木曽山脈北部における最終氷期の氷河の消長と編年
スポンサーリンク
概要
- 論文の詳細を見る
The glacial landforms, consisting of cirques, troughs, moraines, etc., are widely distributed in the vicinity of Mt. Kisokomagatake (2, 956 m) in the northern part of the Kiso Mountain Range. They are obviously classified into the younger and fresh landforms and the older and dissected ones. The older ones are much more extensive than the younger ones. The author discussed the glacial fluctuations and chronology in the drainage basin of Nakagosho-gawa east of Mt. Kisokomagatake, where glacial landforms are well preserved, in order to clarify the developmental period of these two types of glacial landforms (Fig. 1). Geomorphological data (Figs. 2, 11 and 12), geological data (Figs. 3 and 5), sedimentological data (Figs. 710, and Tab. 1) as well as tephrochronological data (Fig. 4 and Tab. 2) were examined to this purpose.<BR>Since about 80, 000 years B. P., three stages of glacial advance were revealed in the drainage basin of Nakagosho-gawa. Consequently, they are included in the Last Glacial Age and are named, from older to younger, Nakagoshodani Stadial I, II and III, respectively. Glacial landform development during each stadial were quite different, as described below (Figs. 2 and 13) : <BR>1. Nakagoshodani Stadial I (80, 000-45, 000 years B.P.) <BR>This stadial is characterized by the largest extension of glacier. Glacier, attained 100 m or more in maximal thickness, extended from the Senjojiki- and Gokurakudaira- Cirque to the east of Shirabidaira (1, 790-1, 600 m a.s.l.) over a horizontal distance of 2.5 to 3.0 km. Glacial snout was located approximately at 1, 550 m a.s.l.. This glacial advance resulted in the formation of large trough (HG) below the above-mentioned cirques as well as the deposition of till (S-1 gravel bed) at Shirabidaira. The total thickness of till composed of ablation and lodgement till amounted to 50 to 60 m. Besides, lateral moraines were formed at Shirabidaira.<BR>2. Nakagoshodani Stadial II (45, 000-30, 000 years B.P.) <BR>Though glacier extended to Shirabidaira again, glaciation was limited in extension and magnitude rather than that of the Nakagoshodani Stadial I. Glacier is inferred to have been a maximum thickness of 50 to 70 m. Only the upper part of the trough was further denudated resulting in the formation of relatively small trough (MG) through this glacial advance. Ablation till (S-2 gravel bed), 20 to 30 m in maximum thick, was deposited and formed lateral moraines at Shirabidaira. End and lateral moraines (a, b) were also formed on the thresholds of both the Senjojiki- and Gokurakudaira- Cirque, preceded and accompanied by the retreat of glacier from Shirabidaira.<BR>3. Nakagoshodani Stadial III (30, 000-10, 000 years B.P.) <BR>Glaciation was much more limited in extension and magnitude than that of the Nakagoshodani Stadial II. The glaciers descending from both the Gokurakudaira-Cirque and the northern half of the Senjojiki-Cirque excavated in the Nakagoshodani Stadial I and II, attained only a maximum thickness of about 20 m. The glacial snouts were at about 2, 290 m a.s.l.. The upper part of the trough, modified during the Nakagoshodani Stadial II, was further slightly denudated resulting in the formation of small trough (LG) through this glacial advance. The formation of lateral moraine (c) in the Senjojiki-Cirque was prior to that of the end and lateral moraines (d-o) in the trough. In the northwestern part of the SenjojikiCirque (A, B), glacier remained up to the latest stage.<BR>Judging from the investigation in the drainage basin of Nakagosho-gawa, it can be regarded that the older and younger glacial landforms obviously classified by the degree of dissection are relevant to the Nakagoshodani Stadial I and to Nakagoshodani Stadial III, respectively. Surface area and equilibrium-line altitude of glaciers, the altitude of glacial snouts, etc., during these two stadials are shown in Tab. 3.
- 社団法人 東京地学協会の論文