> Is this a chicken and an egg thing that may only go back as far as the acts of enclosure.
> Sort of - "well we're gonna enclose the land, so we'll use some prominent landmarks....see
> all them mounds running along that ridge? everything to the south of them belongs to me"

two examples I can think of up here:

Hill of Drimmie stone circle http://www.themodernantiquarian.com/post/44138 a boundary runs straight through it (for the best view of it look at the Streetmap)

Clach Ossian http://www.themodernantiquarian.com/site/3800 in this case it was used as a prominent landmark for 18th century road-building. When the English army were putting down roads through the Highlands after Culloden, the road through the Sma' Glen used Clach Ossian as their "aiming point" from the bottom of the glen. They obviously weren't too clever because they kept aiming for the stone until they got right up to it, then thought "oh bugger" and had to lever it out the way (revealing a cist underneath)

Cheers
Andy