Structural studies since 1954 have included an evaluation of the role of the basement on the geologic evolution of the Bighorn Mountains. The Tensleep and Horn faults and certain N. 10°-15° E. tear faults along the east flank were guided by foliation and shear zones of Precambrian age in the basement. A number of small northwest-trending anticlines with steep southwest limbs may reflect basement control, particularly by locally pervasive steep northeast-dipping foliation. The problem of fracture analysis is critically examined. An important feature is that fracture patterns, revealed by statistical plots, are markedly different in basement rocks when compared with those in the adjacent sedimentary rock cover. Basement fracture patterns are more complex and conta n maxima not present in the sedimentary rock patterns. Upward propagation of basement fractures during Laramide and younger deformation is not a factor here; if it were, the fracture patterns should be the same. It is suggested that the 400-1,000 feet of basal incompetent Paleozoic rocks acted as an insulator with the result that the higher competent cover rocks acted independently of the basement as far as fracture formation was concerned.

Basement foliation is largely discordant with the over-all trend of the range and the northeastward-convex curve of the range cannot be correlated with any single fracture direction. The structure of the range as a whole, therefore, has developed in response to post-Precambrian regional stresses with basement anisotropy exerting an influence on the trends of some smaller scale faults and folds.