Coal availability in the Hilight quadrangle, Powder River Basin, Wyoming: a prototype study in a western coal field
by Carol L. Molnia, Laura R. H. Biewick, Dorsey Blake, Susan J. Tewalt, M. Devereux Carter (US Geological Survey), and Charlie Gaskill (Bureau of Land Management)
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Figure 8. Map of Hilight quadrangle showing areas where overburden for the Main Wyodak coal bed is 0-300 ft thick (white), and areas where this overburden is greater than 300 ft thick (green). For the purposes of this study, we assumed that 300 ft of overburden would be the limit for surface mining. Maximum overburden for the Main Wyodak coal bed in the Hilight quadrangle exceeds 600 ft. 
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U.S. Geological Survey Open-File Report 97-469