The mountains are only about 200M years old. Look at this model of the earth and allow the water in the oceans to remain the same in volume, as the current ocean basins close up the water would be pushed up onto what we now have as dry land. From 200MYA to far previous to that the places that had not yet been uplifted would have been shallow seas.

Alberta has mountains and plains, the plains are currently 2,000 ft above sea level and rather than being uplifted like the mountains on the west side, the waters receded as room was made available by the spreading oceanic rifts.