Well, I do not remember any stone circles, and if there were stones there, they were not taken from the river by us.
I was still eating the remnants of my cheese remnants 3 days later, and cannot remember seeing any cheese or other litter, but then again, we left by one route and saw nothing, and as Phil's photo shows, there appeared to be no visible litter. In fact I remember bundling up anything that I found, which was then carried out.
So I am quite surprised by this.