Abstract
It has long been believed by many geographers in Japan that the Tenryu River beheaded the Toyo River along its middle course near Nakappe town, and then the Ochise River, a tributary of the former, encroached through the wind gap into a fossil valley of the previous main stream of the latter, having remained a valley watershed (Talw asserscheide in German) at Ikeba village. This valley watersched has a length of about 500 meters, sloping gently to eastward. Its west end is terminated by the valley-head of the Shimano-kawa, one of the source streams of the present Toyo River ; and the eastern margin slopes gently into the stream of Tero-sawa, one of the source streams of the Ochise River in the Tenryu drainage basin, forming several alluvial cones.
Through his observations, the writer cannot approve of the opinion of stream-piracy above mentioned. The reasons are as follows:—
1) The breadth of the Ikeba valley watershed is too narrow to assume that it was the previous main stream of the Toyo River.
2) Both sides of the valley watershed show concave slopes with no terrace landforms.
3) The outcrops along the valley floor are all composed of angular and ill-sorted talus deposits without water worn gravels.
So the Ikeba valley watershed cannot be assumed to be the fossil valley of the Toyo River. On one hand, some geologists presumed a fault-line along this valley watershed, the pioneer of whom was B. Naumann, who found the “Medianlinie”, the main geotectonic line in southwestern Japan. Though the writer could not find any actual evidence of a fault, it is rather probable that the valley watershed here was originally formed as a fault valley.
