Abstract
　The geological aspects of the 1989 rock-fall disaster at Tamagawa, the Niu Mountains, Fukui Prefecture were investigated. In order to know what kind of geologic processes prevails in the Tamagawa area, we analyzed the topographic characteristics of the Niu Mountains and the adjacent sea-bottom and the petrologic properties of rocks at the rock-fall site. This area was currently uplifted by geodynamic compression with a very high speed (the average speed during the last 125,000 years is about 0.08cm/yr). Under-cutting of the coastal topography (0.2cm/yr) by the erosive action of sea waves formed steep high cliffs (l00m or taller). At the cliffs, rocks were released from the confining pressure, and many joints parallel to the cliff formed. The cohesive forces along joints are gradually lowered by water-rock interaction. Consequently, rock blocks are successively truncated at the joints. A simple calculation suggests that rock-fall events with similar volume to the 1989 event will take place every 10 years or more along the distance of 1 km of the cliff of this area and that the reccurrence time of rock-fall event with this volume is nearly 2,000 years or more at every site. Knowledge of weathering rates of rocks is necessary to predict when and where future rock-fall events will take place.

【キーワード】：トッブリング，海食，続成作用，：上昇速度，地殻変動
Key words: toppling, coastal erosion, diagenesis, uplift rate, tectonics

*福井大学教育学部地学教室
Geological Laboratory, Fukui University
**福井大学工学部環境設計工学教室
Departmcnt of Architecture and Civil Engineering, Fukui University
***名古屋大学理学部地球惑星科学教室
Department of Earth and Planetary Sciences, Nagoya University
****愛知女子短期大学一般教育学科
Department of General Education, Aichi Women College.