I'm not surprised that there are areas in the Canadian Shield where soils are deeper - the Shield is very old, and erosion is a thing, and soils collect in lowe

I'm not surprised that there are areas in the Canadian Shield where soils are deeper - the Shield is very old, and erosion is a thing, and soils collect in lower lying areas. I'm guessing that the area you're describing has a lot of lakes, or swamps, or both. Might be a bunch of drainage schemes put in place to make waterlogged soils useful for agriculture? One swallow doth not a summer make. And one hollow doth not a viable agricultural soil over an entire geological structure make. (Obviously there's more than one place in the Shield where deeper soils are present).