長野県白馬盆地の伝行山小丘列 : 埋め立て掘り出し過程について
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概要
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There are small but strange hillocks, Kitabayashi, Dengyoyama and Nishidengyoyama from east to west, at the northern end of the Hakuba Basin, northwestern Nagano Prefecture. They stand in a row 970 m long and 150 m wide at the maximum. The row is a little sinuous longitudinally. The hillocks are just on the northern rim of the terrace surface S 2, which is limited on the north by the V-shaped valley of the Kusukawa (the River Kusu). They are 10 to 40 m high above the terrace surface, while the valley is 25 to 30 m deep below the same surface. The hillocks are asymmetrical in cross section, namely the southern slope is gentle against the very steep northern slope. To the north, there are the Tatenoma Mountains, of which the lower slope is very steep down to the valley floor of the Kusukawa. The hillocks and the Tatenoma Mountains are composed mainly of andesitic lavas, tuffites and tuff breccias of the late Tertiary Ohmine Formation. A thin veneer of gravel bed is found on the summits of the hillocks. Six geomorphic surfaces are distinguished in this area, and are named tentatively the S 1, S 2, S 3, S 4, S 5 and S 6 Surfaces. The S 1 Surface is the lowest of the six, and is the flood plain of the present streams of the Himekawa (the River Hime) and the Matsukawa (the River Matsu). The S 2 Surface extends to the south of the hillocks. The process of formation of the hillocks and the Kusukawa valley must have been as follows. (1) A thick gravel bed accumulated in this area, and reclaimed the basin to cover the hillocks entirely. At that time, the hillocks and the Tatenoma Mountains together formed one and continuous mountain mass. In other words, they were not separated from each other by the valley of the Kusukawa. (2) The stream of the Kusukawa, at that time, was on the surface of the gravel bed, and was just above the present course of the Kusukawa. Also it was in contact with the southern foot of the Tatenoma Mountains. (3) A general upheaval of the area opened the next stage. The Kusukawa bagan to cut the river bed downward. At first the stream dug the gravel bed, but it soon reached the hard rocks of the Ohmine Formation. Continual down-cutting of the Kusukawa dug out deeper and deeper the V-shaped valley. (4) The gravel bed that had once reclaimed the basin suffered also from denudation according to fall of the base level, that reached ultimately the horizon of the present S 1 Surface. Parts of the gravel bed were left on the summits of the hillocks and more extensively in the area of the present S 5 Terrace. (5) Then a new gravel bed accumulated to form the S 2 Surface. But it was eroded out again from most of the area except the present S 2 Terrace. Similar deposition and erosion of gravel beds must have occurred again and again. It is suggested by such terraces and gravel beds as those of S 3 and S 4. Processes of accumulation and denudation of gravel beds certainly have produced the strange hillocks that are now separated from the main mountain mass by a stream. The similar process will produce various kinds of unusual land forms. The writers call it reclamation-excavation process. It reminds us of the digging process of hillside channels along some of the lower courses of valley glaciers.
- 信州大学理学部の論文
- 1985-10-31
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