The Upper Cambrian Potsdam Formation (orthoquartzitic sandstone and conglomerate) in eastern Ontario, southwestern Quebec, and northern New York State, was deposited during an early Paleozoic transgression over the Precambrian surface of the Canadian shield. Rock types and sedimentary structures indicate three distinct environments: (1) Terrestrial, talus sheet wash fan (quartzite breccia), Frontenac arch, Thousand Islands region; (2) marine and (or) fluvial channel (cross laminated sandstone), Ontario and northern New York; and (3) low-energy, littoral to nearshore (laminated sandstone and some siltstone) Quebec and New York State. Reference outcrops are cited; bibliography is comprehensive.

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