The history of the Appalachians spans over a billion years, with four collisions forming the Appalachian Mountains. The last collision occurred around 300 million years ago. Originally, the chain would have rivaled the Himalayas at over 5 km tall, with some mountains perhaps reaching as high as 9 km. Three hundred million years of erosion slowly whittled the mountains away; today, they average 1 km, with the highest point being 2 km at Mount Mitchell, North Carolina. Read more →