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' Memorandum to Kathy Welt <br />August 28, 1997 <br />Page 2 <br />3. Landslides and flows of various shapes, ages, and extents, which locally contain <br />hummocky topography, ponds, and scarps, can be observed on the 2963 USGS black <br />and white aerial photos and on the 1975 IntraSeazch color aerial photos of the azes. <br />Landslide scarps and landslide deposits were also mapped by Dunrud (1989) in the <br />azea in the eazly 1980s. <br />4. There aze significantly more landslides in the azea than were observed during my <br />mapping of the azea in the late 1970s. Many movements aze reactivations of <br />pree.Yisting slides during the very wet periods in the mid 1980s and this spring. <br />5. Some of the cracks above Panels 8 and 9 occur in material that seems to have been <br />raised up or bulged in a manner typical of landslide processes, bat not typical of <br />subsidence processes above longwall panels. <br />Jumbo Mountain Sites trsifed <br />The following describes the sites of surface cracks visited on the north side of Jumbo <br />Mountain on August 6, 1997. With the exception of the Muffler Rock slide and the Pane19 <br />cracks, locations aze referenced to the ground crack location map that was compiled after the <br />CDMG inspection of the azea on July 23, 1996 (see Map 1 showing locations of cracks as <br />seen during CDMG [Jim Pendleton and others] inspection and enclosed video by Dunrud <br />made during this field visit; numbers identify video locations): <br />Landslide at Muffler Rock (landslide scarp so called by Larry Mautz: The slide <br />reportedly was first observed in April 1997; the longwall face in Panel 9 passed <br />beneath the area in April 1997. An azcuate mass of rock about 40-feet thick, 50 <br />feet in the north-south direction, and perhaps 500 feet in the east-west direction <br />slid northwazd about 125 feet subpazallel to the dip (about 5 degrees). Much of <br />the material then spilled over an existing landslide scarp (about X00-feet high), <br />which I had mapped in the late 1970s. The material involved in the slide this <br />spring included sandstones, silistones, and clays of the Barren Member of the <br />Mesaverde Formation (Dunrud 1989). We observed broken material rolling <br />down the scarp during our visit. Later, we observed sigtuficant flow north of <br />the toe of the Muffler Rock landslide. The head scarp of the slide this spring is <br />neazly vertical and 30- to 40-feet high; failure occurred along two roughly <br />conjugate sets of joints trending approximately northeast and northwest. The <br />Muffler Rock slide moved northwazd and the longwall face moved eastward. <br />Therefore, the tilt, curvature, and strain produced by the eastw~azd movement of <br />the face and attendant subsidence would create a small impact on the stability of <br />the Muffler Rock landslide scarp, particularly when contrasted with the impact <br />of the very wet spring and summer. <br /> <br />