Shutogashima Senjojiki | Naru Island’s Ancient Rock Platform by the Sea

At the southern end of Naru Island, beyond a beach covered with smooth, rounded pebbles, a vast platform of exposed rock stretches toward a small green islet.

This is Shutogashima Senjojiki, one of Naru Island’s most distinctive scenic and geological sites.

The name “Senjojiki” literally suggests a space large enough to hold one thousand tatami mats.

Reddish-brown rock, unusual formations shaped by erosion, the vivid green of the offshore islet, and water that shifts from emerald to deep blue combine to create one of the island’s most memorable landscapes.

Yet the appeal of Senjojiki goes far beyond its appearance.

The rocks beneath your feet preserve traces of a time when the Goto Islands were not islands at all. Depressions interpreted as the footprints of rhinoceroses or other large mammals, along with fossils of freshwater shellfish, offer clues to an ancient environment of rivers, lakes, and wetlands.

Senjojiki is both a coastal viewpoint and an open-air museum of the earth.

A Vast Stone Corridor Extending into the Sea

Senjojiki begins at the southern end of Shutogashima Beach and extends across a broad, relatively flat rocky area toward a small offshore islet.

From the beach, it resembles a natural stone corridor laid across the sea.

Cracks run in several directions across the exposed surface, while layers of sandstone and mudstone appear along the edges. In some places, erosion has carved the rock into shapes that resemble steps, shelves, or enormous slices of layered cake.

On a clear day, the warm brown and reddish tones of the rock contrast dramatically with the blue sea and the dense green vegetation covering the islet.

Small pools of seawater collect in depressions across the platform. When the surface is calm, these tide pools reflect the sky and reveal tiny marine creatures beneath the water.

The scenery changes with every few steps. Looking back toward the beach, out toward the open sea, or down into the layered rock each offers a completely different view.

The Small Islet That Protected the Rock Platform

The broad, flat surface of Senjojiki was shaped through a long interaction between different types of rock and the power of the sea.

Much of Naru Island is built from the Goto Group, layers of sandstone and mudstone formed from ancient sand and silt.

At Senjojiki, waves and weather gradually stripped away softer material and exposed the broad bedding surface visible today.

The small islet at the end of the platform is made from harder rock created when magma cooled and solidified.

This hard mass of rock acted like a natural shield against powerful waves. The strata behind it were partly protected from erosion, allowing the wide, flat platform to survive while the surrounding coastline continued to wear away.

What appears to be a quiet arrangement of rock and sea is therefore the result of millions of years of geological change.

Why Are There Rhino Footprints on Naru Island?

While walking across Senjojiki, you may notice several rounded depressions in the rock, some measuring around 30 centimeters across.

They are believed to be fossilized footprints left by rhinoceroses or related large mammals.

The idea of rhinoceroses walking across a small Japanese island may sound surprising. The explanation is that Naru Island did not yet exist in its present form when the tracks were made.

The Goto Group formed approximately 22 to 17 million years ago, when the land that would become the Goto Islands was still connected to the Eurasian continent.

At the time, rivers and lakes crossed the landscape. Sand and mud accumulated along their banks and across wet, low-lying areas.

Large animals walking over the soft ground left impressions behind. Those impressions were buried beneath later sediment, hardened into rock, and preserved within the strata.

Much later, tectonic movement separated the land from the continent, and erosion exposed the ancient surface once again.

Fossils of freshwater shellfish have also been discovered around Senjojiki.

Finding evidence of freshwater life and large land animals beside the modern sea reveals just how dramatically this landscape has changed.

The footprints and fossils can easily blend into the natural patterns of the rock. Learning what to look for before your visit—or joining a local geoguide—makes exploring the site feel like a geological treasure hunt.

The Landscape Grows as the Tide Falls

The visible size and character of Senjojiki change with the tide.

As the water recedes, more of the rock platform emerges from beneath the sea. The broad surface becomes easier to appreciate, and the connection between the beach and the offshore islet appears more clearly.

At higher tide, lower sections of the platform disappear beneath the water. Waves move through the cracks and channels, making the rock appear to float between sections of blue sea.

Both conditions are beautiful, but visiting around low tide generally makes it easier to observe the exposed strata and experience the scale suggested by the name “Senjojiki.”

The sea can return quickly, however.

Before walking far across the platform, check both the current tide and the expected conditions during your return. Wind and offshore waves can also send water across apparently dry sections of rock.

Never continue toward the islet when waves are breaking over the route, and avoid the platform entirely during rough weather.

A Beach of Pebbles Polished by the Waves

Shutogashima Beach, located beside Senjojiki, is also worth exploring.

Instead of fine sand, the shoreline is covered with rounded pebbles in shades of white, gray, brown, and black. Waves have polished their edges over time.

The pebble beach extends for approximately 500 meters.

As each wave retreats, thousands of small stones roll against one another, producing a gentle rattling sound that is unique to this type of shore.

During the summer swimming season, changing rooms, toilets, and free cold-water showers are normally available. The facilities are generally open from around mid-July until late August, although exact dates may vary from year to year.

Outside the swimming season, the beach becomes especially quiet.

Even without entering the water, walking along the pebbles, listening to the waves, and looking toward Senjojiki offers a peaceful introduction to Naru Island’s coastline.

A Key Geosite in the Goto Islands Geopark

Shutogashima Senjojiki is one of the representative geosites of the Goto Islands Geopark.

A geopark is not simply a collection of unusual rocks.

It is a place where visitors can understand how geological processes created the land and how the landscape has influenced ecosystems, history, and human life.

At Senjojiki, the story can be read almost entirely from the scenery.

Sand and mud accumulated beside continental rivers and lakes.

Those sediments hardened into sandstone and mudstone.

Tectonic forces separated the land from the continent and divided it into islands.

Waves and weather exposed the strata, while the harder offshore islet helped preserve the broad rock platform.

The surrounding Shutogashima–Nagizaki Coast is also designated as a Nagasaki Prefectural Nature Conservation Area. Its protected landscape includes a submerged coastline, sea cliffs reaching approximately 50 meters in height, and large-scale sheet-like joints.

Do not remove fossils, stones, plants, or marine life.

Even an ordinary-looking fragment may form part of the geological record that makes this place valuable.

More Than a Beautiful Seaside View

Senjojiki offers a different experience from a viewpoint where visitors simply look at a landscape from a distance.

Here, you can step into the scenery.

You can walk across the exposed strata, listen to waves moving through cracks in the rock, observe tide pools, and feel the sea breeze coming from the open water.

Look down, and you may be standing near evidence of animals that crossed an ancient continental wetland millions of years ago.

Look ahead, and the small green islet reveals why the platform beneath you survived the force of the sea.

At first glance, the landscape appears to contain little more than rock, water, and vegetation.

The more you learn, however, the more stories it begins to tell.

That combination of beauty, mystery, and geological time makes Shutogashima Senjojiki one of the most rewarding places to visit on Naru Island.

Before You Visit

Shutogashima Senjojiki is an outdoor natural site with no admission fee or fixed visiting hours.

Visit during daylight and allow enough time to return before sunset.

The rock platform contains wide cracks, uneven surfaces, loose stones, and sections that become extremely slippery when wet. Wear sturdy walking shoes with non-slip soles rather than sandals or heels.

Check the weather, waves, and tide before entering the platform.

Do not walk onto the rocks during strong winds, high waves, heavy rain, thunderstorms, or typhoon conditions. Even during low tide, isolated waves may wash across lower sections.

Children should remain closely supervised, particularly near the edge of the platform and around tide pools.

Parking is available beside Shutogashima Beach.

Toilets, changing rooms, and cold-water showers are generally available only during the summer swimming season, usually from around mid-July to late August.

Prepare drinking water and use facilities around Naru Port before visiting outside that period.

Ferries to Naru Island may be delayed or canceled because of weather and sea conditions. Check the latest operating information before departure and reserve island transportation in advance.