I agree John but those stripes are only a "mystery of stonehenge" because (a) archaeologists uncovered them (looking for transport evidence) and (b) the media production staff look for "simple stories" and find them on blog-sites. The avenue, over its first straight section, descends at a grade of 1 in 43 and, no doubt, in dozens of other places the land surface displays such a grade. Chalk deposits were presumably once completely uncovered and subject to erosion from sea, wind, rain-water and, possibly, heavy water flows from some residual ice-covering. A suitable geo-scientist could go and uncover chalk anywhere outside the Heritage Site, without much restriction, and analyse a few striated surfaces, relate them to local grades and bring out a definitive paper. It's high time this topic gained maturity but would that be too boring a proposition to be funded?