My very good friend Rachael wrote an article for Clarion called "Spec Tech: Making Mountains". It's all about how horribly impractical many mountain ranges are in fantasy novels, the most notorious being Lord of the Rings and it's magical right-angle mountain ranges like the ones that surround mordor. She finishes her story with a great line, "Nature often curves in interesting ways, but rarely does it work at right angles."
Go take a gander at the whole article. It's a great read that does a fantastic job of explaining how mountain formations should look compared to how fantasy writers think they should look.