Ordovician deposits in this area, the Williston basin comprise 2 lithologically contrasted groups of strata: up to 320 ft. of sandstones and shales called the Winnipeg group, overlain by up to 1,000 ft. of limestones and dolomites called the Bighorn group. Cambrian rocks (locally ranging up into the Beekmantown) unconformably underlie the Ordovician in the western part of the basin; depositionally they fringed the Cordilleran trough and consisted mainly of clastic deposits. Silurian rocks, largely dolomite, reach a maximum of 1,120 ft., and succeed the Ordovician conformably. Pre-Devonian erosion cut deeply into the lower Paleozoic pile. Winnipeg deposits reveal the earliest Williston basin outline. Sedimentation began with a central sandstone (Burgen) and passed at the first of several basin enlargements into a shale with peripheral sandstones (Winnipeg shale and sandstone) which, due to the absence of Cambrian rocks on the eastern side, transgressed onto the shield. These sandstones are not, as formerly supposed, equivalent to the earlier (Burgen) sandstone. At the base of the Bighorn group another sandy deposit (Lander-Hecla beds) recorded a further step in sedimentary expansion. After a comparatively small amount of accumulation a far-reaching transgression brought further Bighorn deposits into contact on the W. with the Cambrian. Carbonate sedimentation dominated the remaining half of the lower Paleozoic. In its basin interior development of the Bighorn group consists of limestones, rhythmically deposited evaporitic rocks, and argillaceous beds, known (in upward order) as the Red River (Viola), Stony Mountain, and Stonewall (Sylvan-upper Maquoketa) formations. In the peripheral area (the western periphery only has survived) the Bighorn consists of dolomite but reveals by recurrent changes of composition and texture the influence of the sedimentary regime controlling the central deposits. Age considerations, resting mainly on the objective principle that any stratum is of the same stratigraphical age throughout its extent, lead to the following conclusions. 1) Ordovician rocks in the Williston basin area are a remnant of a once continent-spanning sheet. 2) Everywhere they lie unconformably on Beekmantown or earlier rocks. 3) The Winnipeg group is correlative with the pre-Bromide part of the Simpson group of Oklahoma, though no equivalent of the basal Simpson (joins) may exist in the Williston area. 4) Red River strata (including the Hecla-Lander beds) are correlative with the Viola and upper Simpson rocks of Oklahoma, and, transgressing beyond the Winnipeg perimeter, lie on Cambrian (the classical Caradocian transgression?). This unconformity extends from Alberta to Oklahoma, thus approaching continental scale. 5) Upper Ordovician rocks (Stony Mountain and Stonewall or upper Maquoketa-Sylvan formations) overlie the Red River-Viola with regional conformity, and manifestly represent all Cincinnati stages. 6) Silurian rocks (Interlake-Hunton) rest conformably on the Ordovician but are everywhere affected by post-Silurian-pre-Devonian (Caledonian) or later unconformities.