Change the comma after horizon to a period.
Either add a comma after ruins or add that before it.
I don’t like the latter part of the last sentence; devastation obvious and as powerful just doesn’t sound right. I can’t think of better words at the moment, though. (If I was editing this in someone’s manuscript, I’d ask the author for clarification/elaboration.)
They scanned the horizon. So many crudely made crosses rose up between the ruins that it was pointless to count them all. Whatever happened here had happened long ago, but the devastation was obvious and as powerful as ever.