The Cheam rock avalanche, Fraser Valley, British Columbia, Canada

The Cheam rock avalanche, which occurred about 5,000 years ago in the lower Fraser Valley, British Columbia, is the largest known catastrophic landslide in western Canada (175x10(6) m(3)). A photo-draped digital elevation model of the rock avalanche reveals two morphologically distinct areas, an eastern area of arcuate hummocky ridges separated by flat-floored depressions and a lower western area with a subdued, gently rolling surface. Debris is up to 30 m thick and consists of rubbly, clast-and matrix-supported diamicton derived from local argillaceous metasedimentary rocks. Failure was probably caused by high pore water pressures on a thrust fault that daylights in the source area. Plastic deformation of sediment beneath the rock avalanche debris suggests that liquefaction occurred due to undrained loading when the debris struck the Cheam terrace. Liquefaction also explains the morphology and travel distance of the western debris lobe. The coincidence of well-sorted sands (the Popkum Series soil) with the rock avalanche debris indicates that significant amounts of water flowed over the surface of the landslide just after it came to rest. Sto:lo Nation oral history suggests that the debris may have buried a village, causing the first known landslide fatalities in Canada.