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West Elk Mine the Oliver No. 2 Mine area, and in other azeas where water might be encountered in an area equal to, or greater than, the width of the groposed longwall panels, maximum vertical displacement may be expected to approach 0.8 times the coal-extraction thickness. Modern Methods of Ventilation Under modern methods of ventilation, high capacity fans exhaust return air from the mine. This procedure keeps the mine air pressure less than the outside air pressure. The mine is less likely to "gas up" because methane is continually removed from fractures in this negative-pressure environment. In the case of the Oliver No. 2 Mine, fresh air reportedly was blown into the mine with a 4-foot squirrel-cage blower (Boyd Emmons, Oral Communication, November 6, 1996). In this type ofpositive-pressure environment, the mine workings become more and more gassy if the fan does not have a sufficient air-moving capacity to ventilate sudden inflows of methane. Potential Impacts of Subsidence on Landslides In October 1996, large, composite landslides with numerous scarps were observed between the unnamed drainage west of Box Canyon and Box Canyon and also to the east of Box Canyon. As discussed in Section 2.04.6, the slides are compartmented into essentially separate slides by competent sandstone ledges. Grabens, sag ponds (complete with small wetlands), and tension cracks occur between the scarps. The tension cracks could easily be confused with subsidence cracks, but no mining has been done in the area. The landslides are located in or neaz the northern boundary of the Box Canyon permit revision area. The southern limit of the landslides is located at least 650 feet north of the nearest planned longwall panel-well outside of the conservative angle of draw. I fact of HiQhwav Construction Based on a review of aerial photographs taken before highway construction and on communication with Bob Barrett of CDOT, two apparently dormant landslides were located east and west of the mouth of Box Canyon. In November 1996, however this area is covered by extensive slides with vertical, horizontal, and rotational movements. These extensive slides began when the new highway was under construction on the south side of the North Fork valley. The lower portions of colluvial deposits and toes of the dormant landslides on these steep slopes were removed during construction, which produced steeper, less stable slopes. The oversteepened ground soon became unstable and began to slide. Some of the slides show movement as recently as a week to a few months old as of November 1996. Importance of Baseline Landslide Data Baseline information on landslides in the Box Canyon permit revision area needs to be carefully documented prior to mining. MCC will plan to take aerial photographs of the Box Canyon Lease azea and field verify any landslides seen in the photograph, prior to mining. Some of the landslide features (cracks and grabens) closely resemble subsidence feamres. However, it is Mr. Dunmd's opinion that coal mine subsidence could not cause landslides nearly as severe as those 2.05-130 RevuedJw. l995 PR06; IN6 RN03; RevisedJmi. 1998 PR0.4