Howdo YP,
The causeway is pretty close to the road, it's usually quite boggy up there so a pair of wellies may be a good idea.
tracking down the origins of Wade near drove me around the bend but that's another tale.
Check out this
http://www.themodernantiquarian.com/site/6609/wheeldale_moor_cist.html
The page has a letter from Blaise Vyner giving his theory that the causeway may be a lot older than Roman, there's plenty of folk who don't agree though.
Whilst in the area you should have a look at this
http://www.themodernantiquarian.com/site/6683/old_wifes_well.html
It has a disputed tag on it but I'm don't let that put you off, this well is on the margin of a large site that has substantial evidence of occupation from the mesolithic to the bronze age and is the only decent water source around, plus it's pretty.
A walk up to this place from the causeway is not to difficult
http://www.themodernantiquarian.com/site/2948/simon_howe.html
Whilst your on the road it's worth having a look at the Wheeldale stones, mixed feelings over the antiquity of these but they're worth seeing.
Have a good trip
fitz