Not much is known about the Long Valley. All that can be found right now is that Cera said that you could get to it by following the earth break. She suggested going that way instead of through the Land of the Featherhead Sharpteeth. Cera said that the Long Valley would take them to the Fire Mountain.
However, Littlefoot said that even if the Long Valley did go to the Fire Mountain, it would take longer than going through the Land of the Featherhead Sharpteeth.