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FIRM BEDDED ROCK ., Figure 4. A rockfall. Slides Although many types of mass movement are included in the general term "landslide," the more restrictive use of the term refers to only "those mass movements where there is a distinct surface of rupture or zone of weakness which separates the slide material from more stable underlying material" (Rogers and others, 1974, p. 18). The two major types of landslides are rotational slides and translational slides. Rotational slide A rotational slide is one in which the surface of rupture is curved concavely upward and the slide movement is more or less rotational about an axis that is parallel across the slope (FIgUre 6). The scarp formed at the head of the slide may be almost vertical since the move- ment at that point is almost wholly downward. The toe usually bulges upward, but sometimes flows outward. A "slump" is an elllI1llPle of a small rotational slide (Varnes, 1978; U.S. Geological Survey, 1981, 1982). Translational slide In a translational slide, the mass moves out, or down and out along a more or less planar surface and has little rotational movement or backward tilting (FIgUre 7). The mass commonly slides out on the original ground sur- face. Such a slide may progress over great areas if con- ditions are right. The movement of translational slides is commonly controlled by surfaces of weakness such as faults, bedding planes, and variations in shear strength between layers of bedded deposits, or by the contact between firm bedrock and overlying loose soils (Varnes, 1978). Slide material may range from loose uncon- solidated soils to extensive slabs of rock. A block slide is a translational slide in which the moving mass consists of a single unit, or a few closely related units that move downslope as a single unit (Varnes, 1978). H the slide material is a solid block of 12 a. , ' : . .. ~ --'--r" . . .....:..,.......~.. -. ------- ------- - - ---=- -=- ~ ---=--= ---------- -=------------=---=--=-~ -----=--=---=- - - - - ---=- --=- --=- -=- --=----= - -=-- -=- --=- -=-----..-=- --=- --=- -=- --=--= b. .. '. .'" '7 , ~ ...J.---T .. . '~:..':~' -= : - ---=--=--== - ---=--- - --=--=--= --- c. :J--- . . . ~. . . o I o I 0 ';- f 0 (f)L:-_ . .-->--. . ;l . ' ~"" --- \I .;:~.: ---~ - I '..:. -=--=--- .-A- >..... -=--=- - ~~i .. = A -- -=--=--=--=--= o --- ----- '..- ~ Figure 5. oeles in the process of toppling: a) separating from the main rock mass and then. b) rotating a d falling on the slope below. and c) after brea 'ng into fragments. bounding down the hillsi