Abstract
The Naka River flows into the Pacific Ocean at Nakaminato, Ibaraki Pref., rising at the western foot of Volcano Nasu. Fluvial or marine terraces and largely dissected hills surface distributed along the river can be classified into five groups and are mostly covered by volcanic ash layers, so-called “Kanto Loam”, . which is divided into four horizons in this region. The geomorphological developments of this region are summarised as follows;
1) The landform surfaces along the river are classified into five stages; namely, Hills (DlII), Upper terraces (DuIa), Middle terraces (DuIb), Lower terraces (DuII), and Terraces which are not covered by “Kantô Loam” and Floodplain (A).
2) Two types of the Hills (DlII) are distributed in this area; the Kitsuregawa Hills in the Kitsugawa Basin and the Tomobe, Tokoronuki Hills along the southeastern margin of the Torinoko-Toriashi Mountain Blocks. The former are composed of lacustrine sands or gravels and the latter of the marine sands.
3) After the formation of the Hills, the Naka River has cut the present valley across the Torinokoa. Toriashi Mountain Blocks and has flown into the Pacific Ocean.
4) Nasunogahara alluvial fan at the southern foot of, Volcano Nasu is divided into three levels correlated to Upper, Middle and Lower terraces in the lower drainage basin of the river. The Lower of these surfaces of three levels develop intersecting each other.
5) The terraces distributed in the lowest drainage basin are also classified into three groups; Upper, Middle and Lower terraces. The Upper terraces are coastal plain. The Middle terraces are distributed in the valley cutting the Upper terraces and the Lower terraces disapper under the floodplain 20km. up from the present river mouth.
