The St. Lawrence Lowlands are primarily made up of sedimentary rocks, specifically limestone, shale, and sandstone. These rocks were formed over millions of years from the accumulation of sediments deposited in a shallow sea that once covered the region.

Here's a breakdown:

* Limestone: Formed from the accumulation of marine organisms' shells and skeletal remains.

* Shale: Formed from fine-grained sediments like clay and silt.

* Sandstone: Formed from sand grains cemented together.

These sedimentary rocks are relatively soft and easily eroded, which is why the St. Lawrence Lowlands have a relatively flat landscape with fertile soils.