The position of individual folds or groups of folds within the regional structural pattern of the Wyoming foreland area is considered to be due to the response to stress of either: (1) fractures in the pre-Cambrian basement; or (2) heterogeneity of the basement complex. Deformation of the overlying rocks resulted from adjustment along one of these controls acting under tangential stress.

An integral part of the development of some folds is a localized fault system known as epi-anticlinal faulting. The epi-anticlinal fault systems have been attributed to tension in the rising anticline; or to locally applied tangential stresses comparable to the action of a plunger in the immediate vicinity of the fold. An analysis of the existing fault system in the Elk Basin and Pilot Butte folds has been made from data obtained by critical examination of electric logs. Reconstruction of the fault planes in space indicates that the idea of a tensional origin for the faults is only partially true; and that the local plunger action can not be demonstrated.

One portion of the fault system developed as slippage between strata, and followed fractures across the bedding to propagate upward toward the surface at higher angles. Other faults apparently originated as a pair of conjugate fractures, on either or both sets of which movement became appreciable. Some of the faults are due to adjustment in the hanging wall block of an earlier fracture.

The fault systems discussed appear to be limited to Mesozoic strata, and particularly the Upper Cretaceous rocks. Folds located in the Big Horn, Wind River and Powder River basins from which erosion has stripped the younger rocks overlying the central portion and exposed the Paleozoic rocks, have no epi-anticlinal fault systems. These folds may be bounded by a high angle reverse fault which parallels the steep limb of the fold.

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