0n the ancient mega-landslide along the Three Gorges Dam Reservoir, Yangtze River, China

The Three Gorges Dam, which is one of the biggest projects this Century in the world, is being constructed at Sandonpong in the middle reaches of the Yangtze River. The authors observed many ancient mega (to gigantic) landslides (in the broad sense, including slope failure and rock fall) along the both banks of the dam reservoir. Three Gorges, namely Qutang, Wu and Xiling Gorges, are composed of landslide blocks and their topography, is famous as one of the attractive spots of the Yangtze River cruise. Geology of the reservoir area is mainly composed of the Paleozoic-Mesozoic stratified sedimentary rocks, and that explains the geologic structure and mass-moved deformed structure which can be observed clearly. At the immediate upper stream of the Three Gorges, we can observe a horizontally stratified semi-consolidated silty layer about 20 meters thick, which made up the fluvial terrace-like morphology. It seems that the Pleistocene sediment was deposited in a stagnant water area and it suggests the existence of an ancient natural landslide-dam downstream. Furthermore, the basement geology of the Fenjie City and Baidicheng Town are composed of ancient landslide debris deposits. Sediment accumulation and reactivation of ancient landslides in the Three Gorges Dam reservoir are very difficult problems for this big project. There is some high potentiality that the ancient mass movement deposits distributed along both sides of the Yangtze River, will trigger reactivated landslides with the storage of water. The detailed investigation of ancient landslides is an important subject for this big project