The Cambrian Potsdam Group in southwestern Quebec comprises the quartzo-feldspathic Covey Hill Formation (up to 2000 ft (610 m) thick), and the unconformably overlying and overlapping Chateauguay Formation, which is subdivided into the lower Cairnside Member (up to 500 ft (152 m) thick) of pure quartz sandstone and the Upper Theresa Member of interlayered quartz sandstone and dolostone. Petrographic study concentrated on Covey Hill and Cairnside lithologies from three deep cores near Montreal. Traditional interpretation of Potsdam Sandstone as a pure quartz blanket deposited by transgressing seas applies only to the Cairnside Member.The Covey Hill Formation comprises subround-round sands, sparse rounded gravels, scattered penecontemporaneous clay clasts, and rare thin mud layers. Juxtaposition of well and poorly sorted laminations of different sand sizes, mica-clay, and heavy mineral concentrations, attests fluctuating depositional energies. Presence of silt-free clayey matrix and scattered coarse grains set amongst much finer sands attests marked predepositional eolian activity. Composition of quartz, fresh and partly weathered K-feldspar, micas, and stable heavy minerals is inferred to reflect a prolonged predepositional history during which unstable minerals were destroyed. Hematitization of biotite flakes, and other diagenetic modifications, are inferred to result from pedogenesis in an alluvial environment of deposition.