Geography and Geology
The islands form a chain along a fault zone running from north-northwest to south-southeast. This geological structure has shaped their distinctive landscapes. Steep fault scarps are particularly prominent along the western coasts of Kitagishima and Shiraishijima, reflecting the geological processes that helped form the Seto Inland Sea region.
Most of the islands consist of granite, while Kamishima is formed from rhyolite. The terrain is generally mountainous, with little flat land, so settlements have traditionally developed along the coast and on hillsides.
The archipelago lies near a boundary between tidal waters. Shiraishi Strait, between Takashima and Shiraishijima, is especially known for its strong tidal movements. Water levels and currents change noticeably with the tides, creating a dynamic seascape.
History of the Kasaoka Islands
Archaeological Sites from Ancient Japan
Numerous archaeological sites dating from the Paleolithic, Jōmon, and Yayoi periods have been discovered across the Kasaoka Islands. Paleolithic sites are particularly numerous on islands such as Akijijima and Takashima compared with the mainland, while remains from the Jōmon period onward have been found throughout the archipelago.
The Ōdomari archaeological site on Takashima indicates that a substantial settlement engaged in salt production existed there from the Kofun period through the Nara period. On Ōbishima, a ritual site dating from the Nara to Kamakura periods has yielded objects such as sancai-glazed pottery and bronze mirrors.
These discoveries are believed to provide evidence that envoys traveling to Silla and Tang China, as well as officials bound for Dazaifu in Kyushu, stopped at the islands during their voyages and performed religious ceremonies for safe passage.
Legends and Political History
According to accounts in the Nihon Shoki and the Kojiki, Emperor Jimmu established a temporary palace on the islands during his legendary eastern expedition and lived there for several years.
During the Heian period, the Manabe clan is said to have controlled the archipelago and supported the Taira clan during the Genpei War. From the Kamakura period onward, the Suyama clan gained influence. Later, during the Muromachi and Sengoku periods, the Mōri and Kobayakawa clans expanded into the area, using the islands as bases for their naval forces.
In the early modern period, the islands came under the control of the Fukuyama Domain in Bingo Province. Ports were developed, and the archipelago grew in importance as a stop along Seto Inland Sea shipping routes. During the Edo period, extensive land reclamation took place around Kasaoka Bay and the islands, joining several areas to the mainland and increasing their economic significance.
Industries and Local Culture
Agriculture, Fishing, and Stone Quarrying
For centuries, residents cultivated crops on the islands’ hillsides, and cattle raising was once an important activity. As the population grew and farmland expanded, however, keeping cattle became more difficult. Fishing and maritime transport consequently became increasingly important to the local economy.
Kitagishima and Shiraishijima also became known for their stone quarries. During the early modern period, the area was particularly famous for its sardine catches, which supported the livelihoods of island communities.
Tourism and Cultural Diversity
As fishing, shipping, and the stone industry declined, tourism became increasingly important across the islands. In 2005, a regional revitalization initiative introduced Shimaben, a specialty bento lunch featuring local ingredients. The product received the Minister of Agriculture, Forestry and Fisheries Award.
The islands’ location between Shikoku and Hiroshima has also contributed to a distinctive blend of regional cultures and dialects. Local speech is based on the Okayama dialect but shows influences from both Shikoku and Hiroshima. For example, the word kyōtē, meaning “frightening” or “scary,” is used on Kitagishima and the Tobishima islands, while residents of Manabeshima and Mushima use otoroshī for the same meaning.
Explore the Islands
Yokoshima
Yokoshima was once a separate island, but land reclamation during the early modern period connected it to the mainland. Reclamation work took place during the Kan’ei era (1624–1645). A small island at its southern tip, known as Ōtonosu, was also incorporated into the mainland through reclamation.
Kamishima
Kamishima lies approximately three kilometers south of Kasaoka Port. It was once an inconvenient offshore island that could only be reached by small, hand-rowed boats. The construction of Kamishima Bridge in 1970 connected it to the mainland, and land reclamation in Kasaoka Bay in 1990 completed the connection, so it is no longer an island in the geographical sense.
The island is home to Kamishima Shrine, which is recorded in the Engishiki Jinmyōchō, an important Heian-period register of Shinto shrines. The shrine reflects the area’s long religious history.
Takashima
Takashima lies south of Kamishima, approximately 12 kilometers from Kasaoka Port. Nearby subsidiary islands include Sashidejima, Akijijima, Inazumijima, and Kodakashima. Takashima was designated a Place of Scenic Beauty by the national government in 1944.
The island contains numerous archaeological sites, some dating back to the Paleolithic period. According to legends recorded in the Nihon Shoki and the Kojiki, Emperor Jimmu established a temporary palace here during his eastern expedition.
Shiraishijima
Shiraishijima is an inhabited island approximately 16 kilometers from Kasaoka Port. A flat area reclaimed during the Edo period occupies the central part of the island, where a substantial harbor was developed.
A granite rock formation about 10 meters high stands near the center of the island and is designated a Natural Monument of Japan. Shiraishijima has long served as an important port in the Seto Inland Sea and was frequently used along the routes traveled by feudal lords from western Japan during their obligatory journeys to Edo.
Kitagishima
Kitagishima is the largest island in the Kasaoka archipelago and lies approximately 25 kilometers from Kasaoka Port. Its mountainous terrain leaves little flat land, so the main settlements developed along the northern and southern coasts.
The island is famous for Kitagi Stone, a highly regarded granite used in prominent structures throughout Japan. Examples include Osaka Castle, Kyoto’s Gojō Bridge, and the torii gate at Yasukuni Shrine in Tokyo. The island’s quarrying heritage remains an important part of its identity.
Tobishima: Ōbishima and Kobishima
The Tobishima Islands consist of Ōbishima and Kobishima, located approximately 25 kilometers from Kasaoka Port. At low tide, a sandbar famously emerges between the two islands, allowing visitors to see a striking natural feature shaped by the changing tides.
An ancient ritual site on the sandbar has yielded archaeological artifacts, suggesting that the area had religious significance in earlier times. The sandbar has become smaller in recent years, although the cause of this change remains unclear.
Manabeshima
Manabeshima is an inhabited island approximately 30 kilometers from Kasaoka Port. It retains the atmosphere of a traditional fishing village, making it an appealing destination for visitors interested in the slower pace of island life.
In the 1980s, the island served as a filming location for the movie Setouchi Shōnen Yakyūdan (Setouchi Schoolboy Baseball Team). Manabeshima is also known as an “island of flowers.” In the past, the cultivation of cold-resistant chrysanthemums was an important local activity.
Mushima
Mushima is an inhabited island at the southernmost point of Okayama Prefecture, approximately 33 kilometers from Kasaoka Port. Before World War II, it was renowned for transporting live fish and was said to rank first in Japan for this trade. The industry gradually declined as motorized vessels became widespread after the war.
The island is also famous as the setting of Seishi Yokomizo’s mystery novel Gokumon-tō (The Devil’s Island). When the novel was adapted into a film in 1977, scenes were actually shot on Mushima.
Transportation and Access
The Kasaoka Islands have long been connected to important maritime routes across the Seto Inland Sea. Today, however, the small harbors make it difficult for large vessels to enter, so scheduled passenger ferries from Kasaoka Port and marine taxis are the main means of access.
The ports on the individual islands are generally small and shared by passenger boats, fishing vessels, charter fishing boats, and recreational craft. As a result, transport services and harbor facilities are limited, and visitors should check ferry schedules and connections when planning an island-hopping trip.
The waters south of Mushima mark a boundary with Hiuchi-nada and are known for their strong currents, which can pose a risk to navigation. Lighthouses and signal stations have been installed on Mushima to help guide vessels. Meanwhile, aging facilities and a shortage of parking at Kasaoka Port remain challenges for the region’s transport infrastructure.
Discover the Islands of the Seto Inland Sea
The Kasaoka Islands offer a rich combination of scenic coastlines, rugged granite landscapes, traditional fishing villages, and archaeological sites that reveal the area’s long history. Each island has its own character, from the celebrated stone quarries of Kitagishima to the old village atmosphere of Manabeshima and the dramatic tidal sandbar between Ōbishima and Kobishima.
Whether exploring island heritage, enjoying the coastal scenery, or discovering local traditions, visitors can experience a quieter and distinctive side of the Seto Inland Sea. Traveling by ferry between the islands is an opportunity to appreciate the archipelago’s varied landscapes and the enduring connection between its communities and the sea.