Moss, you're absolutely right about the "strings" - they are exceedingly intriguing, and thank you for looking up the source of the information. It seems significant that these ropes/strings were laid over the primary mound because that maybe suggests that the place was already significant and had been marked by the primary mound, with subsequent activities that recognized and augmented the specialness of the place. It's very tempting to think that the strings were connected in some way to a post that was erected on that spot.

I'm also intrigued by the composition of the primary mound, which was described as "gravel". Not an obvious material to use and without knowing any better, I'd guess that it was retrieved from the bed of a stream. Another water connection? If the stream was the Kennet, the source of the "sacred river" that runs to the sea, then the gravel could symbolically represent the all-enveloping waters that figure in the creation myths ... and then all the different local soils, plus sarsens, above would represent the Avebury people's world, created from the waters, and standing proudly above them.

Enough speculation!