[HiTech RedNeck]

And in the meantime, if someone could have hypothetically asked the ancient people who put this up what they might think of this... they might have said “Cool. We wish we could be there for the dinosaur barbecue when you do.”

It says a “tomb” — is the actual burial in the ground, beneath? With the original stones, now replaced by that picnic table, being the markers? I can’t believe they managed to cart off an actual entombed body without noticing it.

[PLMerite]

I believe the theory is that there was a mound of earth built over the "chamber" created by the stone slabs.

Six thousand years is long enough for the mound to weather away.