The eastern side of the Williston basin is bounded by the pre-Cambrian rocks of the Canadian shield. These pre-Cambrian rocks gradually slope westward toward the center of the basin. Paleogeological maps show the successive eastward overlapping of the seas upon this shelf in Cambrian and Ordovician times and the successive off-lapping during each period from Silurian throughout the remainder of the Paleozoic era. The Mesozoic era again saw a general increasing eastward overlap of sediments in each new period. A study of the lithology and isopach relationships indicates that the center of the basin has not always been located along its present axis and that the seas had a variety of generally westward connections.

Structures in the pre-Cambrian in Manitoba, North and South Dakota can be traced into the sedimentary basin and can be followed in the sediments both through structural and lithological studies. Where these structures, as well as structures originating in the sedimentary basin, cross lithological changes as shown on isopach and paleogeological maps, there is considerable promise of oil accumulation. That such accumulation may be present in certain areas is indicated in some cases by surface evidence.

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