The Lower–Middle Ordovician Beekmantown Group of the St. Lawrence Lowlands (eastern Ontario and southwestern Quebec) consists of basal inner-shelf dolomitic sandstones (Theresa Formation) and inner- to middle-shelf carbonates (Beauharnois and Carillon Formations). The stratigraphic relationship of the three formations has been considered in the past as a normal vertical succession. Integration and reinterpretation of the existing lithostratigraphic and biostratigraphic data support a new chronostratigraphic relationship, particularly between the Theresa and Beauharnois Formations, as well as correlations with the outer-shelf deposits preserved in the Philipsburg tectonic slice (PTS). Variations in the lithostratigraphic pattern are pronounced along a cross sectional depositional profile from inner shelf to outer shelf. The data demonstrate that the Beekmantown Group is thickest in the region south of Montreal and gradually becomes thinner toward eastern Ontario (Ottawa embayment) and the region north of Montreal (Laurentian highs). The lower boundary of the Theresa Formation is diachronous, becoming younger toward the craton. In the region south of Montreal, Theresa Formation sandstones are temporally equivalent to the lower part of the middle-shelf carbonates assigned to the Ogdensburg Member of the Beauharnois Formation. The Theresa Formation sandstones pinched out toward the outer shelf and replaced by Ogdensburg-like facies. The latter apparently merges with the outer-shelf carbonates of the Wallace Creek and Morgan Corner Formations. The outer-shelf strata of the Hastings Creek and Naylor Ledge Formations correlate with the upper part of the Ogdensburg Member. The Naylor Ledge Formation is truncated by the Sauk-Tippecanoe unconformity and overlain by the lower Whiterockian (?Rangenian) Luke Hill Formation. The onset of the Sauk-Tippecanoe unconformity is older in the outer-shelf rocks of the PTS and becomes younger toward the inner shelf, where it separates the middle Whiterockian Carillon Formation from the Chazyan strata of the St. Lawrence Lowlands.