Boso Peninsula, Central Japan was studied topographically and geologically.
The result is as follows:
(1) The base of the Iioka upland is constituted by the Inubo Group which is divided into three formations; the Na-arai, Iioka and Toyosato Formations in ascending order. The Inubo Group is overlain by the Katori Formation with slight unconformity. The Katori Formation is conformably overlain by the newly named Byobugaura clay. The Kanto Loam overlies the Byobugaura clay with unconformity, and is covered with sand hills (dunes) in some places (Fig. 1).
(2) The geomorphic surface of the Iioka upland can be divided into seven forms, namely, tableland, shallow-depression, gentle-slope, steep-cliff, extended-valley, extended-channel and valley bottom. Among them, a new geomorphological term: extended-channel which forms between the extended-valley and valley bottom is proposed (Figs. 3, 4 & 7).
(3) The stage of the drainage system (valley topography) in the Iioka upland is younger than that in the Shimosa upland. Many tributary streams of the Takada river show mainly north trend (Figs. 1 & 3). These branches go down to the gently-sloping area.
(4) The littoral deposit is observed in the valley bottom.
(5) The feature by summit level map seems to be nearly the same with that of the base of the Katori Formation. This means the evidence of basal movement since the end of the last glacial epoch.
(6) The valley-in-valley is indicated in the longitudinal profile of the main river bed. A coast line in 6000 years ago can be traced from the data of erosional speed of 30m/60 years (KAWASAKI, 1954) and at that time the sea-level was about 4 meters higher than that of the present time from the equation of longitudinal profile of the old river (Fig. 9).
(7) The height of the Iioka upland at the time of regression of the last glacial epoch was exactly lower than that of the present time. A value of ascending velocity of sea-level after the last glacial epoch is estimated 1-1.3cm/year. As the upheaval velocity of the Iioka upland was smaller than that of sea-level after the last glacial epoch, the “transgression”was taken place on the Iioka upland area. The sand hills (dunes) were formed at that time. After the last transgression this upheaval was discontinuous in the Iioka upland.