Sebayashi Ripple Marks are a valuable geological heritage site located along National Route 299 in Kamihara, Kanna Town, Tano District, Gunma Prefecture. Also known locally as “Sazanami-iwa,” or Ripple Rock, the site is famous for the wave-like patterns and dinosaur footprints preserved on the surface of the rock.
This unusual geological outcrop allows visitors to observe both landforms created by natural forces and traces left by prehistoric animals. It has also been designated a Natural Monument by Gunma Prefecture.
The exposed cliff is approximately 15 meters high and 11 meters wide, with around 50 depressions of various sizes visible on its surface. These are believed to be dinosaur footprints dating back approximately 120 million years to the Cretaceous Period of the Mesozoic Era.
The preservation of traces left by dinosaurs that once walked across this ancient landscape makes Sebayashi Ripple Marks one of Kanna Town’s most distinctive sightseeing and geological attractions.
The rocks containing the Sebayashi Ripple Marks belong to the Sebayashi Formation, which was formed during the Cretaceous Period approximately 120 million years ago.
At that time, this region looked completely different from the mountainous landscape seen today. Scientists believe the area was a coastal or tidal-flat environment near the sea or a river estuary. Sand and mud carried by rivers and the sea accumulated over time, eventually forming layers of sandstone and mudstone.
The delicate wave-like patterns preserved on the surface of the rock are known as ripple marks. These patterns were originally created by water currents, waves, or wind moving across sandy surfaces such as beaches and tidal flats.
The same type of ripples can still be seen on beaches and sand dunes today. What makes the Sebayashi site remarkable is that these ancient patterns were preserved through geological processes and eventually became part of the rock itself.
The Sebayashi Ripple Marks were discovered in 1953, when construction work on National Route 299 exposed the rock face. At first, the site attracted attention as a geological formation preserving ancient wave patterns.
In 1985, however, researchers determined that some of the large depressions on the rock surface were actually dinosaur footprints.
This discovery was highly significant for paleontology in Japan and is recognized as the first confirmed discovery of dinosaur footprint fossils in Japan.
Two general sizes of footprints have been identified, and both are believed to have been left by bipedal dinosaurs.
The largest footprint measures approximately 57 centimeters long and 25 centimeters deep. It may have been left by a large carnivorous dinosaur standing around 5 meters tall. The smaller footprints are thought to have belonged to smaller theropods approximately 1 to 2 meters tall.
The arrangement of the footprints allows visitors to imagine dinosaurs walking across the tidal flats of this region more than 100 million years ago.
Dinosaur footprints are often preserved with clearly defined three-toed shapes. The footprints at Sebayashi, however, have relatively indistinct outlines.
This is thought to be because the dinosaurs did not walk directly on the sandstone layer that can be seen today. Instead, they walked across a layer of mud that once covered the sand.
As the dinosaurs walked across the muddy surface, their weight pressed impressions down through the mud and into the underlying layer of sand. More mud and sand were subsequently deposited on top, eventually hardening into rock and preserving the depressions.
These indirect or undertrack-style footprints are considered particularly unusual examples of dinosaur trace fossils.
Later tectonic movements and mountain-building processes caused the geological layers to tilt dramatically. The formation was eventually exposed as the steep rock face visible today, creating a rare opportunity to observe the ancient layers directly.
The discovery of the Sebayashi Ripple Marks helped establish the region’s reputation as a “Dinosaur Kingdom.” Dinosaur bones and teeth have also been discovered in the surrounding area, providing important evidence of prehistoric life in this part of Japan.
Nearby, the Kanna Dinosaur Centre features exhibits of dinosaur and other prehistoric fossils, as well as hands-on activities such as fossil excavation experiences.
Visiting the Dinosaur Centre after seeing the actual geological site provides a deeper understanding of prehistoric life and the geological history of the Earth.
Facilities around the Sebayashi Ripple Marks include parking areas, restaurants, and souvenir shops, making the site relatively easy for tourists to visit.
An automatic audio guide is also available at the site, providing information about the geological layers and the dinosaur footprints.
Other natural attractions can be found nearby, including Shigasaka Pass, Ōse Falls, and Kujū Falls. Surrounded by the deep mountains of Kanna Town, Sebayashi Ripple Marks can be incorporated into a broader sightseeing route exploring the area’s natural landscapes and geological heritage.
Sebayashi Ripple Marks are a remarkable natural heritage site that preserves evidence of Earth’s environment and dinosaur activity from more than 100 million years ago.
The combination of ancient wave patterns and dinosaur footprints preserved on the same rock face offers a fascinating glimpse into a distant world. Standing before these traces provides a tangible connection with the immense span of Earth’s geological history.
When visiting Kanna Town, Gunma Prefecture, be sure to include Sebayashi Ripple Marks in your itinerary. These quiet traces of prehistoric life preserved in the mountains offer a memorable opportunity to experience the scale, mystery, and wonder of Earth’s ancient past.