Detailed geological mapping has revealed the stratigraphic sequence of Neogene and Quaternary sediments with andesite in Nakanokawa and its vicinity．The sediments are divided into six geological units ： the Jigokudani Formation，Sonoha Gravel，Mikasa Andesite，Osaka Group，older trrace deposits，and alluvium，in ascending order．Of these units，the first three are assigned to the Middle Miocene，and the Osaka Group is Late Pliocene to Early Pleistocene in age．The older terrace deposits and alluviumare also of late Middle Pleistocene and Holocene age respectively．
The Jigokudani Formation crops out in such three separate areas as east of Mikasa-onsen，east of Nakanokawa，and the area along Tsukigase Road．East of Mikasa-onsen，it is composed of tuff and conglomerate，whereas it consists of alternating beds of sandstone and mudstone with lignite east of Nakanokawa．Along Tsukigase Road，it is composed mainly of tuff with tuffaceous sandstone and mudstone．Unfortunately，the stratigraphic relationship among the sediments in these three areas are unknown．
The Sonoha Gravel，which is widely distnbuted in the study area，unconformably overlies the Jigokudani Formation．It is composed mainly of rounded to subrounded cobbles and pebbles．The Sonoha Gravel is dated at between 15.5 Ma and 13.1 Ma，based on radiometric dates of the underlying Jigokudani Formation and overlying Mikasa Andesite．The latter is represented by black augite hypersthene andesite．Pyroclastic rocks usually occur just below the andesite．
The Osaka Group in the study area is composed of unconsolidated mud，sand and gravel with thin volcanic ash layers．Its distribution is divided into western and eastern areas by the intervening Sonoha Gravel．The Osaka Group in the western area has already been described by Kawamura（1993），and thus that in the eastern area is only described here．It consists mainly of irregularly alternating beds of gray mud and pebble gravel，and contains thin intercalated volcanic ash layers，of which the Kaseyama II Volcanic Ash Layer is traceable as a maker bed．The older terrace deposits unconformably overlie the Osaka Group．They consist mainly of subrounded to subangular cobble and pebble gravels，and occur in and around the Midorigaoka Filtration Plant．
The Neogene and Quaternary sediments generally dip gently west to west-north-west，but a very slight syncline is recognizable in Nakanokawa．Of the three faults found in the study area，the Onigatsuji Fault is the most prominent，and runs east to west along the southern margin of the area mapped．
The present detailed mapping has also shown that the geological map recently published by Ozaki et al．（2000）contains many stratigraphic problems in the distribution and division of the Neogene and Quaternary，as far as the study area and
the adjacent Nara Hills are concerned．These problems are also discussed here．