The early Middle Ordovician Chazy Group forms part of an 8000-ft-thick, mostly marine, sedimentary sequence in southern Quebec. It includes the lower Laval quartz arenites and shales, the middle Laval shaly limestones and calcarenites, and an upper part of shaly limestones, dolomitic shales, and shaly dolomites which grade into the overlying "Pamelia" Formation in the Montreal area. The areal distribution and thickness of the Laval Formation are irregular, and lateral facies changes are common. On a faunal basis the formation can be divided into an upper Rostricellula plena assemblage zone, correlatable with - the upper Chazy of the standard section, and a lower Bolboporites americanus assemblage zone with lower and middle Chazyan faunal aspects. The lower part of the Bolboporites americanus zone comprises quartz-sandy and shale beds which contain an autochthonous benthonic fauna of ichnofossils, and the shaly limestones and calcarenites of the upper part have varied allochthonous brachiopod-bryozoan-trilobite-ostracod-echinoderm fauna. Calcarenite developments (St. Martin lithofacies) of the Laval Formation are characteristically crossbedded and indicate chief current directions from the NW. They contain, in the uppermost parts of the Bolboporites americanus zone, widely scattered coral bioherms. The species found in these structures are identical with those found at equivalent levels of the Champlain Valley Chazy. In the Montreal area a gradational contact exists between the upper part of the Laval Formation and the Rostricellula-bearing "Pamelia," and also between the "Pamelia" and the Lowville. The Chazy-Black River contact is placed at a minor disconformity within the "Pamelia," rather than at the base of it. The part below the unconformity is correlated with the lower phase of the Ottawa Valley Pamelia.