Before the Frank Slide

The Frank Slide occurred on the east limb of the Turtle Mountain Anticline, which was thrust up along the folded and splayed Turtle Mountain Fault. Easterly dipping, Paleozoic limestones and dolomites then rested on sheared, weaker, Mesozoic clastic rocks and coal strata. Cordilleran glaciers steepened the eastern flank of Turtle Mountain but left buttressing kame moraines. These were eroded by the Crowsnest River, which was pushed against Turtle Mountain between its North and South Peaks by the growth of the alluvial fan of Gold Creek. Blairmore Group mudstones and shales beneath the moraines were susceptible to toppling. Photographs of the east slope of Turtle Mountain before the Frank Slide show disturbed vegetation, uneven topography, steep slopes, and rock fall deposits, all consistent with active slope movements. The Frank Slide may have been triggered by the freezing of melting snow in rock joints and by coal mining. Our calculations show that, while individual stopes may have been unstable, the mine pillars and the coal mine itself were stable. Numerical simulations of the coal mining in the Frank Mine suggest mining reduced the strength of the east slope of Turtle Mountain by less than 10% before the Frank Slide.Key words: Rocky Mountains, Frank Slide, coal mining, river erosion, toppling.