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E!'eCerr4t9~z 4.7_J03 PayF 3-2 cliffs that may have rock fall potential depending on bedrock integrity. Geologic hazards and steep slope areas within the LBA tract are shown on Figure 3-1, Geologic Hazards Map. 3.2.2 Environmental Consequences Alternative 1 -Proposed Action The actual leasing of the LBA tract would impose no topographic change on the area. If this tract is subject to future underground longwall mining, there would be subsidence as a result of mining activities. Subsidence would be most notable on ridges and steeper slopes, particularly cliff areas, where cracks might open on the order of a few inches to possibly one foot wide and 25 to 50 feet deep (see Section 2.9, Reasonably Foreseeable Development Scenario). Fewer cracks would occur in drainage areas than on ridges because the drainage areas are more stable and any alluvial/colluvial materials found in these areas tend to be more yieldable than some of the brittle bedrock found on the ridges. Subsidence from longwall mining could contribute to the movement of existing landslides and rock falls. Topographic changes caused by subsidence with longwall mining are often unnoticeable to the naked eye. As longwall mining proceeds under a particular area, there would be some cracking on the surface. As mining concludes or proceeds away from the area, this surface cracking tends to disappear, although the elevation of the area would be lowered. In certain areas, such as where unconsolidated material occurs, the material tends to stretch but may or may not rupture when subsidence occurs, depending on the amount of overburden present. Other than lowering the land surface, the long-term effects of subsidence on surface topography would be minimal, and even unnoticeable to most casual observers. Some residual cracks may remain in the more brittle bedrock material on ridges or cliffs. Overall, the topography above subsided longwall mining workings would be similar to the pre-mining topography, albeit lower in elevation. Subsidence from underground mining could aggravate, and perhaps even accelerate, the existing landslides and rock falls in the area. Other natural factors may cause an acceleration of impacts, these factors being separate from subsidence. For example, in an extremely wet spring, the moisture from snowmelt and spring rains could cause these natural landslides and rock falls to move and shift. This seems to have been the case in the North Fork of the Gunnison River Valley in the mid 1980s during a period of intense precipitation and moisture. It is difficult to assess whether the naturally occurring landslide or other unstable areas have been aggravated by subsidence. There are no existing structures or facilities located above the West Flatiron LBA tract, so subsidence caused by mining activities would not be a factor in this matter. No structures or facilities are planned for construction or operation on the LBA tract. The unstable nature and slope steepness occurring over much of the lease tract preclude surface use. Cumulative impacts: The entire North Fork Valley region east of the Town of Paonia has numerous existing natural landslide and other unstable areas. These natural features would contribute to future changes in the topography of the area. Likewise, continued mining at the West Elk Mine would contribute to subsidence of the topography under the mining area. Encironm~ntal EsSSessma~;t vilest v-Sa:Sra? LBA Trac# Gunnison Ccur;y. Gclorado