Part 6/11:
By 300 million years ago, the formation of the supercontinent Pangaea took place due to another massive collision, known as the Alleghanian Orogeny. This was the last major mountain-building event which shaped the region, after which the Appalachians began to stabilize.
Transition to a Passive Margin
After the Alleghanian Orogeny, the landscape continued its evolution as Pangaea started to fragment around 200 million years ago. The mid-Atlantic ridge emerged during this time, leading to the continuous stretching of the Eastern margin of Laurentia, now known as North America. The result was a passive continental margin that no longer experienced the tectonic activity typical of younger mountain ranges like the Rockies.