Here's a breakdown:

* Sedimentary rocks are formed from the accumulation and cementation of sediments, such as sand, silt, clay, and organic matter.

* The St. Lawrence Lowlands are a vast plain, formed over millions of years by the deposition of sediments from rivers, glaciers, and the sea.

* This process created layers of different sedimentary rocks, including limestone, sandstone, shale, and conglomerate.

While other rock types exist in the region, sedimentary rocks are the most prevalent and defining characteristic of the St. Lawrence Lowlands.