The southern part of the Boso Peninsula is characterized by straight to gently curved coastline consisting of beaches and rocky shores. The east coast of Tateyama Bay is formed by a medium to coarse sandy beach bordered on the north by 50 m high cliffs of the Daibutsu Cape fringed by a narrow boulder beach. The southern border of the bay is formed by the island of Okinoshima connected by a low sandy bar to the Japan Self-Defense Forces base, constructed by landfill around a small island of Takenoshima. From Okonoshima to Sunozaki, the coast is formed by sandy beaches occasionally interrupted by rocky promontories. Neogene siltstones crop out along the shore at Sunozaki Cape, forming a more diverse coastline with shallow, short bays and scattered small rocky islands. From the village of Ito to Ainohama, the coast is formed by a long sandy beach. The southeast coast of the peninsula from Ainohama to Chikura is characterized by a rocky shore bordered by a zone of low rocky reefs. The northeastern coast of the area is formed predominantly by sandy beaches.

The sea floor around the peninsula slopes down to about 20 to 30 m, forming a shallow platform 1 to 1.5 km wide. A second, deeper platform surrounds the first at a depth of 100 m. It is about 5 to 10 km wide but is most extensive near the Sunozaki Cape, from which it extends west for about 20 km. The platform is cut by several submarine valleys believed to have been dissected by rivers during a lower stand of sea level. At the edge of the platform the sea floor slopes steeply down to depths of 600 to 1,000 m.

Sediment distribution in Tateyama Bay is shown in text-fig. 8. The central part of Tateyama Bay is occupied by silt and sandy silt. The bottom along the northern fringe of the bay, to a depth of 10 to 15 m, is formed by rocky .reefs surrounded by coarse sand. Along the eastern and southern shore of the bay, fine to medium sand extends from the beach to depths of about 10 to 15 m, where it merges with the central bay silt. An area of coarse sand lies northwest of Okinoshima at a depth of about 30m.

The sea bottom from Okinoshima, around Sunozaki Cape to Ainohama town, consists of bedrock outcrops surrounded by fine to coarse sand. Information available for the east coast of the area, from Ainohama to Maruyama , is very scarce but suggests that the bottom sediments are predominantly sand and rock.

Large rivers are concentrated in the northern part of the area. The Heguri and Shioiri Rivers empty into Tateyama Bay. The Nuruishi, Maruyama, and Seto Rivers flow into the sea along the northeastern coast. The Tomoe River is the only large river on the south coast.

Sea water temperature off the southern Boso Peninsula is influenced by the warm Kuroshio current flowing from the southwest. The main branch of the current flows past the southwest coast at a speed of 1 to 3 knots. A smaller branch, moving at about 0.5 knots, enters the Tateyama Bay from the north, and flows clockwise to join the main branch near Sunozaki Cape. The strength and direction of the flow of the Kuroshio current are variable and occasionally the cooler water masses of the northern Oyashio current extend to the southern part of the peninsula, thus adversely affecting the organisms living on the east and south coast of the area.

In Tateyama Bay, February and March are the coldest months (13°C), and August and September are the warmest (25°C) (Hamada, 1963). The predominant wind direction in the area is southwest. The maximum tidal range is 175 cm.