The writer has studied the genesis and evolution of yamahida (ravines) on the steep slopes of the western Tanzawa mountains, nearly 100km. west of Tokyo. His research was first aimed at seeking for the cause of yamahida genesis, and so fractured and jointed zones in lithology, debris-avalanche scars on slopes and so forth in this area were surveyed. This preliminary report, however, is primarily restricted to the fractured and jointed zones only, of which all those extending along the yamahida-bottoms and others dipping at the angle of more than 70° excluding the above-mentioned ones were dealt with. Distribution maps of these zones were also made. (Figs. 3-U1, U2, M and L). It has become clear as a result of this survey that a majority of yamahida in this area show a distribution pattern quite similar to that of the fractured and jointed zones. Thus this correlation may well validate that the existence of such fractured and jointed zones is a major cause of yamahida genesis at least in this area.