The Richelieu River Valley, in the St. Lawrence Lowlands, lies on sedimentary rocks which are some 450 million years old and which formed during the Cambrian period of the Paleozoic era. The bedrock of this area consists of clay schists and sandstone of the Utica group, clay schists and sandstone of the Lorraine group (sub-formation of Breault) and calcareous clay schists of the Lorraine group (sub-formation of Chambly).
The typical Utica group consists of black fissile clay schist, usually with a high concentration of mica flakes. Its hardness suggests high quartz content.
The rocks in the Lorraine group (Breault sub-formation) cover most of the area. They consist of dark grey clay schist as well as sandstone lentils up to 5 cm thick.
The Lorraine group (Chambly sub-formation) contains the most recent sedimentary rock in the region, a series of clay and limestone schists that are redder towards the top.
Clay schists are sedimentary rocks of dynamic origin, formed by the splitting of existing rocks. Calcareous schists are sedimentary rocks formed by the accumulation of animal or plant matter in bodies of water.
This region also bears the marks of the Quaternary geological era. Immediately after the last ice age, the whole St. Lawrence Valley and its waterways became a vast inland sea (the Champlain Sea) that stretched as far as today's Lake Champlain.