In a book a read in my 20's (which was a hideously long time ago!) I heard stone and sea being described as "permanence at rest and permanence in motion" - it was a mental image that stuck with me.
I think we should add the sky as another form of permanence as well - although I don't know how I would describe it in a way that would differentiate from the sea - maybe "permanence in colour" or "permanence above".
I like this image because of the balance of the 3 permanences!
You can find more shots of the sky at Sky Watch Friday.
Enjoy the skies!