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Abstract

Distinctive suites of landslides occur in five stratigraphic-structural provinces in the Foothills area of southwestern Alberta. The Porcupine Hills are characterized by slumps and earthflows on slopes steepened by fluvial activity. The ridges of the Rocky Mountain Foothills have a low frequency of landsliding due to slope angles that are generally lower than bedding-plane dip angles. Extensive landsliding around the Mokowan Butte upland is likely due to shearing of bedrock beneath the Lewis thrust and glacial oversteepening. Glaciolacustrine valley fills form the floors of interridge valleys within the Foothills. These glaciolacustrine sediments fail as rotational slumps and flows. Rockslides and rock avalanches cluster along major thrust faults in the eastern Rocky Mountains. Glacial steepening and the exposure of cliff-forming Proterozoic and Paleozoic carbonates and clastics overlying recessive clastics, particularly along thrust faults, are identified as significant destabilizing factors. A mass-wasting feedback loop is suggested, cliff-forming massifs driving failure in underlying recessive rocks, which in turn triggers failures in the massif. Creep is suspected as a factor in footwall slope instability. Landsliding has likely been a prime agent in the retreat of the mountain front for at least the past 2.6 m.y. Recession rates of 0.2 cm/yr can be computed for this period.

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