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<br />Memo to Herry/Boulay <br />Jumbo Mountain Landaliding <br />page 4 <br />saplings and scrub oak, suggesting ages of <br />these estimates of age are based on my <br />grossly flawed, the comparative age of <br />active landsliding demonstrate that <br />landsliding have affected this area. <br />Subsidence Cracking on Jumbo Mountain <br />several decades. While <br />experience and may be <br />these features and the <br />recurrent episodes of <br />At several locations during our tour of the site I examined <br />cracking which appeared indicative of subsidence troughing without <br />landslide translational movement. Sites designated on the MCC maps <br />as #2, #3 and #8 fall into this category. These cracks are <br />generally smaller in width (2 to 6 inches), and demonstrate no <br />translational or lateral offset between the opposite faces of the <br />cracks. Their orientation tends to sub-parallel the axis of the <br />panel or the longwall face. <br />Relationship of Subsidence and Landslide Occurrence <br />Because of the proliferation of landslides throughout the North <br />Fork valley, the Division has repeatedly expressed a concern <br />regarding the possible reactivation of ancient, historic and modern <br />landslides during mine permit review. This same concern is <br />commonly expressed by land owners and the public. The Division has <br />included permit conditions requiring every mine with mapped <br />landslide deposits above projected subsidence to monitor the <br />stability of known landslide deposits to determine whether any <br />reactivation were to occur. The Bear Mines and the West Elk Mine <br />were both required to monitor known landslides adjacent to <br />Somerset. While these landslides subsided several feet no <br />reactivation has occurred. The Orchard Valley mine (Bowie #1) was <br />required to monitor known landslides in Stevens Gulch and adjacent <br />to its Portal facilities. None of these landslides evidenced <br />reactivation. These same requirements have also been placed on <br />underground coal and hardrock mines in other areas of the State of <br />Colorado. To date, while they have been subsided, none of the <br />monitored landslides have evidenced any reactivation. At the same <br />time, every one of these mines has continued to evidence landslide <br />occurrence, both in and outside subsided areas within the permit <br />and adjacent areas. Based upon this experience, I conclude that <br />subsidence does not appear to be a significant determinant in the <br />reactivation or initiation of landslide activity. <br />Land Use Implications of Landaliding <br />As mentioned previously, landsliding is prolific throughout the <br />North Fork valley. Landowners in the valley are aware of the <br />