You're likely thinking about the Canadian Shield, a vast, ancient geological formation covering much of central and eastern Canada. It's not that the Canadian Shield is a former mountain, but rather that the mountains it once contained were eroded away over millions of years, leaving behind a flat, low-lying landscape.

Here's a breakdown:

* Ancient Mountains: The Canadian Shield was once a region of towering mountains, formed by tectonic activity billions of years ago.

* Erosion: Over time, wind, rain, ice, and other weathering processes gradually wore down these mountains, leaving behind a landscape of exposed bedrock.

* Low Elevation: The Canadian Shield today is characterized by low elevations, numerous lakes, and rocky outcrops. This is because the mountains have been eroded to their roots, leaving behind a relatively flat surface.

Therefore, you can drive across the Canadian Shield without experiencing significant elevation changes because the mountains that once existed have been worn down to a low elevation.

It's important to note that the Canadian Shield is not completely flat. There are still hills and ridges, but they are generally low-lying and spread across a vast area.