FRONTISPIECE .-Northward view across the North Fork Gunnison River showing the geologic and topographic setting in the Somerset coal mining district, Gunnison County, Colo. The rugged topography shown is characteristic of the Somerset mining district. The ridge in the left foreground overlies the mine workings that were selected for one of the subsidence studies. The Mesaverde Formation comprises the area from the light-tan ledges and cliffs in the foreground (Rollins Sandstone) to the ridge crests in the middleground. The coal-bearing portion of the Mesaverde is portrayed by the pinkish-red coloration in the left foreground, caused by the burning of the coal beds, which , in turn, has caused extensive subsidence. The Mancos Shale underlies the Mesaverde Formation. The light-tan ledges and cliffs above the Mesaverde (background) are the rocks of the Ohio Creek Formation. The subdued terrain in the background, with its extensive landslides and green aspen groves (right background), is developed on rocks of the Wasatch Formation, which overlies the Ohio Creek Formation. Some Middle Tertiary

TopographyThe central Utah area is a part of the Colorado Plateaus physiographic province. It is characterized by high plateaus bounded by steep cliffs cut by deep canyons, by precipitous rock ridges, by rock pinnacles and monuments, and by isolated mountain peaks. These are described briefly below.At the north edge of the area is the Roan (or Tavaputs) Plateau, which is divided into east and west parts by the canyon of the south-flowing Green River (called Desolation Canyon to the north and Gray Canyon to the south).The plateau is bounded on the south by the precipitous Book Cliffs.Elevations on the plateau reach nearly 3,140 m above sea level west of the Green River and 2,900 m east of the river. The plateau is dissected by deep canyons that contain streams tributary to the Green River or that flow generally southward across the Book Cliffs.At the west edge of the area is the Wasatch Plateau, which is bounded on the east by high cliffs similar to the Book Cliffs. Elevations on the higher drainage divides of the Wasatch Plateau range between about 2,900 m and 3,445 m above sea level.At the southern end of the area are the north-northwest-trending Henry Mountains, which are characterized by high domal peaks several miles apart separated by broad dissected saddle areas. Mount Ellen, the northernmost and highest peak, is 3,540 m above sea level. In the central part of the area is the north-northeast-trending San Rafael Swell, an uplifted area about 40 km wide and 113 km long marked by bare rock ridges, pinnacles, and narrow

Coal mine subsidence is the local lowering of the ground surface caused by coal mining. Subsidence processes above underground mines consist of a gradual downwarping of the overburden into coal extraction panels, which causes depressions, or a sudden collapse into individual mine openings, which causes pits. Subsidence type (depressions, pits), amount, areal extent, rate, and duration are controlled by (1) thickness of coal mined, (2) mine geometry and mining methods, and (3) thickness, lithology, structure, and hydrology of the bedrock and surficial material in the mining area. Subsidence in surface mining areas is caused by compaction of rehandled overburden material (spoil), dewatering of aquifers near surface mines, and (or) stress and strain readjustments. Depressions (troughs), cracks, and pits (sinkholes) can occur under either underground or surface mining procedures. Maximum vertical displacement in depressions above extraction areas large enough to cause maximum subsidence in the western United States ranges from about 45 to 90 percent of the thickness of coal mined. However, pits may be deeper than the mining thickness, where the caved material can move laterally into adjacent mine openings. In surface mining areas, although little detailed information is available in the western United States, subsidence may range from a few percent to perhaps as much as 10 percent or more of the thickness of the reclaimed spoil. The limit angle, which determines the area of subsidence above underground mines, ranges from about 5° to 30° (from a vertical reference) above underground mines in the western United States. In surface mining areas, however, the subsidence area can range from nearly equal to the mining area, where compaction of rehandled spoil occurs, to many times the mining area, where aquifers are dewatered and undergo compaction. The time or duration between mining and complete subsidence above underground mines commonly ranges from a few months to a few years, where downwarping occurs above extraction panels, to many years or decades, where pillars are not mined. The duration between mining and the occurrence of pits (sinkholes), however, can vary from a few decades to many decades or even centuries. The time necessary for depressions and pits in surface mining areas is not well known but apparently depends on factors such as methods of emplacing and grading and rate of wetting the rehandled material, rate of dewatering of aquifers near the mine, and stress-strain readjustments.

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