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Affected Environment and Environmental Consequences extend into colluvium, alluvium, and weathered bedrock. As demonstrated by the existing West Elk mining experience, tension cracks in unconsolidated sediment and weathered bedrock will likely heal a few years after mining ceases. Tension cracks that form in durable sandstone outcrops were observed to grow in length and width for up to one year. No changes were observed thereafter (Agapito 2005). Maximum crack depth is a function of lithology and the depth of the neutral surface. Shale and other weak rock types tend to attenuate fracture propagation. Dunrud (WWE 1998) estimates a maximum fracture depth of 50 to 100 feet in a single seam. Where double-seam extraction takes place, additional surface fracture depths could extend 100 to 200 feet below ground surface. Surface tension cracks caused by subsidence would most likely occur on ridges and steeper slopes, particularly cliff areas, where cracks might open on the order of a few inches to possibly 1 foot wide and 25 to 50 feet deep for a single seam. Where both E and B Seams are mined, this estimate could double (Agapito 2005, see Appendix B). Fewer cracks would occur in drainage areas than on ridges because the drainage areas are more stable and any alluviallcolluvial 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 aggravate the movement of existing landslides and rock falls. There are only two reports in the North Fork Valley of confirmed mining-induced landslides or rockfalls. If subsidence were to induce these activities, the result would be a feature that may not be readily identifiable from naturally caused mass movements. 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- mising topography, albeit lower in elevation. Chapter 3 Soils The direct disturbances resulting from subsidence versus that from potential post-leasing surface use are expected to have notably different impacts. Assuming the RFMP (Section 1.8.1), impacts to the soil resource due to subsidence would include cracks and other surface manifestations in areas of shallow overburden, where surface rocks are brittle, or where soils are shallow over bedrock. Soil cracking is most likely to occur at the ends of individual longwall panels and over the gate roads where the land surface is left in a tensional state after mining. Subsidence cracks that might develop in soil or colluvium tend to self-heal due to sloughing and natural filling by soil material. This type of disturbance to soils at the surface are likely to heal a few years after mining is complete. Subsidence has potential to affect surface water channels and basins and could result in increased rates of erosion. Soil erosion within drainage basins and resultant sediment loading may be increased until ground movements associated with subsidence stabilize relative to natural conditions. Post-lease surface use of approximately 60 acres could occur over the life of the lease as a result of access roads, exploration, installation of mine support structures, and related activities. Site- specific locations of potential surface activities are unknown, but are expected to be distributed throughout the LBA tract. Effects of post-leasing activities will be evaluated under NEPA on their own merits if/when activities are specifically proposed. These activities would be conducted in accordance with regulations administered by the CDMG. These regulations require detailed surface use plans that ensure the soil resource is salvaged prior to surface use and replaced as part of the reclamation processes. Measures would be developed and required as necessary to amend nutrient levels, control erosion, alleviate compaction, and mitigate other soil resource impacts, including those resulting from subsidence. However, even with implementation of these management practices, some soil loss may occur and disturbed soils may temporarily or permanently exhibit reduced productivity. At other sites, revegetation has been 3-12 Dry Fork Lease-By-Application FEIS