Mines and prospects-ContinuedMines and prDspects-Continued Chalk Creek mining district-Continued Chalk Creek mining district-Continued Other mines and prospects-Continued Other mines and prospects-Continued

This report describes the stratigraphy and structure of an area of about 5,000 square miles in northeastern Wyoming and adjacent parts of Montana and South Dakota. The area includes the northern end and part of the western side of the Black Hills uplift and the adjoining part of the Powder River Basin.About 11,000 feet of sedimentary rocks ranging in age from Mississippian to early Tertiary are exposed in the area, not including surficial deposits of Tertiary(?) and Quaternary age. The oldest rocks crop out in the southeastern part of the area and consist of 500 to 600 feet of light-gray cherty limestone that makes up the Early Mississippian Pahasapa limestone. Unconformably overlying the Pahasapa limestone is the Minnelusa formation of Pennsylvanian and Permian age. It is 650 to 800 feet thick in drill holes and at outcrops in the southeastern part of the •area, and it consists of interbedded lightgray and pink sandstone, gray sandy limestone and dolomite, some red shale and siltstone, and local beds of gypsum and. anhydrite. The Permian Opeche formation overlies the Minnelusa formation unconformably and comprises 60 to 90 feet of red fine-grained sandstone, siltstone, and silty• shale. Next in order is the Permian Minnekahta limestone, which is composed of light-gray and light purplish-gray thin-bedded limestone about 40 feet thick. The Spearfish formation, 450 to 825 feet thick, BLACK HILLS UPLIFT, WYOMING, MONTANA, SOUTH DAKOTA Weston County. It is composed of dark-gray shale, at places sandy and silty, interbedded with bentonite and numerous limestone and siderite concretions. The formation is divided into several members, which from base to top are the Gammon ferruginous member, 0 to about 1,000 feet thick; the Mitten black shale mem~r, 145 to about 870 feet thick; and the upper part of the Pierre shale including the Monument Hill bentonitic member 150 to about 220 feet thick, and the somewhat younger Kara bentonitic member about 100 feet thick. In the northern part of the area, the Gammon ferruginous member contains a zone of sandstone and sandy shale about 100 feet thick known as the Groat sandstone bed.The Fox Hills sandstone overlies the Pierre shale and is 150 to 200 feet thick. .Yellowish-gray sandstone and gray shale make up the Fox Hills sandstone at most places except in Carter County, Mont., where as much as 100 feet of very light gray cliff-forming sandstone crops out at the top of the formation and is mapped separately as the Colgate member .. Nonmarine sedimentary rocks overlie the Fox Hills sandstone and make up an essentially conformable sequence of sandstone, shale, •and coal beds that includes the Lance formation of Late Cretaceous age, the Fort Union formation of Paleocene age, and the W•asatch formation of Eocene age. The Lance formation is 1,600 feet thick in northern Weston County, Wyo., but thins northward to about 500 feet in Carter County, Mont. The overlying Fort Union formation, 1,500 to about 2,100 feet thick, also thins northward. It is divided into the Tullock member at the base...

Tunnels are among the most expensive of engineering structures. They can, and often do, present great difficulties in design and construction. For several decades the techniques used in tunneling have lagged behind those used in other fields of engineering. An engineer, for example, can design a bridge so that it will adequately and economically fulfill its proper function. The design and construction of underground openings commonly is a far different situation. The stresses surrounding a planned underground opening are generally unknown quantities, as are the nature and behavior of the rock mass itself. Therefore, judgment and trial and error prevail; design, in its true sense, becomes impossible; and safety factors can be uneconomically large or dangerously small. Geological factors exert a decisive influence on the difficulties and costs of tunnel construction. Tunnel hazards unanticipated sources of expense and delay are caused largely by the divergence of structural details of a given rock mass from the statistical average for similar rock masses. This variation in rock structure results in erratic tunneling costs; the cost of a tunnel may be considerably higher than the average cost of similar tunnels constructed in similar bodies of rock. A competent experienced geologist usually can predict the kinds of difficulties that would be encountered in different parts of a proposed tunnel, but he seldom can quantitatively evaluate the difficulties. On such a basis, preliminary estimates for materials and equipment for constructing a tunnel commonly involve considerable guesswork. This guesswork usually results in the procurement of unnecessary supplies to offset the possibility effacing an emergency with inadequate provisions. The present (1972) state of tunnel art combines experience and intuition with theoretical and practical principles to design and construct underground openings. We believe that in the future the fullest use of rock-mass information, together with improved theory, will result in greater economy, safety, and confidence in tunnel construction. It is toward this goal that the present report is directed. The Straight Creek Tunnel is about 55 miles west of Denver. The proximity of this tunnel to the research center of the U.S. Geological Survey in Denver offered an unusual opportunity to utilize the personnel and facilities of the Survey on the problems of defining the environment of the proposed tunnel. The authors were able to consult with, and have the services of, experts in nearly every field of geology. As a result, research investigations in the tunnel area were conducted by many survey personnel. The investigations performed in conjunction with the basic geologic investigations, such as geophysical and groundwater research, are described in separate chapters by those who directed the research. At the dedication of the first of the twin bores on March 8, 1973, the Colorado Division of Highways, in accordance with a resolution passed by the General Assembly of Colorado, officially named th...

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