The rocks of the Wirral – Big Lake area occupy a transitional zone between a Silurian – Lower Devonian volcanic sequence of the Coastal and Mascarene–Nerepis Belts and turbiditic rocks of the Fredericton Trough. Most of the rocks of the Wirral – Big Lake area were included by previous workers in the "Pale and Dark Argillite Divisions of the Charlotte Group." It is proposed that the term Charlotte Group be discontinued because the subdivisions of this group do not conform with and locally cut across formational boundaries. Moreover, both Ordovician and Silurian strata were included in the Charlotte Group.The Coastal and Mascarene–Nerepis volcanic belts probably represent an ensialic island arc sequence that developed simultaneously with the deposition of sedimentary strata of the same age in the Wirral – Big Lake area and in the Fredericton Trough. It is suggested that these turbiditic sediments represent a behind-the-arc basin. The stratigraphic succession and structure of the map-area reflect several distinct stages in the closing of this basin.

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