REFERENCE LAYERS

The geological map of Yokosuka is more detailed (1:50,000) than the surrounding areas (1:200,000). The surrounding areas are classified by rock type — how the rock formed — into four categories: Sedimentary, Igneous, Accretionary, and Metamorphic. For Yokosuka, the pop-up instead shows lithological information — what the rock is actually made of — printed in smaller text beneath the bolded stratigraphic formation name.

ROCK TYPES

【Sedimentary rocks】Formed when eroded fragments of pre-existing rock, minerals, and organic material are deposited in layers and compacted or cemented together over time.

【Igneous rocks】Formed when molten rock — magma below the surface or lava above it — cools and solidifies into crystalline or glassy minerals.

【Accretionary rocks】Formed when ocean floor sediments, basalt fragments, and seamounts are scraped off a subducting plate and added onto the edge of the overriding plate at a convergent margin.

【Metamorphic rocks】Formed when pre-existing rock is transformed by intense heat and pressure — without melting — causing its minerals to recrystallize into new structures.

SRATIGRAPHY (from oldest)

∙ Hayama Group (16.5 - 12 Ma), incl. Morito, Abuzuru, Oyama, Kinugasa & Yabe Formations

∙ Miura Group (15 - 2 Ma), incl. Misaki, Hatsuse, Zushi & Ikego Formations

∙ Kazusa Group (2.4 - 0.45 Ma), incl. Uragō, Nojima & Hayashi Formations

∙ Sagami Group (0.6 - 0.4 Ma), incl. Miyata & Yokosuka Formations and Obaradai & Misaki Sand and Gravel Deposits

∙ Alluvium, Reclaimed land, & Earthwork materials (〜 today)