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I I I I I I I I I I I I I I I I I I I areas is straightforward in most places, the physical parameters that determine frequency and extent of rock movements are poorly understood. Risk determination at a site, if possible at all, is dependent on (1) complicated physical analysis involving size, shape and mass of potentially mobile rock particles; (2) texture of the slope surface down which rocks might move; (3) local ground-moisture conditions (including man-made modi- fications); and (4) local potential for freeze-thaw cycles, intense rainfall, and ground shaking (seismicity). Debris avalanches and debris slides. Debris avalanches and debris slides are most common on south-facing, moderate to steep slopes or places where colluvium, particularly that of granitic origin, is relatively thin (typically less than 2 m thick). The most important difference between debris avalanches and debris slides is their rate of movement. Debris avalanches result when an extensive thin sheet or elongate chute of colluvial material is rapidly mobilized en masse. Debris slides, on the other hand, are intermediate in rate between debris avalanching and soil creep. In either case, it appears that either process takes place when a threshold involving ground water content and intergranular pore water pressure is ex- ceeded. Mode of debris movement is then dependent on slope and composition of the debris (Johnson, 1970; Rodine, 1974). Field work in the Big Thompson Canyon shows that colluvium derived from granitic rocks is typically more permeable than that derived from metamorphic rocks owing to less well- developed soils and preponderance of granule- to boulder-size clasts on steep to very steep slopes. However, small debris avalanches and debris slides have been observed in colluvium derived from metamorphic rock in which larger flaggy clasts make up most of the deposit and vegetation cover is sparse. 29