When exactly was this? I have always pictured a giant block of ice slowly sliding diagonally down the middle of the country 🤣
It’s the opposite.
The glaciers removed the flat sediments and revealed the non-flat metamorphic and igneous rocks.
Without glaciers, the Canadian Shield would be part of the North American platform and would be flat.
Most of midwestern US wasn’t covered with glaciers, which is why it’s part of the North American platform and is flat. So is the US great plain and the Canadian prairie.
> I have always pictured a giant block of ice slowly sliding diagonally down the middle of the country 🤣
That is essentially accurate. The ice sheets involved were absolutely enormous – they were [a kilometer or more thick](https://xkcd.com/1225/) in those areas – and thus extremely heavy. They more or less steamrolled the area.
But it wasn’t that mountainous even before that. Everything between the Appalachians and the Rockies, aside from a few highlands in Arkansas, Kansas, Oklahoma, and Missouri (the Ozarks and Ouachita mountains) has always been lowland; much of it was even covered by shallow seas during times of high sea level (which is part of why it has rich soil today).