 This area has a depth of before reaching the mountains in the west of the region, but reaches in the vicinity of Rimouski, only to shrink again in the eastern limits of the region.This riverside land disappears entirely between the municipalities of Sainte-Félicité and Grosses-Roches.In the western part of the territory, this littoral plain is interrupted by inselbergs, ridges reaching that can reach 200m in height, typical of the riverside land of Bas-Saint-Laurent

In the Quaternary, the region was marked by glaciation.