[Southern Southern Alps] Direct Ascent of the Marubon-dake East Ridge | 32km Core Loop with 'Dense Bamboo Grass + Residual Snow' over 3 Days and 2 Nights from Sumatakyo

A ridge in the deep southern part of the Southern Alps that requires judgment due to a lack of records.
I made a direct ascent of the Marubon-dake East Ridge from Sumatakyo.

This ridge is supposed to have a traverse path to Tenchi.
However, as of 2019, no reliable passage records had been found for over 10 years, making it effectively a 'direct ascent route'.

Bamboo grass taller than a person.
Dense bushwhacking from 1700m onwards.
Post-holing in residual snow from around 1900m.

In this article, I will record:

・The situation of the Marubon-dake East Ridge at that time
・The transition of the traverse path
・The realistic physical burden of the dense bamboo grass + residual snow period
・Points to note when exiting toward Kuroboshi-dake

I will record these.

This content is for those who want to walk through the 'series of judgments' typical of the deep southern region.

Mountain Trip Overview

Schedule: April 25–27, 2019 (3 days, 2 nights)
Mountain Area: Deep Southern Alps
Starting Point: Sumatakyo Onsen
Distance: 32.4km
Cumulative Elevation: ± approx. 3,437m
Activity Time: 21 hours 03 minutes
Course Difficulty Rating: 84 (Hard)
Physical Difficulty Level: ★★★★★ (Tent equipment + steep ascent + bushwhacking endurance required)

Main Peaks

・Marubon-dake
・Kuroboshi-dake
・Mae-kuroboshi-dake
・Shimonishikouchi-yama
・Futatsu-yama

Overall Course Map

Starting from Sumatakyo. Direct ascent of the Marubon-dake East Ridge from the Senzu Dam, then a loop through Marubon-dake, Kuroboshi-dake, and Mae-kuroboshi-dake.

The core section is the direct ascent of the Marubon-dake East Ridge.

Reason for choosing this mountain

As of April 2019, there were almost no records of the traverse path to Tenchi.
Even when I searched, I could only find records of direct ascents.

'If you want to play it safe, go for the direct ascent.'

That was my judgment.

Later, rope maintenance was carried out on the traverse path by volunteers, and passage records increased temporarily.
I myself was able to reach Tenchi comfortably when I walked the Fudo-dake East Ridge in 2021.

However, as of 2026, records have become sparse again.
There are also reports of some ropes being cut and being forced into precarious traverses.

The Deep South is a place where "information never stays fixed."
I feel that this is also a defining characteristic of this mountain region.

Course Overview and Impressions

Day 1 | Sumatakyo to Camping on the East Ridge

■ Sumatakyo to Senzu Dam

About 10km of forest road. The tunnels are pitch black, so a headlamp is essential.

It is important not to exhaust too much energy here.

■ Marubon-dake East Ridge Trailhead

The Ammoku Suspension Bridge is aging, making it a bit nerve-wracking.
However, the real crux lies beyond that.

Judging the traverse to be prone to collapse, I made a direct ascent from the right.

The steep slope about 200m directly above the trailhead felt like the most dangerous section of this trip.
There is a risk of slipping due to loose rocks.

The moment I reached the ridge, I finally felt relieved.
I camped near the Ammoku Col.

Day 2 | Dense Bushwhacking + Residual Snow + Marubon-dake

■ 1700m to 1900m Bamboo Grass Hell

Bamboo grass taller than a person.
The trail is almost completely buried.

I advance as if "swimming" while grabbing the bamboo grass.
It is very exhausting with heavy gear.

■ Residual snow above 1900m

Now, I'm post-holing.
The areas around tree roots are hollow, so checking with trekking poles is essential.

A mentally draining section.

■ Arrival at Marubon-dake

Finally, the
Marubon-dake summit.

The summit sign faces the East Ridge side.
I couldn't help but say out loud, "I made it."

■ Marubon-dake to Kurohoshi-dake

After that, I passed through Kamoshika-daira and headed toward Kurohoshi-dake.

The area directly below Kurohoshi-dake is loose scree.
Be careful with route finding when it's foggy.

Be careful with your direction on the descent from Kurohoshi-dake.
There are more red tapes, but don't let your guard down.

Second night near the heliport.

Day 3 | From Maekurohoshi-dake to the maintained trail

Light snowfall during the night.
The sea of clouds and sunrise from the heliport were impressive.

The path is maintained after Maekuroboshi-dake.
It becomes mentally much easier all at once.

Returned to Sumatakyo via the forest road.

Summary of the current state of the Marubon-dake East Ridge

*As of April 2019 + reference to records from around 2026

Traverse path

・Prone to collapse
・Reports of partially severed ropes
・Few passage records

→ Gathering information in advance is essential

Direct ascent section

・Steep slopes
・Loose rocks
・Risk of slipping
・Difficulty increases with tent gear

Not recommended for beginners.

Above 1700m

・Bamboo grass taller than a person
・Unclear trail
・High physical exhaustion

Above 1900m (during residual snow season)

・Post-holing
・Hollows underneath
・Poles recommended

Suitable for

・Those with tolerance for bushwhacking
・Those who can navigate using topographic maps
・Those with experience in steep climbs with tent gear
・Those who can make the decision to turn back

Reflections

I had considered descending on the second day, but the result was a 3-day 2-night trip as planned.

In the deep southern region, a "generous plan" is the correct approach.
I felt that once again.

The Marubon-dake East Ridge is a ridge that tests your
"resolve" and "judgment" more than your physical strength.

Even so, I want to walk it again.

A quiet ridgeline.
Three days without meeting anyone.

While I mainly hike in the mountains of Shizuoka, I occasionally check my current location in the Deep South.
I am grateful that I was able to return safely this time as well.

Reference Links

▶︎ Click here for a 3-day, 2-night grand loop walking through the Deep South | Sumatakyo to Mt. Kuroboshi Loop

▶︎ Mt. Fudo East Ridge | Round trip from Sumatakyo | Core route record (41km / ±5,130m)

▶︎ Click here for a long trek in the Deep South during mid-winter | Baratani-no-kashira 30km round trip