The Lower – lower Middle Ordovician Beekmantown Group of the St. Lawrence Lowlands is a variably thick pertitidal succession of dolostone, limestone, quartzose carbonate, and subordinate siltstone and shale that is gradationally bound by the Potsdam Group below and unconformably to conformably by the Chazy Group above. It is here considered to include three regionally extensive formations, a basal Theresa, a middle Beauharnois, and an upper, redefined Carillon. A principal reference section is established in the subsurface. The Theresa Formation is a transgressive succession, above Potsdam Group siliciclastics and below Beauharnois Formation carbonates. It is dominated by quartz arenite and quartzose dolostone; skeletal fossils are rare (usually gastropods), whereas trace fossils are abundant. The Beauharnois Formation is divided into two members, a basal Ogdensburg and an upper Huntingdon. The Ogdensburg Member is sandy, especially in its lower part, and relatively more fossiliferous than the Huntingdon Member. Both members include fossiliferous and nonfossiliferous, stromatolitic and oolitic, coarse-grained dolostone and subordinate limestone and reflect the development of a relatively wide peritidal carbonate platform. The Carillon Formation is a widespread unit that marks the onset of Middle Ordovician Taconic orogeny at the continental margin. It consists mostly of cyclic packages of laminated and burrowed, fine-grained dolostone and limestone, and as well siltstone and shale. Thin fossiliferous beds are dispersed in its upper part.