The St. Croix Belt of southeastern Maine and southwestern New Brunswick is part of a distinctive terrane situated between the Avalon platform and Miramichi arc—two important physiographic components of the Late Precambrian – early Paleozoic Iapetus Ocean. It bears lithologic and stratigraphic similarities to both of those tracts but is identical to neither. Formerly mapped entirely as the Cookson Formation, it is now divided into four formations of Cambrian through Early Orodovician age assigned to the Cookson Group. These rocks record periodic influxes of terrigenous debris into a deep anoxic basin and may have been part of the west-facing continental slope of the Avalonian continental block. The St. Croix Belt has been strongly affected by both pre-Silurian and Early Devonian (Acadian) folding. Facies changes rather than tectonic sutures are used to explain the relationships between the St. Croix Belt and its neighbouring pre-Silurian tracts.