Tachikawa Walk: Slopes, River Terraces and a Flat City’s Hidden Tilt

Flat-looking Tachikawa tilts about 55 metres. Why its slopes run only north–south, what the Ice Age left, and the mystery of the “Tachikawa Fault”.

日本語で読む（Japanese version）

Everyone says Tachikawa is a flat town. I thought so too. Walk from the station’s North Exit to Showa Kinen Park and you will not meet anything you could call a hill. That must be why it is known as a town for bicycles.

But when I looked up neighbourhood after neighbourhood in the elevation data published by the Geospatial Information Authority of Japan (GSI), the numbers were not flat at all. At the city’s northwestern edge, Nishisuna-cho rises above 120 metres. At the southern edges of Nishiki-cho and Shibasaki-cho, near the Tama River, it drops below 70. The town of Tachikawa (立川) tilts by more than 50 metres from northwest to southeast.

Then I came across a curious sentence written by the people who compile the city’s history. In Tachikawa, they wrote, when you move east or west you hardly notice any slopes, but when you move north or south, at a certain point a slope suddenly appears. The slopes, in other words, have a direction.

Why does a town called flat lean? And why are there slopes only from north to south? I decided to walk the ground beneath Tachikawa’s feet to find out.

Is Tachikawa really flat? A 55-metre tilt from Nishisuna-cho to Nishiki-cho

Let me start with numbers.

Tachikawa Monogatari (“Tachikawa Stories”), the newsletter of the city’s history-compilation team, says the highest point in the city is Nishisuna-cho in the northwest, at 125 metres, and that the land falls gradually from there to the southeast. The terrain consists of two parts, it adds: the lowland along the Tama River, and the Musashino Plateau one step higher.

To check, I looked up several neighbourhoods in GSI’s elevation data — a fine grid measured by airborne laser, one metre square. Nishisuna-cho 7-chome: 119.6 metres. Izumi-cho, where the city hall stands: 94 metres. Shibasaki-cho 3-chome, just outside the South Exit of Tachikawa Station: 83.4 metres. Akebono-cho 2-chome on the North Exit side: 83.3 metres, so the north and south sides of the station sit at almost exactly the same height. Head south from there, and Shibasaki-cho 6-chome is 70.8 metres, Nishiki-cho 6-chome 73.7.

From 125 metres at Nishisuna-cho to 70 metres by the Tama River: a difference of about 55 metres. That is about the height of an 18-storey building, which the people of Tachikawa climb and descend every day without noticing.

Let me work out the gradient as well. From Nishisuna-cho to Nishiki-cho is a little over 7 kilometres in a straight line on the map. On average, that is a fall of 7 to 8 metres for every kilometre. How gentle is that? According to the city’s cultural-heritage guide, Edo’s Tamagawa Aqueduct (the 17th-century channel that carried water to Edo, old Tokyo) drops about 92 metres over the roughly 43 kilometres from Hamura to the Yotsuya Okido gate — a little over 2 metres per kilometre. So Tachikawa’s land tilts more than three times as steeply as Edo’s water supply. Would an Edo engineer be startled to hear it, or envious? I wonder.

Even so, few people feel this tilt in their bodies. Seven metres over a kilometre’s walk is not a slope you can see. But if the pedals feel a little heavier on the road north than on the road south, it may not be only your legs.

This flatness also decided Tachikawa’s fate. The city’s walking-route guide lists, among the reasons the army airfield was placed here at the end of the Taisho era (the 1920s) — alongside nearness to the capital and the railway — “a vast, flat piece of land”. Tilted by 55 metres it may be, but it was flat enough for a runway. The town later called “the capital of the sky” began with the character of this ground.

So where are the slopes? Look at the map again and the answer is gathered, surprisingly, in one place.

The Tachikawa and Aoyagi terraces: a giant staircase left by the Tama River

Almost all of Tachikawa’s slopes are on the southern edge of the city, where the land drops in steps towards the Tama River.

The history newsletter explains an escarpment, a gaisen (崖線), as “a continuous cliff separating lowland from plateau, or a higher terrace from a lower one”. A terrace is the trace left as a river, over a very long time, cut down into the land and moved its course one step lower each time. The flat surfaces left uncut line up like the treads of a staircase.

Tachikawa’s stairs have names. A land-classification survey published by Saitama Prefecture says the Tachikawa plateau, spreading out like a fan with Ome at its apex, is made up of river terraces: the Tachikawa surface, the Aoyagi surface, the Haijima surface and others. The widest tread is the Tachikawa surface; Tachikawa Station, the city hall and Showa Kinen Park all sit on it. The tread one step lower, to the south, is the Aoyagi surface. The cliff dividing the two is the Tachikawa escarpment.

As I wrote in the Yagawa and hake article, the Tachikawa escarpment is a long cliff running from Ome towards Chofu, about 15 metres high around Tachikawa. The same survey says the Aoyagi and Haijima terraces were formed about 20,000 years ago, in the coldest part of the later Ice Age — a time when the sea was far lower than today and rivers cut deep into the land. Tachikawa’s slopes are a parting gift from the Ice Age.

What are the treads themselves made of? A national earthquake-survey report, drawing on boring records from central Tachikawa, says the top 2 metres or so are volcanic ash carried and laid down by the wind, and beneath that the Tachikawa gravel bed, about 10 metres thick, continues at an almost even thickness. A gravel bed is a layer of round stones and sand carried by a river. In other words, the town of Tachikawa is built on a 10-metre board of gravel, with a 2-metre rug of red earth spread on top. Trace them back, and the office blocks by the station are standing on the stones of an old Tama River bed.

The Aoyagi surface has a different face from the Tachikawa surface. According to issue 14 of Tachikawa Monogatari, the Owada site — a large village of the middle Jomon period (Japan’s prehistoric hunter-gatherer era) — lay on the Aoyagi surface, one step below the Tachikawa surface. Springs rise at the foot of the cliff. People first chose the lower tread, close to water.

The height of the step varies a great deal from place to place. A local resident’s blog about walking around Hagoromo-cho says the step between the Tachikawa and Aoyagi surfaces there is only about 2 metres. GSI’s figures agree: Hagoromo-cho 1-chome is 81.4 metres and 3-chome 77.1 metres — a gentle difference. In Shibasaki-cho and Fujimi-cho, by contrast, the cliff is clearly high. The same single escarpment is a wall in the west and a mere step in the east.

Incidentally, another famous cliff begins in Sunagawa to the north: the Kokubunji escarpment, known as the green cliff of Seijo and Futako-Tamagawa in Setagaya. Documents from Musashino and Chofu cities say this escarpment begins in Tachikawa. Tachikawa is a town that holds the beginnings of two cliffs: the Tachikawa escarpment in the south, and the Kokubunji escarpment stretching away to the northeast. I wrote about this in detail in the Kokubunji article, so I will leave it at one line here.

No slopes east to west, only north to south: the names and origins of Tachikawa’s slopes

Here I return to that curious sentence from the start.

At the beginning of an article on Jomon pottery, the history newsletter asks its readers: in daily life, when you move along roads east or west you hardly feel any slopes — but moving north or south, do you notice that at a certain point there is a fairly large difference in height, a slope? The reason is that the escarpments of the river terraces made by the Tama River run almost exactly east to west.

The slopes have a direction because the cliffs have a direction. Walk parallel to an escarpment and you stay on the same tread the whole way. Only at the moment you cross it do you go up or down a step. So Tachikawa’s slopes almost all run north to south, and they appear together around the Shin-Okutama Kaido road and the Nekawa stream.

The exceptions are properly noted, too. In a footnote, the same newsletter adds that this does not apply to people living in Wakaba-cho and Saiwai-cho, because the Kokubunji escarpment runs from northwest to southeast to the east of them. A city historian naming neighbourhoods to put his readers right. An honest footnote.

Quite a few of those north–south slopes have names. (In Japanese, a slope is saka, and in names it often becomes -zaka.)

The best known is probably Sanchu-zaka in Fujimi-cho, a slope along the cliff where, in the Pacific War air raids, many people died in an air-raid shelter. I told that story in the air-raid article, so here I will only give the name.

Near Sanchu-zaka is Baba-zaka. According to an article in a Tachikawa community paper, once you cross the Babazaka-shita Bridge over the Zanbori River, the road splits in two: right is Baba-zaka, left is Yokocho-zaka. Another article in the same paper says Tachikawa is mostly flat and convenient for cycling, but heading north from the Shin-Okutama Kaido you have to climb, and although there are several slopes — Sanchu-zaka, Baba-zaka and so on — choose the wrong one and you will end up pushing your bike before you reach the top. If your bicycle has no gears, it recommends the road under the monorail; the further east you go, the gentler the slopes. Advice from a local who has learned it the hard way.

The slopes’ names survive in the names of bridges too. Tachikawa City’s plan for extending the life of its bridges lists, over the Zanbori River, the “Sanchuzaka-shita Bridge” (1969) and the “Babazaka-shita Bridge” (1970). Saka-shita means “below the slope”: a bridge at the foot of a slope is called “below-the-slope bridge”. I rather envy the decisiveness of whoever named them.

At the eastern end is Kaigara-zaka, “Seashell Slope”. The Nekawa Kaigarazaka Bridge, at the end of the Nekawa Green Path, was named after the nearby Kaigara-zaka, according to the city’s walking-route guide. I could not find a reliable source on where the slope’s own name came from.

Names of slopes hide in an even older layer of place names. Issue 2 of Tachikawa Monogatari, drawing on Place Names of Tachikawa, compiled by the board of education in 1988, maps the place names of Edo-period Shibasaki village. Among them are Sakaue (top of the slope), Sakashita (foot of the slope), Ido-zaka (well slope), Mama-shita, Hake, Taki-no-ue (above the falls), Taki-no-shita (below the falls), Takitsubo (plunge pool) and Dai-no-shita (below the plateau). Mama and hake are both old words for a cliff. The many names with taki, “waterfall”, are probably a memory of water once falling from the cliff. The people of Tachikawa were naming cliffs and water and living with them long before the slopes got their names.

Tachikawa’s slopes by the numbers

• 125 — metres: the elevation of Nishisuna-cho, the highest point in the city. About one-thirtieth of Mount Fuji. No sense of a summit whatsoever.
• 55 — metres: the rough difference in height between Nishisuna-cho and the low ground by the Tama River. Eighteen storeys, climbed and descended a little every day.
• 7–8 — metres: the average fall of Tachikawa’s land per kilometre. More than three times the Tamagawa Aqueduct. A town steeper than the aqueduct.
• 10 — metres, roughly: the thickness of the Tachikawa gravel bed beneath the city centre. The buildings by the station stand on the stones of an old riverbed.
• 20,000 — years ago: when the Aoyagi terrace was formed. The history of the slopes is a good deal longer than the history of the town.
• 4.5 — metres, roughly: the height of a cliff, lower on its west side, in the city centre. Who made it has not yet been settled.

The wanderer’s quiz

According to the City of Tachikawa’s documents, in which direction can you walk through the city and barely feel a slope?

1. East–west; (B) north–south; (C) it is the same in every direction.

The answer comes near the end of this article.

The story of that last number, the 4.5-metre cliff, comes later. First, I want to stop at an even stranger place, where a river seems to climb a hill.