Jimit, I've walked up those ramps right up onto Salisbury Plain in my capacity as gofer for Pilgrim who did a lot of work on the various proposed routes on behalf of the Stonehengineers.

When you view the ridge looming up even from miles away it looks extremely daunting and that exact spot looks very obviously the spot that would have been aimed for since it's a bit lower than what lies to each side. How old the ramps are I have no idea (I got the impression they were at least partly natural) but that gap seems the obvious one whether ramps existed or not.

I reckon stones could have been dragged or stonerowed up there along the line of the ramps, i.e. by going diagonally and thereby reducing the gradient. On the other hand going straight up on either side would be quite impossible IMO.