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Two areas in the Big Horn Mountains are given as examples of the beauty and simplicity of the drape-folding-over-basement-block model of structural interpretation. The Piney Creek thrust is interpreted as an uplifted fault block with sedimentary strata draped over its edges. Structural cross sections of the Horn area show how sedimentary layers are continuously folded over a faulted basement.In both areas, the role of a ductile stratigraphic unit, the Gros Ventre shale of Cambrian age, is important in partially decoupling the basement from the overlying sedimentary veneer and in providing a slide surface for gravity phenomena. Gravity glide masses probably are more widespread than is generally recognized in the foreland province and should be considered a characteristic element of the foreland tectonic style.The construction of three-dimensional models of the basement surface, based on detailed geologic mapping, and the comparison of observed structures in the sedimentary cover rocks with experimental draping over the model are effective tools for gaining a mechanical understanding of the development of structures in the foreland province. on July 7, 2015 memoirs.gsapubs.org Downloaded from 132 JOHN C. PALMQUIST 108° 107°

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