Petrological features of gravels of serpentinized peridotites in the Nagahama Formation of the upper Kazusa Group (middle Pleistocene), distributed in the central Boso Peninsula, are examined in detail. Studied peridotites are serpentinized to various degrees, but their protoliths were inferred from textures and relic minerals; harzburgites frequently containing plagioclase are dominant, and lherzolites (with plagioclase) and dunites are subordinate. Forsterite (Fo) contents of olivine are high, around 91, and Cr/(Cr + Al) ratios of chromian spinel are intermediate, mostly from 0.4 to 0.6. The geological and petrological characteristics of the serpentinite gravels indicate that they were derived from the Mineoka Belt (Circum-Izu Massif Serpentine Belt), southern Boso Peninsula.
     Serpentinite gravels in several strata, including the Nagahama Formation, from the early Miocene to Recent in the Boso Peninsula, show very similar petrological features (particularly chemical composition of chromian spinels), so the gravels were possibly derived from mantle peridotites of a common origin. Almost homogeneous peridotite bodies have continued to be uplifted at the fore-arc region of the Honshu arc (the Mineoka Belt) throughout the time after protrusion near/at the trench in the early Miocene.
     Northward streams were active in the central part of the Boso Peninsula due to uplift of the southern part at the time of sedimentation of the Nagahama Formation. The ‘Kamogawa Graben’, which topographically divides the Mineoka Belt and the northern area now, had not been developed in the middle Pleistocene.

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