A study of the chaotic deposits at the basal part of the type Arakawa Formation (lower Middle Miocene) is made on the river beach outcrops of the Ara-kawa. Lower part of the chaotic deposits consists of pebbly mudstone with many mudstone pebbles, and turbidite. Upper part of the chaotic deposits consists of mudstone conglomerate, composed of mudstone pebbles and mudstone matrix, and exotic blocks of the Miocene sediments. These chaotic doposits are probably subaqueous debris flow origin. The exotic blocks are, based on the lithofacies and molluscan fossils contained in them, correlative to the Kozono Formation, underlying the Aarakawa Formation, and/or to the Ogawamachi Formation, same geologic age with the Kozono Formation, in the Kanto Mountains to the south of the Hiki Hills.
Chaotic deposits are not only found in the type Arakawa Formation, but also in other many places of the basal part of the Arakawa Formation. As a whole, existence of a chaotic sedimentary unit through the basal part of the Arakawa Formation can be concluded, in the southernmost part of the Hiki Hills area adjacent to the north of the Kanto Mountains.
The sedimentary chaotic unit is originated with the shifting of the depositional area of the Miocene sediments, and with rapid subsidence of the depositional area probably connected with the upheaval of the northeastern part of the Kanto Mountains, adjacent to the south of Hiki Hills.

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Stratigraphy and geologic structure of the Miocene series of the Kodama area are reexamined by detailed field observation and also by laboratory studies.
This area is geologically subdivided into three subbelts: northern Namanoyama-Suwayama, middle Kibe-Niragayato, and southern Shiroishi-Kamigo subbelts. Distribution and structure of the Miocene sediments are different among these subbelts.
The Miocene sequence in this area can be divided into following four formations in ascending order: Kozono, Kodama, Namano and Suwayama Formations. On the basis of plankton microfossils and molluscan fossils, it may be said approximately that the Kozono Formation is late Early ～ early Middle Miocene, the Kodama Formation is early Middle ～ middle Middle Miocene, and the Namano and the Suwayama Formations are late Middle ～ Late Miocene in age.
The Miocene sediments of the Kodama area and of the Hiki Hills area, adjacent to the southeast of the Kodama area, have some similarities with each other in the lithofacies and the geologic structure.

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Pollen analytical studies were conducted on the Sayama Formation in Sayama Hills. The Fossil pollen assemblages were characterrized by Tsuga, Picea, Pinus, Fagus. Paleovegeteation can be presumed to be coniferous forest with decidous broard-leaved trees of the cool temperate zone level.
This part of Sayama Formation corresponds to the Metasequoia C ～ D Zone of Osaka Group in Kinki region, South-western Japan.

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