Abstract
The Agatsuma, a tributary of the Tone, flows in the northwestern part of Gumma Prefecture, Central Japan. In this drainage basin there are many volcanoes, such as Mts. Asama, Azumaya, Kusatsu-Shirane and Haruna, activities of which have considerably influenced the topographic development of the region.
The author tried to clarify the geomorphic history of this volcanic region with geomorphological and geological investigations. The results are summerized as follows.
(1) The Agatsuma drainage basin is divided into Tsumakoi Highland and Nakanojo Basin by the older volcanic zone which ranges meridionally in the central part of the basin.
(2) In consequence of the birth of Volcano Asama, the ancient Agatsuma valley opening southward into the Komoro Basin was dammed up by volcanic products and the Ancient Tsumakoi Lake was formed in its upper part. Owing to headward ersosion of the older volcanic zone from east, the lake was drained eastward and its floor has been dissected and transformed into terraces. After that, volcanic products covered again the highland.
(3) From longitudinal profiles of lower terraces of the Agatsuma it is inferred that the older volcanic zone has been upheaved relative to Tsumakoi Highland and Nakanojo Basin. This fact coincides well with geological structure and results of geodetic survey in the region.
(4) Small hills in the central part of Tsumakoi Highland have been elevated by folding which had succeeded up to stage of Ancient Tsumakoi Lake.
