The full answer is probably one for Ed or Leanthar to answer, but in the old lore, this location was known as Rangers' Vale. Calling it Folian's Vale is likely a nod to the fact that a lot of Rangers follow Folian....and I think it also keeps us somewhat safe from possible copyright violations. Anyway, there's very little in the old handbook about the place, but I'll paste it here for reference (adjusted for current references).
The [Silkwood] Forest's verdant sprawl is located in central Mistone. The forest is fed by the lakes Ravine and [Allum], which are the sources of the [Ro river and the River of Shallows]. The abundance of water makes this forest especially thick with foliage, and perhaps this is why the Rangers' Vale (now Folian's Vale) is hidden away deep in the [Silkwood]. Rumors abound that once a year, on a date determined by the phases of the moons, all the rangers in Mistone gather in Aeridin's grove in the Vale for a meeting....
So yes, at least, there is supposed to be a temple to Aeridin there. It may also be revered ground by Folianites, though not necessarily "holy" or sacred.
