the Plate, compared with the other areas of Tanzawa. Next week, I’ll show off my catch here. 😊

The field for hunting

Lots of catch here!

At the entrance of Tokawa Park, there is Tanzawa Visitor Centre. The contact address is

Hadano Visitor Center for Tanzawa-Oyama Quasi-national Park
丹沢大山国定公園・自然公園秦野ビジターセンター

1513 Horiyamashita, Hadano, Kanagawa Prefecture, 259-1304
〒259-1304 秦野市堀山下1513

Phone: 0463-87-9300

https://www.kanagawa-park.or.jp/tanzawavc/

Slicing a cake: Kuzuha Greenery 葛葉緑地 2

There are several options for the access to Kuzuha Greenery 葛葉緑地. It has ample car parking. You set your car navigation to Kuzuha Ryokuchi (Kuzuha Greenery), drive your car along Route 246, and reach your destination. Or, if you’re willing, you can walk from Odakyu Hadano Station 小田急秦野駅 to the Forest. It’s about 30-40 minutes’ walk of mainly paved bus roads … I guess it would not be much fun … The third option is riding a commuter bus from Hadano Station. Take any service from #2 bus stop for the Station and leave at Miyagami Stop 宮上. It’s less than 10 minutes ride. Walk back about 100-150m to the direction the bus came from the Station, and cross the traffic light named Miyagami to the opposite side of the bus route. Then, walk in the direction we’ve come from and turn right at the first corner. Go straight. In front of you is the entrance of the Kuzuha Greenery with the panel of an aerial photo I showed you last week.

At the entrance

Before entering the Forest, please check a house on your right. It is a café named Forest Café and a gallery for local artists. I’ve been there during this Golden Week Thursday, but the place was reserved for a private party. We could not try the place. Actually, not only we but also several other groups were turned down the entry due to the event, which means the place is popular. If you have a chance, it would be a nice place for lunch, especially with organic local veggies. (It’s closed for Wednesdays and Thursdays.)

Forest Café

At the entrance of the forest, you find a narrow pedestrian road going down rapidly towards Kuzuha River 葛葉川. This is a characteristic feature of Kuzuha Greenery. Basically, Hadano 秦野 is a basin which is an alluvial fun created by several rivers running down from Tanzawa Mountains. Before, Kuzuha River took a straighter route running down from the slope of Mt. Ninotoh (ASL 1144m) 二ノ塔. About 60 thousand years ago the plate tectonics of crashing Philippine and North American plates pushed up the land mass of present-day Oiso Hill 大磯丘陵 that was/is on the Philippine Plate. The alluvial fan north of Oiso Hill received huge pressures and several faults were created like creases of linen. At least 3 fault lines, Hadano 秦野, Shimojuku 下宿, and Mitsuya 三屋 lines crossing Kuzuha River were activated and created waterfalls. The upper stream of each fall had a gentler slope where water ran slower. It meandered in the alluvial fan and eroded the riverbed until the fall disappeared when the inclination of the riverbed reached the degree of pre-tectonic change. In this way, the valleys have become deeper as it went down near to the point of a gone waterfall. Inevitably, From the Forest entrance to the river, the pedestrian road is rapidly going for approximately 30m down.

Down, down, …

Going down, we meet a cozy garden with a mixture of native and imported flowering trees. When we stroll along the path, we soon meet Kuzuha House that is an admin office cum meeting place for locals. You enter the house and pick up several info leaflets. Oh, some of them are also downloadable from their HP here. The staff of the House is kind and friendly. They also know the latest about the Forest. Er … speaking of the “latest,” Kuzuha Greenery is in the process of recovery from the devastations due to storms in recent years. In addition, oak tree wilt is rampant and killed several large oaks in the Forest. So, as of May 2023, the path to upper stream of Kuzuha River where fireflies dance in late May to early June is closed for safety concerns + rejuvenation of firefly population. By the same reason, the access to lower stream of the River within the Greenery is also closed. Consequently, the place we can enjoy now is about ½ of Kuzuha Greenery. Having said that, the place has lots of places with richness of living creatures + the planet earth.

Inside the Kuzuha House where community can gather.

They also have a small natural museum.

The Kuzuha Greenery is managed to control different environment among the spots. The place we enter via a steep slope intentionally leaves local trees and some lower shrubs some of which are introduced. Relatively speaking, its floor is wetter and tends to receive less sunlight. The ground herbaceous are those that love such environment. On the left of Kuzuha House to the upper stream direction, there is an open space named Camphor Tree Space. It has several picnic benches that are popular for weekend picnics. The space is surrounded by large trees, some of them are imported variety. The number of trees is smaller than the garden we came from. The floor has more sunlight, and so, the place has the herbs for drier places such as sedges. We can enjoy such difference in just 10 or so meters apart. That’s fun. In front of Kuzuha House, there is a suspension bridge called Kuzuha Suspension Bridge くずはのつり橋. Let‘s walk this to cross Kuzuha River. The other side has more “wild” atmosphere that would reflects less human intervention. Going straight from the bridge and we enter residential area surrounding the Greenery. Today let’s turn right from the bridge to go down the steps. We’ll meet another special feature of Kuzuha Greenery.

After descending the slope,
we enter such cozy path to Kuzuha House.

Camphor Tree Space.
Could you see the difference from the above photo?

Kuzuha Suspension Bridge

The other side is like this.
I guess we’ll see a spectacular autumn colors here ...

In early May, the step was adorned by Pourthiaea villosa.

We go down the steps and arrive at the bank of Kuzuha River. Unless the weather is bad, the River is, frankly, small. There are several points whose water depth is probably less than 20cm. Though, the valley is deep. That‘s the result of plate tectonics. Both side of the river has steep slope, or cliff where vegetations cling to. We walk a couple of minutes from the end of the steps, and on our left we find a large geological formation with plates attached. That‘s special for Kuzuha Greenery. This is one of the few places where we can observe natural cross section of Hadano Basin 秦野盆地. The bottom is named Kuzuha-dai Gravel Layer 葛葉台礫層 which was formed about 120-130 thousand years ago. It contains round rocks that came from Tanzawa Mountains, i.e., the evidence of the former riverbed. Above it, there are several whitish layers that have volcanic rocks from the large eruptions piled up in a short period of time. Between these white layers there are thick brown layers gathering small particles of volcanic ashes, yellow sands from China, and the other things. Among them, the layers with the name starting with “K” received the contribution of violent and continuous Hakone eruptions of 130-80 thousand years ago. A layer some 5 m above the river bottom has the name starting with “O” which is the evidence a huge eruption of Mt. Kiso Ontake 木曽御嶽山 98 thousand years ago that threw out pumice far and wide. Then, the very top immediately beneath the vegetation is Iwakura Gravel Layer 岩倉礫層 where before the tectonic push created waterfalls the river ran there. We can know how the earth is doing!. Wow.

We reached the river.

The slice of a cake

Deutzia scabra is clinging on the cliff.

Yearly, Kuzuha House organizes one year training course for nature observation instructors. The lecturers are pros from museums and research centers of the Prefecture. Though all the sessions are in Japanese, if you’re interested in it, please check their HP. The seat is limited, first-come-first-served base. The cost is 3000 yen (for 2023), and the application starts every March. Checking the member of the lecturers, what we can learn from the training would be useful in all over Kanagawa Prefecture. What Kuzuha House provides about the nature of Kuzuha Greenery is based on their study of the place. They concluded the Kuzuha Forest is a very rich place. I could feel it. Please try.

If you need to make a contact with Kuzuha Greenery, below is the address:

Kuzuha-no-Ie くずはの家
1137 Soya, Hadano City, Kanagawa 257-0031 

〒257-0031 神奈川県秦野市曽屋1137番地
Phone/Fax 0463-84-7874
kuzuhaie@city.hadano.kanagawa.jp