"Come up with a bit of evidence that they were 'sacred' to prehistoric people"

Fair do's. What constitutes evidence is the thing that needs sorted. Otherwise the presence of flint scatters could be used, which could be taken of evidence of nowt more that someone having been there with some flint. It wouldn't confer sacredness. The cloutie aspect is useful, but the inclusion of such places on a prehistoric list would have to be based on not much more than a hopeful implication of continued tradition, or at least continuation of the memory of a tradition. But before sacredness could be determined, would a defintion of 'sacred' be needed first?

I'm assuming a definition isn't needed for 'pre-historic', nor for 'people' ;)