Geology

After the intense folding that raised the Laurentians during the pre-Cambrian era, the sea receded from the Canadian Shield. Sediment stopped accumulating and the continent became exposed to erosion. We do not know how long the land remained exposed but it was certainly a long time. When the folding forces abated and the region subsided sufficiently for the sea to flood the region once again, all the former peaks had been worn down. It was on these peaks that the sea discordantly deposited the sediments that are today the St. Lawrence Plain. The rocks of the Laurentians and the St. Lawrence Plain were thus formed during distinctly different geological eras a long time apart.

This territory is part of a major physiographic unit known as the St. Lawrence Lowlands. This wedge-shaped plain is bounded to the north by the Laurentians and to the southeast by the Appalachians.

The rocks of the St. Lawrence Lowlands are largely masked by major deposits of clay, sand, pebbles and so on. These deposits have helped to level the land over which they are spread and their distribution has resulted in a whole range of soils, offering a variety of possibilities for agriculture.

At the western tip of the Island of Montreal is a group called Potsdamian sandstone. This Cambrian group has been divided into two distinct formations: to the north is the Châteauguay formation, to the south the Covey Hill formation. Sainte-Anne-de-Bellevue is part of the Covey Hill formation. Most of the formations described in the past are now obscured by buildings. The red sandstone outcrops were limited and at ground level, but in most cases, stratification was visible. The main significance of this group is that the dips average 5° to the south. Because outcroppings located immediately to the north belong to the Beekmantown group and have quasi-horizontal dips, there must be faulting between these two formations.

The Covey Hill formation consists of thin beds of siltsone and sandstone, mostly coarse-grained ranging to conglomerate. The conglomerate chips, unmistakably quartz, are up to 2.5 cm in diameter in many places, though rarely larger. Also found in the sandstone beds are grains of feldspar and opaque black minerals. The chips are well rounded and the sands range from subrounded to subangular. Compaction varies from friable to excessive.

Generally speaking, Potsdamian limestone contains no fossils. However, there are worms known as Skolithos and giant tracks classified as those of "Clomachtinites and Protichnites".