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Memo to Boulay <br />Jumbo Mountain Landaliding <br />page 6 <br />Recurrence of Jumbo Mountain Landslide Activity <br />My field and aerial photographic examination of the Jumbo Mountain north <br />slope landslide complex discerned evidence of recurrent modern (since <br />white man's occupation) activity. In several areas, predominantly in the <br />middle to higher portion of the landslide mass, closed extensional <br />troughs are present. These undrained graben-like topographic troughs are <br />caused by downslope movement of landslides pulling away from the upslope <br />headscarps. The most impressive of these features examined measured <br />approximately 30 feet across and twenty feet in depth, extending for a <br />length of approximately 100 feet. Several of these troughs have been the <br />locus of "natural" stock watering ponds. Unfortunately, this practice <br />provides infiltration water on the landslide mass, which is not conducive <br />to stabilization of the landslide materials. One such pond is referenced <br />by Mr. Mautz as having been dewatered by the 1996 landslide. Judging <br />from the size of tree growth and the degree of smoothing by weathering <br />of the graben side slopes, I estimate that these larger features are in <br />excess of a century in age. I also examined several smaller troughs with <br />widths of five to ten feet and depths in the five foot range. These <br />smaller grabens were less weathered, retaining smoothed but steep-sided <br />trapezoidal shapes, and were less vegetated with small aspen saplings and <br />scrub oak, suggesting ages of several decades. While these estimates of <br />age are based on my experience and may be grossly flawed, the comparative <br />age of these features and the active landsliding demonstrate that <br />recurrent episodes of landsliding have affected this area, separated by <br />periods of relative stability. <br />Subsidence Cracking on Jumbo Mountain <br />At several locations during my 1996 and 1997 tours of the site, I <br />examined cracking which appeared indicative of subsidence troughing <br />without landslide translational movement. These cracks are generally <br />limited in width (2 to 6 inches), and demonstrate no translational or <br />lateral offset between the opposite faces of the cracks. Their <br />orientation tends to sub-parallel the axis of the panel or the longwall <br />face. In contrast, landslide cracking is prolific throughout much of the <br />Jumbo Mountain north slope, contrastingly characterized by extensive <br />translation and offset across crack planes. <br />Relationship of Subsidence and Landslide Occurrence <br />Because of the proliferation of landslides throughout the North Fork <br />