Mystery Photo (the first in a series)

I thought that this might be fun! Just to give you some help for this one - the photo was taken in the Ile-de-France, northwest of Paris, in late October/early November. And it's not a picture of a pile of rocks!

I can't promise prizes for guesses or even for a correct answer; I'll work on that if I get responses to this or future posts in the series :)