The Manitoba Mines Branch in 1969 and 1970 drilled a number of core holes in the Ordovician and Silurian strata in the Interlake Area of Manitoba. This core has provided a complete lithologic section from the top of the Middle Ordovician, Winnipeg Formation, to the base of the Middle Devonian, Ashern Formation. Previous studies were limited to poorly exposed outcrops, and a relatively small number of quarries. Data obtained from this core permit placement of most of the strata exposed in the outcrops and quarries into their correct stratigraphic position, and define the lateral variations in both thickness and lithology of the stratigraphic units involved in this study. The carbonate depositional environment varied from infratidal (basinal) to supratidal. Deposition is believed to have occurred primarily in an infratidal to intertidal environment on a slowly subsiding basin with numerous transgressions and regressions.