When we walk water source forests in Tanzawa 丹沢, it looks like Kanagawa Prefecture cannot have water shortage. Mountain torrents are here and there. Lots of rain washes the soil down when the afforested area has not had enough care … water is abundant. Hmmmmm … recently I noticed lots of rain does not necessarily guarantee water supply. One example we can find in our prefecture is communities in Oiso Hill 大磯丘陵. Let’s do a little hike there this week and next, to find historical ponds our ancestors dug to tackle water problem there.
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| It
happened to be down-pouring in the morning when we visited Oiso Hill the other day … Believe me or not, it’s a gigantic cinnamon tree grew in the garden of local resident. |
Oiso Town 大磯町 is more familiar with its seaside features. Oiso Beach 大磯海岸 is the first modern swimming beach in Japan, opened in 1883. In the 19th and early 20th centuries, the beachside was populated by villas of rich and powerful for Japanese national politics. Of this connection, the place was a playground for Yukio Mishima 三島由紀夫, a grandson of the third colonial minister for Sakhalin. These days, Haruki Murakami 村上春樹 lives near Oiso Beach. All sound pretty much gorgeous, don’t they? When we look north of the JR Oiso Station 大磯駅, the hill is almost over us. Within 10 minutes’ walk from the station, we are on the seashore. A flat zone between Oiso Hill and the beach is quite limited. That’s the reason why the celebrities of the last centuries loved the scenery of the town, for sure. But geologically speaking, the place has a challenging feature especially for water. Nowadays, the town is receiving water from Sagami River so that the problem is solved. Before having modern water supply system, for ordinary folks who tilled the soil for living in Oiso, securing water was a critical issue. The area has lots of rain. So, the problem for commoners in Oiso was how to “safeguard” it.
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| Oiso
Beach in winter. Once it was a surfing place for Haruki Murakami. |
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| A sunrise in Oiso Fishing Harbor |
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| JR Tokaido Line 東海道本線 runs along almost the edge of Oiso Hill, where the south of the railroad is farmlands. |
When we observe the southwest direction from Bodai Pass 菩提峠, about an hour walk from Yabitsu Pass ヤビツ峠, we notice there is a kind of land mass between a flat alluvial fan immediately below us, i.e. Hadano City 秦野市, and another urban expansion beyond, i.e. Odawara City 小田原市. The hilly area between 2 cities is Oiso Hill. The hill itself is divided by an active fault line, Ikusawa Active Fault Line 生沢活断層, where currently Tokaido Shinkansen Bullet Train 東海道新幹線 is running. We can infer the birth of Oiso Hill from this scenery. Kanagawa Prefecture situates itself on the crush line between Eurasian and Philippine plates. About 5 million years ago, the tectonic squeeze pushes up the land mass on Eurasian Plates, and raised the sediments on the seafloor along the friction line of two plates. It created the base of Tanzawa Mountains and Oiso Hill 丹沢山地の衝突現象. This tectonic movement continues even now that is the Ikusawa Fault Line. The sediments from the sea continued to be raised till around 120 thousand years ago, which is now directly observable in the southeast side of the fault line called Shonan Daira 湘南平 of Oiso Hill where Komayama Park 高麗山公園 locates. Around 500 thousand years ago, Mt. Hakone and Mt. Fuji erupted and started to spew out tephra and lapilli. They piled up over the raised ground by plate tectonics, especially on the west side of the Ikusawa Fault Line. The push-up and volcanic tephra eventually created the peak of Oiso Hill, aka Mt. Takatori 鷹取山 at ASL 222m. Thus, the current soil of Oiso Hill is made of the raised seabed and volcanic projectiles whose date is relatively recent. That’s the reason why humans came here about 10 thousand years ago found the hold of local water tricky.
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| Observing
Oiso Hill from near Bodai Pass. The flat land below us is Hadano City, then there is Oiso Hill separating Hadano from Odawara over there. Could you see the Oiso Hill is divided by a small fissure on the left? It’s the Ikusawa Active Fault Line. The left of the fault line is Shonan Daira which is very popular for family picnic. The higher mountains at the background of this photo is Hakone 箱根. |
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| Observing
Shonan Daira from the estuary of Hanamizu River 花水川. Hanamizu River originates from Mt. To 塔ノ岳 and Mt. Oyama 大山 of Tanzawa, and yet Oiso Town, the west of Hanamizu River, had water problem thanks to Oiso Hill. |
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| The
point where Hanamizu River meets with Sagami Bay 相模湾 is a peaceful place … |
When farmers dug the ground for wells in Oiso Hill, they could easily be blocked by large lapilli. Although the highest altitude of the Hill is not much, the way its created is exactly the same as steep Tanzawa mountains, i.e. plate tectonics and volcanos. The slope of smallish Oiso Hill is sharp, and it’s very near to the sea. Unless the ground is extremely absorptive, rain drops reached to the ground can escape rapidly to the sea. Conglomerate of Oiso Hill is exactly the opposite of spongy soil. Frequent rain along the warm Oiso coast line can nurture a good forest for celebrities to admire, but it does not necessarily guarantee stable water supply.
To see how it actually is in Oiso Hill, we visit the peak of Mt. Takatori 鷹取山. There are several trekking routes one of which is introduced by the Oiso Town in the link above. Though, this time we start from Higashino-ike Pond 東の池, and descend to Hiratsuka City 平塚市 where we can actually observe a gigantic lapilli that prevents rain from becoming an underground reservoir. To go to Higashino-ike directly, we take a commuter bus service of Kancyu Bus Hira-31 or -32 (平‐31, 平‐32, time table here) from #2 bus stop in the north exit of JR Hiratsuka Station 平塚駅. After about 30 minutes ride, we get off the bus at Ikusawa Bus Stop 生沢, next to Higashino-ike Pond. To the south, we are greeted by frequent visits of Shinkansen Bullet Train, and Hiratsuka TV Relay Station Tower of Shonan Daira. Now, let’s continue next week our adventure to the forest that has difficulty to be a water source any way.😉
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| #2 bus
stop in the northern exit of Hiratsuka Station |
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| Ikusawa Bus Stop |
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| A Bullet Train and the TV tower to the south of the bus stop |
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| It was raining anyway. |
The Oiso Hill covers Oiso Town, Hiratsuka City, and Ninomiya Town 二宮町. The Hill in Ninomiya Town is occupied mainly by private golf courses so that if you find problems during your hike, you’d better make a contact with either Oiso Town or Hiratsuka City. Their contact addresses are
Oiso
Town Hall 大磯町役場
183
Higashi-koiso, Oiso-cho, Naka-gun, Kanagawa, 255-8555
〒255-8555 神奈川県中郡大磯町東小磯183
Phone ： 0463-61-4100 FAX ： 0463-61-1991
https://www.town.oiso.kanagawa.jp/ikkrwebBrowse/inquiryPC/Sec.do;jsessionid=AC09D1499780CCBD42365B009AB24F7A?mode=pc&inquiryId=53&ref=www.town.oiso.kanagawa.jp/isotabi/
Hiratsuka
City Hall 平塚市役所
9-1
Sengen-cho Hiratsuka City, Kanagawa, 254-8686
〒254-8686
神奈川県平塚市浅間町9番1号
Phone：0463-23-1111 Fax：0463-23-9467
https://hiratsuka.custhelp.com/app/ask/p/64/
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