This paper describes slope forms and landslide activities in the area underlain by Tertiary mudstone of the Teradomari and Shiiya Formations of Niigata Prefecture, Frequency distribution of angle for randomly sampled slopes in the area showed that the angle of 15 degrees was defined as a criterion for classifying of these slopes. Two types of landslide have been identified in the area: (1) Rapid-type landslides, characterized by their higher speed and larger morphological change, occur in the weathered zone of the mudstone, and (2) Slow-type landslides having a characteristics of their slower speed and smaller morphological change occur in the colluvial deposits derived from previous landslides. The investigation for a large number of slope profiles of these landslides indicated that the rapid-type landslides originally arised on a steeper slope segment than 15 degrees and the slow-type landslides were restricted to a gentler slope segment than 15 degrees.
