A plaque explained when and why this area, including the viewpoint from which this photo was taken, was cleared. Unfortunately, I've forgotten what it said – I'd presume it was for the Millennium, but the work looks more recent than that.
The archway is a bridge, rather than merely a gap in a dry stone wall. I'm also presuming (without evidence) that it was erected to serve the nearby watermill, a sawmill which is still in use.