Laserfiche WebLink
• current movement, also occurs in many places above the lower bench within the limits of the <br />old landslide shown on Figure 1. These observations suggest that the azea has had a long and <br />probably continuous history of slope movements. <br />Nature of Lineaments <br />Several lineaments were noted on the aerial photographs and aze shown in Figure 1. These <br />lineaments appeaz to delineate the upslope extent of past movements of the old landslide mass. <br />"There are also two prominent lineaments which form drainage lines intersecting the central part <br />of the slide mass. There is no evidence that any of these lineaments aze the surface expressions <br />of faults in the underlying bedrock. In fact, structure contours on the base of the 'B' seam <br />(Figure 8) aze relatively lineaz and evenly spaced, suggesting that there is no significant faulting <br />of that stratum in this area. It should be noted that a smoothing program was used to plot those <br />structure contours. The initial plot, prior to smoothing, showed anomalous features near the <br />northern margin of the azea, beneath the old landslide deposits of the lower bench. The <br />anomalies may be due to errors in elevations recorded for some of the older borings. However, <br />since this azea is well to the north of the proposed longwall mine there would not appeaz to be <br />• any point in further investigation. <br />DISCUSSION <br />The available evidence indicates that the Jumbo Mountain landslide is an ancient feature. The <br />lower bench portion of the landslide mass does not conform to the mound-like shape which <br />usually characterizes the lower part of a large slide. It is likely that this bench represents only <br />a small remnant of the original mass, the rest having been removed by erosion, over a pericd <br />of hundreds or thousands of yeazs. <br />The North Fork Gunnison River valley has a long history of slope instability of which the Jumbo <br />Mountain landslide represents only one example. Because the Jumbo Mountain landslide had <br />the effect of reducing the overall valley slope in this azea, the potential for additional lazge-scale <br />sliding on Jumbo Mountain is less than for other parts of the valley which have not been <br />similazly affected. <br />r~ <br />LJ <br />-11- <br />0:\Off\30\JPT\ARCO-Rpt.lkl <br />