The systematic surface structure-mapping procedure of the Montana part of the Williston basin is comprehensive and consists of three phases: planning, data-gathering, and integration. The final product is a morpho-tectonic map that combines the physiographic data of an area with the geologic data in a regionally consistent interpretation. Applied to the Williston basin, this comprehensive method has been successful in defining positive and negative trends within the tectonic framework of such well-known features as Cedar Creek anticline, Freedom dome, Weldon fault, and Blood Creek syncline. In the course of defining the local features, a tectonic theory was gradually evolved for the Williston basin suggesting that the local structural trends within the basin are the resul of reactivation of a pre-existing set of basement zones of weakness oriented most commonly at approximately N. 45° E., N. 70° E., N. 45° E., and N. 75° W. Some areas display also a N. 20° W. trend. Paleozoic reactivation of the trends is suggested by geomorphic data and by subtle deflections in the trend of surface structures or by alignment of positive features to form culmination. The Laramide reactivation, concentrated along the northwest-oriented trends, dominates the present surface structure and appears to have been superimposed on the older structures.

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