コロラド・ロッキー山脈，フロントレンジ，ナバホ氷河周辺の“プロテーラスランパート”の再検討，および後期更新世〜完新世のナバホ氷河の復元

Abstract

The air-photo interpretations and the casual field observations in the Navajo Glacier area, the uppermost reaches of the South St. Vrain Creek, Colorado Front Range, led to the speculation that the Triple Lakes “protalus ramparts” which were identified by Benedict (1968) can be classified into the following three different landforms:(1) a ridge located at the highest altitude (U1) regarded as a protalus rampart, while another in the center can be subdivided and classified into (2) a protalus rampart (U2: upper unit) and (3) a lateral moraine (L: lower unit). Several lines of evidence support the above classification. Firstly, the lower unit (L) has a flat surface with vegetation cover. Secondly, the lower unit (L) continues to an arcshaped ridge downvalley. Thirdly, the upper unit (U2) has the same sinuosity as that of U1 and the talus slope but the lower unit (L) does not. Finally, the lower unit (L) has fine matrix whereas the upper unit (U2) does not.Multiple relative dating methods, such as lichenometry (lichen diameter, percent lichen cover, and ratio of green lichens/{brown and black lichens}), weathering rind thickness, SGW I (surface granite weathering index of Benedict, 1985), and pit depth, provided the estimated age of Triple Lakes (3, 000-5, 200 y. B. P.) for both the moraine and the protalus rampart classified above. However, soil profiles and soil color indices (Buntley-Westin and Hurst color indices) indicate that the moraine is older than the protalus rampart. It is probable that the lichenometric and weathering feature data collected from the lateral moraine surface have been influenced by rockfall activity which modified the moraine surface during the Triple Lakes period. The protalus rampart (U2) is estimated to have been formed in the Triple Lakes Stade, as Benedict (1968) states. However, the moraine seems to be of the Satanta Peak Stade (10, 000-12, 000 y. B. P.), which is recognized elsewhere in the Colorado Front Range (Birkeland et al., 1987).Using this result together with the method for estimating glacier ice thickness of Porter et al.(1983), this study has also attempted to reconstruct the Navajo Glacier in the Late Stade of Pinedale and the Satanta Peak and Triple Lakes stades. The glacier is believed to have terminated at an altitude of 3, 250-3, 300m in this valley in the Late Stade of Pinedale (-14, 000 y. B. P.)(Madole, 1969), at which time the area and thickness of the glacier are estimated to have been 5.6km2 and 250m, respectively. In the Satanta Peak Stade (12, 000-10, 000 y. B. P.), the glacier retreated to 3, 480m, and its area and thickness decreased to 1.0km 2 and 100m, respectively. The Triple Lakes glacier terminated at an altitude of 3, 680m in the cirque, with an area of 0.2km 2 and a thickness of 25-30m.