Hmmm- interesting one. It looks like a recent-ish drainage stream- have a look at the stones that make up the banking- similar pattern/structure to the dry stane dyke (from what I can see in the middle piccy). What was on the right of the stone that was laid across the stream? Any signs whatsoever of old fences/walls etc? I've come acoss umpteen old fences stretched across rivers/streams serving no other purpose that boundary marking. The pierced stone looks like an old gate post, but many old gate posts were re-used monoliths.