I don't know how it works elsewhere, but in my experience around here, a site of such importance would have been cordoned-off by a fence, or simply some minimalist wired strung between metal posts. And then likely topped-off with a sign behind plexi, made of metal, or on a bronze plaque.
Or the entire area would be clearly part of a designated park and thus easily understood you don't put your own property on it. I mean, not even a typical 'Murican would put up such an elaborate structure at a regional wilderness, park, or designated historical site. Well, at least not a Californian, anyway...but the rest of the country things we're odd, so there's that. |:S|
Because yes, to be fair...the photos are "just a pile of rocks in a depression," which could easily be explained-away as a natural phenomena unless you're into this sort of stuff.
So my question is: was it clear this entire area is some sort of historic park and also clearly fenced-off from everywhere else so you knew crossing that fence held some significance (at minimum, "private property")?