That's Route 224, and the game describes it to be a miniature version of Sinnoh thanks to its geographical features. The hard part is getting there because you always have to get past uber-annoying Golbats!
Go back there when you've gotten the event item Oak's Letter. The route will extend all the way to the end of the map (I know it's long), but there ain't any Wild Pokémon there. Shaymin (Lv. 30) is at the end.
Based from an NPC in the game, this is why you need Oak's Letter:
In the past, Floaroma Town was a desolate and barren wasteland. All the flowers people tried planting didn't grow. Then, someone (probably Oak) gave thanks for the blessings of nature. Since then the land had erupted into a flower-filled wasteland. Of course, the deity Oak thanked was Shaymin. :p