西中国地方の岩海（Felsenmeer）の数例について

A Study on Some Felsenmeere Found in West Chūgoku, Japan

今村 外治

廣島大學地學研究報告

Geological report of the Hiroshima University

In Japan, Felsenmeer had not yet been reported until several years ago. Very recently, however studies on some examples of Felsenmeer and Blockstrom have been rapidly promoted and discussed from palaeoclimatological standpoint, presenting very interesting problems on the Japanese Quaternary history.
The writer has recently observed following 7 remarkable examples of Felsenmeer newly discovered in Hiroshima Prefecture and previously known in Yamaguchi Prefecture, all in West Chūgoku Mountains, Japan:
A (in Hiroshima Prefecture)
1. Felsenmeer at Yoshida, Kui-cho, Mitsugi-gun.
2. Felsenmeer at Hotoke-dani, Mitsugi-cho, Mitsugi-gun.
3. Felsenmeer at Omine-yama, Hisayamada, Onomichi City.
4. Felsenmeere at Kami-ichi and Kajiya-gochi, Miwa-cho, Futami-gun.
5. Felsenmeer at Yano, Konu-gun.
6. Felsenmeere on the flat-top and southern slope of Mt. Noro, near Kure City.
B (in Yamaguchi Prefecture)
7. Felsenmeer at Makura, Isa-cho, Mine City.
In this paper, the results of the o bservation on each Felsenmeer are described, discussing on the origin and geological age of them. The characteristic features of the Chūgoku Felsenmeere are summarized as follows:
1) Most of the Felsenmeere lie, at the height of 300-500 meters, while a few others of nearly 250 meters or of 600-800 meters.
2) Most of the Felsenmeere are formed in small and shallow valleys dissected on the mid-slope of middle level mountains some of which continue to the ones at mountain ridges while a few Felsenmeere are found on a flat-top mountain of Chūgoku Peneplain.
3) Each of the Felsenmeere is always accumulation of abundant large boulders in nearly same size of from several ten centimeters to one meter or more. The boulders are always fresh and hard in character without suffering any notable weathering or decomposition. The form of the boulders is subangular or roundish, but not angular.
4) The Felsenmeere distribute frequently in granitic and granodioritic regions but are also found in quartz-porphyry and rhyolite ones. It does not always that distinct joints can be remarkably observed on the country rock.
5) There are not a few cases where Blockbildung of Felsenmeer is thought to be performed in situ or at the same places as blocks are found. Accordingly, Stromung was not always neccessary factor or condition to make Felsenmeer and similar topographic features.
6) Though such abundant boulders of Felsenmeere in West Chugoku were probably produced under some colder climatic conditions than the present, the writer can not now find any stratigraphical and topographical positive proof to support above deduction.
7) The same mode of occurrences and identical topographic features of Felsenmeere suggest that most of the Felsenmeere in Hiroshima and Yamaguchi Prefectures were built up at about the same and past time. The age of the formation of our Felsenmeere is clearly after the deposition of Khotachi gravel beds probably of the latest Pliocene or the earliest Pleis-tocene, and more strictly speaking, after the intense decomposition of gravel beds into soft reddish soil, akin to the "Kusari-ishi" in East Japan and before the early Late Yayoi earthen-ware stage of the Japanese pre-historic age (about 100 years B.C.).
The Kusari-ishi in East Japan has been supposed to be formed in pre-Shimosueyoshi stage, Shimosueyoshi stage possibly corresponding to Riss /Würm interglacial age.

日本語

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676