Chapter 2 - Frank Rockslide, Alberta, Canada

ABSTRACT The landslide from the east face of Turtle Mountain that destroyed the southern end of the town of Frank in the Crowsnest Pass area of southwestern Alberta, Canada, has become a classic example of mass movement because it is one of the largest landslides for which eyewitness accounts and a contemporary scientific examination exist. Most accounts rely on reports published shortly after the slide's occurrence in 1903. In these accounts the slide was reported to have moved down the dip of a steeply inclined set of joints and has therefore been classified variously as a rockfall or block flow. Later regional and geological mapping has shown that the crest of Turtle Mountain lay close to the axis of the Turtle Mountain anticline. A new detailed map shows that the slide mass lay on the steeply eastward-dipping limb of the anticline and the crest of the fold lay very close to the crown of the slide. The slide probably took place on bedding surfaces with the orientation of the scarp and lateral margins of the slide controlled by joint sets. A surface of rupture close to the toe of the slide followed a minor thrust above the Turtle Mountain fault. The phenomenon is better classified as a rockslide avalanche. Shear tests on bedding planes in blocks taken from the slide debris give values for coefficients of friction on surfaces showing flexural slip close to those calculated for the slide mass with a factor of safety of 1. This strengthens the interpretation of the new failure mode proposed for the Frank slide.