It's a lovely warm sandstone. We don't know very much about how gates were hung in the past - the best way is to look at old paintings. The pierced stone has clearly been dressed on the upper cusp and this suggests that it is probably fairly recent. It's not the kind of place to put a gate though.

The stone across the stream is just as interesting and is probably worth checking to see if there is any trace of a soil line at either of the ends. The tall stone wall in the background is very neat ...

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.