Excellent exposures in an area of multiple drape folds in the northwest corner of the Big Horn Mountains provide data for the construction of a three-dimensional model of the upper surface of the Mississippian Madison Formation. In the model, the vertical and horizontal scales are the same. Comparison of deformed and undeformed Madison surfaces indicate that displacements required by the deformation vary from place to place on the surface in both magnitude and direction. Calculations made from field measurements show that there is not enough fracturing or thinning in the Madison layer to account for the indicated displacements. Also, reasonable...