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Memo to Boulay <br />Jumbo Mountain Landsliding <br />page 5 <br />landslide scarp. Photographs dating from September, 1983 show .the <br />Muffler Rock scarp to be relatively stable, with a veneer of grass <br />and some relatively mature evergreen trees. A portion of ground <br />(approximately 1/4 acre) above and south of the main scarp, appears <br />to be surrounded by a minor head scarp, perhaps five feet in <br />maximum height. This settled block suggests that a wedge of ground <br />(soil and bedrock) immediately upslope from the main scarp was in <br />the process of failing in 1983. These artifacts, common to <br />landslide masses, suggest the Muffler Rock landslide had <br />experienced an extended period of relative stability, perhaps <br />spanning several decades. However, a block adjacent to the main <br />scarp was in the process of moving downslope. <br />Aerial photographs dating from November, 1995, show increasing <br />instability of the soil horizon of Muffler Rock. Scree chutes have <br />developed between tree stands. The grass has been disrupted within <br />these chutes, which are now the locus for downslope sliding of <br />small sized rocks and debris. The scarp surrounding the settled <br />block noted in the 1983 photographs appears to have increased in <br />height. <br />July, 1996 photographs show the Muffler Rock main scarp becoming <br />more unstable. The upper third of the western portion of the main <br />scarp is losing its soil veneer and trees. The talus and debris at <br />the foot of the scree chutes is forming into lobes. <br />The final photograph examined, dated July, 1997, shows the majority <br />of the main Muffler Rock scarp to be devoid of trees. The scarp is <br />now largely debris and talus covered. Perhaps 80$ of the trees <br />have been removed by the landslide. The western two thirds of the <br />settled block has settled and been decapitated by sliding laterally <br />onto the main scarp. The eastern third of the settled block now <br />contains several head scarps, suggesting that it is extending and <br />sliding laterally towards the main scarp. The debris pile at the <br />toe of the main scarp has overrun several acres of scrub oak. <br />In summary, the occurrence of subsidence may have accelerated the <br />occurrence of this most recent episode of instability of the <br />Muffler Rock scarp. Evidence of increasing instability is present <br />before the actual passage of the longwall face beneath the scarp in <br />April of 1997. In my opinion it would be impossible to <br />definitively determine whether subsidence had triggered this latest <br />episode of mass wasting of Muffler Rock. <br />Recurrence of Jumbo Mountain Landslide Activity <br />My field and aerial photographic examination of the Jumbo Mountain <br />north slope landslide complex discerned evidence of recurrent <br />