A 19th century slide, involving an estimated 25 × 106 m3 of rock, devastated Rubble Creek Valley, 80 km north of Vancouver, B.C. Breaking away from a headwall composed of late glacial dacitic lava, the slide travelled as much as 4.6 km with a maximum drop of 1060 m, thus moving on an average slope of 8.5°. Velocities, estimated from the superelevation of the slide as it moved around curves in the valley, exceed 20 m/s (72 km/h) and sliding was probably completed within 10 min. Tree-ring data indicate that the slide occurred in the fall or winter of 1855–1856. The trigger mechanism has not been identified, but the presence of an exceedingly steep original slope of the lava front, attributed to ponding against latest Pleistocene ice occupying the valley below, was clearly a contributing factor. Both the precipitous headwall and a second ice-dammed lava front are considered to be potential sources for new slides. Some evidence suggests that previous slides have occurred here since the last glaciation, about 11 000 years ago. A court ruling barring residential development in the area devastated in 1855–1856 on the grounds of future hazard to life seems justified.

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