The author has been studying the causal relationship between natural hazards due to geological processes and the effects of heavy rainfalls on weathered granite during past five years. It was recognized that there was a close relationship between the present topography and the condition of weathered rock. The purpose of this paper is to discuss the relationship between a form of rock weathering and the topography of Hiroshima Prefecture and adjacent areas. As a study model, six layers excluding regolith and "idomasa" (colluvium composed of weathered granite) were clearly defined in the horizon of weathered granite: Chemical Weathered Layers I, II and III, and Physical Weathered Layers I, II and III, downwards. The present field investigations indicate that the deep weathered zone is found on the flat surfaces in different elevations such as on the Setouchi, the Sera and the Kibi Surfaces. Further considerations have been extended to the distribution of the deep weathered areas, the formation processes of the present topography and the geological time in which the deep weathering processes were active. It is proposed that the crustal movements which were due to the compression in a N-S direction during the early stage, and in a E-W direction during the later stage have strongly influenced the development of the landforms since the Pliocene.
