Kantō Plain

The  is the largest plain in Japan. It is in the Kanto Region of central Honshū.  The total area 17,000 km2 covers more than half of the region. It extends over Tokyo, Saitama Prefecture, Kanagawa Prefecture, Chiba Prefecture, Gunma Prefecture, and Tochigi Prefecture.Encyclopedia of Japan, Kōdansha

A number of plateaus make up a large part of the plain.  These large plateaus are divided into smaller ones by shallow river valleys.  Volcanic ash from surrounding volcanoes are thought to have been deposited on these plateaus.Nihon Daihyakka Jiten, Shōgakkan

What is unusual is the overall tilt of the plateaus and hills.  The entire area is slightly bent or slanted and forms a basin centered in the Tone River and Tokyo Bay. Tectonic changes are still happening, as the plain’s central region gradually sinks lower towards sea level.Encyclopedia of Japan Tokyo: Kōdansha

 References 

Category:Honshū