The marine park lies within two major geological formations. The area comprising the Saguenay Fjord and the north shore of the St. Lawrence is part of the Canadian Shield, a crystalline rock basement formed from 600 million to 4 billion years ago. The south shore of the St. Lawrence and the mid-estuary islands are part of the Appalachians, a mountain range composed of sedimentary rock hundreds of millions of years old. Between these two formations lie the St. Lawrence Lowlands, formed of more recent sedimentary rock.
The terrain and landscapes of the estuary and fjord have also been sculpted by a variety of geological processes: collapses due to movement of the Earth’s crust, the passage of glaciers and submersion beneath the ocean.