Page 480 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

444 part III The earth–Atmosphere Interface
▲Figure 14.25 talus slope. Rockfall and talus deposits at the base of a steep slope along Duve Fjord, Nordaustlandet, Svalbard. Can you see the lighter rock strata above that are the source for the three talus cones? [Bobbé Christopherson.]
mass-wasting event was great enough to classify it as an earthflow, caused by sandstone formations resting on weak shale and siltstone, which became moistened and soft, and eventually failed with the weight of the overlying strata (Figure 14.27).
Figure 14.24b shows an earthflow in the Mach- iche River valley, in Trois-Rivières, Québec, known as the St. Boniface landslide. It occurred in late April 1996, causing approximately 7 million m3 of sediment to slide into the Machiche River valley, damming the river. This type of retrogressive earthflow can occur within sensitive glaciomarine sediments common to the St. Lawrence Lowlands and Ottawa Valley regions. In these flows, the headwall erodes back into the val- ley side, and the landslide debris flows toward the river, away from the scarp. In January
2005, more than 300 mm of rain in less
than a week saturated the mountainside
in North Vancouver, British Columbia,
resulting in mudslides that caused mil-
lions of dollars of damage and claimed
one life.
▲Figure 14.26 Debris avalanche, Peru. A 1970 debris avalanche falls more than 4100 m down the west face of Nevado Huascarán, burying the city of yungay, Peru. The same area was devastated by a similar avalanche in 1962 and by others in pre-Columbian times. The towns in the valley remain at great risk from possible future mass movements. [George Plafker, USGS.]
individual soil particles are lifted and disturbed, whether by the expansion of soil moisture as it freezes, by cycles of moistness and dryness, by diurnal temperature varia- tions, or by grazing livestock or digging animals.
A dramatic example of a debris flow that did not kill occurred in Banff National Park, Alberta, in August 1999. The failure was unexpected and resulted from an intense rainfall that was unre- corded because local weather stations were outside the area of focused precipi- tation. Debris blocked the highway for 24 hours, and delayed thousands of travel- lers in Banff for a few days while cleanup occurred (Figure 14.24c).
Creep A persistent, gradual mass move- ment of surface soil is soil creep. In creep,
▲Figure 14.27 the Gros Ventre earthflow near Jackson Hole, Wyoming. evidence of the 1925 earthflow is still visible after more than 89 years. [Steven K. Huhtala.]