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2005 Subsidence and Geologic Field Observations Box Canyon, West Flatiron, Apache Rocks, and South of Divide Mining Areas No subsidence impacts were observed at Twin Ponds (ponds P27-2 and P27-3). These ponds are located above the headgate entries of longwall panel 15 in 900 to 925 feet of overburden to the B-Seam in alluvium and colluvium with an estimated thickness range of 50 to 75 feet. As observed in July 2004, no subsidence effects were noted on pond P23-4. The estimated thickness of the clay and silt alluvium beneath the pond is 15 to 30 feet. In addition to pond P23-4 being underlain by material that is unusually thin (the thinnest observed in the traverses this year), it is also located above the eastern solid coal boundary of mined longwal] pane] 22, where tensile strain is predicted to be at a maximum level. Based on these observations at Twin Ponds and pond P23-4, longwall mining is not expected [o impact stock ponds in Sou[h of Divide mining area. The other ponds visited in July 2005 were underlain by thicker material that had at least as much clay present as is present beneath pond P23-4. The duration of subsidence cracks (length of time that subsidence cracks are noticeably present) is a function of the hardness and durability of the material in which they occur. Subsidence cracks in su~cial material, such as colluvium and alluvium, heal and seal up more rapidly than cracks in any type of bedrock. Subsidence cracks in soft bedrock in the western part of the Apache Rocks mining area, which were locally abundant on ridges above longwall panel 13 when it was mined in 1999, are either no longer visible or are much less visible than they were in July 2004. The cracks in this soft, weathered sandstone have filled and sealed up in response to the forces of weathering, erosion, and deposition, in the six years since longwall panel 13 was mined. The subsidence cracks in the hard, durable sandstone at Apache Rocks, however, are very similar in appearance to when they first were observed in 2000 (Dunrud 2000). Weathering, erosion, and deposition will be slow in this area. These cracks are expected to be evident for many years or decades to come. No mining effects were observed on the landslides of the Box Canyon and Apache Rocks mining areas. For example, the landslides located on the southwest flank of West Flatiron above mined longwall panels 14 and 15 were not noticeably affected by either subsidence or mine-related seismic activity. However, the rockfall area at the head scarp of the first east drainage of Sylvester Gulch above mined longwall panel 22 showed renewed activity shortly after it was 831-032.710 Wright Water Engineers, Inc. Page 2 November 2005