The smallest physiographic region of Quebec is a triangular lowland bounded by the edge of the Canadian Shield to the northwest, the great Champlain fault, bordering the Appalachian Highlands to the east, and the Adirondack Mountains in New York State, to the south. The underlying rocks are sandstone, shale and limestone of the Ordovician, Silurian and Devonian periods. The strata are gently dipping, or lie in low, broad, dome-like folds, traversed by faults, some of which are of considerable throw; but compared with the highly folded measures of the Appalachians, they seemed relatively undisturbed. The present surface is low and flat, especially around Montreal (100 feet), but it rises in the neighbourhood of Quebec City to about 300 feet. This uniformity is broken by the Monteregian Hills extending in a line from Montreal to the Appalachian Highlands: Mount Royal (769′), St. Bruno (712′), Beloeil (1,437′) Rougemont (1,250′), Yamaska (1,470′) and Johnson (875′). They are extrusions of igneous rocks that forced their way up during the Devonian period, when orogenic movements were active.
There need be little wonder that the relief is so low. The unfolded sediments were easily removed by the numerous cycles of erosion, the base-level being furnished by the antecedent St. Lawrence River. The hard igneous rocks of the Monteregian Hills stand as monadnocks. But here also glaciation left its marks. The course of the St. Lawrence was altered. Upstream from Montreal, the river forms a series of impounded waters: Lakes St. Louis and St. Francis, and many rapids: Lachine, Cedar, etc., Downstream, the glacier excavated a deep trough on the site of lake St. Peter, now being filled by river deposits. In front of Quebec City, the St. Lawrence had to find a new course and its cliffs are still very steep. After the melting of the ice the whole area was invaded by the Champlain Sea, whose deep clay deposits form valuable agricultural soils.