Thanks for mentioning the Appalachians. I'm going to geek out on them a little bit now. The Appalachians may not be as big now as the the Rockies but they were plenty big enough to hold up westward expansion for a couple hundred years. Why aren't they tall? Because they are old. Why aren't they as long as some other ranges? Because they are old.

How old? They began forming more than 1.2 billion years ago and peaked during the formation of Pangea 500-300 million years ago. Were they bigger? When Pangea was breaking up they were as tall as the Himalayas and the Alps are today. Longer? The Highlands of Scotland were part of the Appalachians, the Little Atlas mountains in Morocco were part of the Appalachians. At their longest they spanned 3 modern continents and acted as a continental divide for a Supercontinent.

What were animals doing when they were forming? Land animals didn't exist, sea animals were just beginning to evolve bones. Why is there so much coal under the Appalachians? The organisms that break down plant material didn't exist yet. Ancient forests (not modern tree based forests, trees didn't exist yet, liverworts, mosses, rhododendron, and ferns) were buried whole with no decomposition. All of the carbon held in those forests was trapped under a massive amount of earth for hundreds millions of years.