The existence of stream piracy can be inferred in the Niijuku Pass between the Shiroishi River, a tributary of the Abukuma River, and the Ôtaki River which belongs to the water system of the Mogami River. At the pass, a wind gap has formed. Tributaries of the Ôtaki River are developing on the little fan. A shallow valley and knick point are found at the upper stream of the Ôtaki River. The absence of a flat valley floor along this river which forms the eastern part of the Yonezawa Basin is recognized. These facts can be considered the result of stream piracy.

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Geological survey on Minami Zao volcano has been carried out in order to constract the evolutional history of the volcano and for further petrological investigation.
Minami Zao volcano is situated on the Pacific side of Nasu volcnic zone in Miyagi Prefecture, northeastern Japan, consists of six cones, and occupies approximately 100km². The evolutional history can be divided into the following three stages: Stage I. Nonomoriyama, Manokamidake, and Eboshidake cones were built on the eastward inclining basement in this order, and Sugigamine cone was formed on the northwestern elevated basement probably at the same time of the formation of the Nonomoriyama cone. Andesite lava flows are dominante, and andesitic pyroclastic flow deposits are found in the successions of the Manokamidake cone. Stage II. Following volcanic activity began to the west of the Manokamidake cone resulting in formation of Byobu cone. First, basic lava had been erupted and was followed by andesitic lavas and pyroclastics. Stage III. To the south of the Byobu cone, volcanic eruptions began and formed Fubo cone consisting of basic lava and pyroclastics.
After the activity or during the late to last activity, two large calderas formed by erosion on the eastern side of the ridge between the Byobu and Fubo cones, and the eruption centers of the Manokamidake, Eboshidake, Byobu and Fubo cones became vague.
The topography of the basement leads to the volume evaluation of only 9.4 km³ for the volcanic body. This can be attributed to the elevated basement under the volcano.

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