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Analyses of the surface effects of past underground coal mining in the Sheridan, Wyoming, area suggest that underground mining of strippable coal deposits may damage the environment more over long periods of time than would modern surface mining, provided proper restoration pro­ cedures are followed after surface mining. Subsidence depressions and pits are a continuing hazard to the environ­ ment and to man's activities in the Sheridan, Wyo., area above abandoned underground mines in weak overburden less than about 60 m thick and where the overburden is less than about 10-15 times the thickness of coal mined. In addi­ tion, fires commonly start by spontaneous ignition when water and air enter the abandoned mine workings via sub­ sidence cracks and pits. The fires can then spread to unmined coal as they create more cavities, more subsidence, and more cracks and pits through which air can circulate. In modern surface mining operations the total land surface underlain by minable coal is removed to expose the coal. The coal is removed, the overburden and topsoil are replaced, and the land is regraded and revegetated. The land, although disturbed, can be more easily restored and put back into use than can land underlain by abandoned underground mine workings in areas where the overburden is less than about 60 m thick or less than about 10-15 times the thickness of coal mined. The resource recovery of modern surface mining com­ monly is much greater than that of underground mining pro­ cedures. Although present-day underground mining technology is advanced as compared to that of 25-80 years ago, subsidence resulting from underground mining of thick coal beds beneath overburden less than about 60 m thick can still cause greater damage to surface drainage, ground water, and vegetation than can properly designed surface mining operations. This report discusses (1) the geology and surface and underground effects of former large-scale underground coal mining in a 50-km2 area 5-20 km north of Sheridan, Wyo., (2) a ground and aerial reconnaissance study of a 5-km2 coal mining area 8-10 km west of Sheridan, and (3) some environ­ mental consequences and problems caused by coal mining. INTRODUCTION Thick coal deposits, representing hundreds of billions of tons of low-sulfur subbituminous coal, are present in the Powder River Basin of Wyoming and Montana (fig. 1). The coal occurs beneath overburden composed of weak bedrock and surficial deposits that range in thickness from a few meters to as much as 900 m. A con­ troversy currently exists over the relative merits of mining strippable coal deposits by under­ ground and surface methods. Many mining and land-use planners contend that the many billions of tons of strippable coal in the Powder River Basin can be mined more safely and much more completely by surface methods than by under­ ground methods, particularly in areas where thick coal beds occur beneath soft bedrock and surficial material less than 60 m thick. On the other hand, environmental groups are con­ cerned that surface mining might be more dam­ aging to the environment than would under­ ground mining. Surface mining operations disturb the total land surface of the mining area during the mining cycle (figs. 2, 3). The topsoil and remaining over­ burden commonly are removed by earth-moving scrapers or by large draglines; the coal is then removed by draglines or power shovels, and trucks. In modern surface mining operations, in contrast to those of the past, the overburden is replaced and graded, the topsoil is replaced, and the land is revegetated (figs. 2, 3). In a few years or a few decades, depending on the climate and on EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA 108° 107° 106° 105° 104°

Effects of coal mine subsidence in the Sheridan, Wyoming, area