The Rapides River flows from north to south for 48 kilometres (30 mi), falling from an altitude of 640 metres (2,100 ft) to sea level.

A few hundred yards from its mouth there is a great mass of iron ore encased in the norite and labradorite rock of the region.

[7] Part of the basin is in the Zec Matimek, which covers 144 square kilometres (56 sq mi) of the western side.

The coastal plain in the south is less than 2 kilometres (1.2 mi) wide, and is fairly flat, rising to about 60 metres (200 ft) of elevation.

[7] The bedrock is mainly magmatic, deformed to some extent, including migmatite, anorthosite, gabbronorite and granodioritic or granitoid gneiss.

In the piedmont and the coastal plain large amounts of silt and clay were deposited by the Goldthwait Sea after the glaciers retreated.

[7] The rivers and streams in the northern plateau follow angular courses dictated by fractures in the bedrock, which are fairly rectilinear and constrained by narrow valleys.

Ombrotrophic peat bogs cover 0.95% of the area, mostly on the coastal plain with its flat relief and fine sediments.

[9] The forest cover is dominated by black spruce (Picea mariana), balsam fir (Abies balsamea) and to a lesser extent hardwoods such as paper birch (Betula papyrifera), trembling aspen (Populus tremuloides) and balsam poplar (Populus balsamifera).