Previous descriptions of the Chazy Group of the St. Lawrence Lowland of Eastern Canada based on the study of the surface exposures and the logs of seven holes, indicated the Chazy to be composed mainly of calcareous shales, shaly limestones, limestones, and some sandstones.The Chazy sediments, as indicated by subsurface data from 30 additional wells, vary laterally from limestone with some shale and sandstone, in the belt bordering the Precambrian Shield, to predominantly dolomite with some limestone, shale, and sandstone in the basinward region southeast of the St. Lawrence River. The character of the sediments suggests a platform or foreland type of deposit, the product of a transgressing sea.The absence of dolomite in the surface exposures and the similarity of the sandstones to those of the older Beekmantown and Potsdam Groups, has misled geologists in logging, and subsequent interpretation, of the subsurface data in a number of wells, particularly in the Three Rivers area. Recognition of these mistakes makes it possible to explain the apparently anomalous thickness of the Chazy, as reported in published logs of these wells.Additional evidence is also presented bearing on the Chazy–Black River relationship.

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