The high terraces along the middle course of the Kiso River were investigated with a view to clarify the Quaternary history of the area. It was also expected that the study would contribute towards the history of the Atera fault separating the northeastern mountainous area and the southwestern upland area. Among the terraces studied the following terraces, from the higher to the lower, i. e. the Jizomine, the Kawai, the Uyama and the Goshodaira terraces are described.
The Jizomine terrace ranges from 900m to 1300m in altitude, and is about 500m high above the present river floor. It is associated with a gravel bed covered with a “loam” bed about 110cm thick. This gravel bed may be correlated with the Toki gravel bed in the lower course of the Kiso River.
The Kawai terrace, ranging from 880m to 900m in altitude, is 170m to 190m higher than the present river bed. The gravel bed of this terrace contains pebbles of the Jizotoge volcanics which cover the Jizomine gravel bed.
The Uyama terrace is 670m high, and is 140m higher than the present river bed. The gravel bed of this rerrace is overlain by a 30cm thick “loam”, which, in turn, is covered unconformably by the Wamura mud flow. This gravel bed is correlated with the middle bed of the Akatsuchidaira gravel.
The Goshodaira terrace is 670m high above the sea level, and the relative height from the present river is 130m. The gravel bed of this terrace, about 30m thick, contains pebbles of the Jizotoge lava as well as those of the Kuragoehara lava, and is overlain ay a bed of the older “loam”. The Uyama and the Goshodaira terraces are considered to be roughly contemporaneous with the older Ontake volcanism.
The Wamura mud flow covering the Uyama gravel bed is rather thin being three meters. It is easily distinguished from the Kisogawa mud flow in thickness and length of the flow.
The difference in height of the higher terraces on both sides of the Atera fault suggests that the fault has been active since the earlier part of the Quaternary age.
