Complex sentence: By comparison, the Appalachians started forming about 460 million years ago, and the Rockies a mere 70 million years ago--when the Appalachians started forming, the St. Francois range was already twice as old as the Appalachians are today.
Split into simpler sentences: By comparison, the Appalachians started forming about 460 million years ago, and the Rockies a mere 70 million years ago. When the Appalachians started forming, the St. Francois range was already twice as old as the Appalachians are today.