下総台地
Shimōsa Plateau sits above the , a geological group under the Bōsō Peninsula. The Shimōsa Group dates from the Tertiary and Pleistocene periods, and is roughly divided into the Jizodo, Yabu, and Narita formations. The geological group consists primarily of sedimentary layers of loose sand, and secondarily of clay silt and pebbly-sand layers. Above the Shimōsa Group are more recent rich, fertile layers of volcanic ash. The Shimōsa Plateau yields great deposits of fossils, notably molluscan fossils, as the plateau was originally submerged under water.