The grassy knolls around the site suggest that the locals hunted and farmed in olden times.#1, I think Fedora is on to something. ;')
So they actually found the whole of Dealey Plaza, not just the grassy knoll. . .
Seriesly, that sounds plausible. Undoubtedly this publicity will encourage more excavation at the site, so hopefully if there's more to be found it'll turn up soon enough.
I'm curious about what's involved in the "will tree" tradition mentioned. Of course this tradition probably stems from generations much, much later than the original site, but if the tradition at least reflects an accurate interpretation of the function of the structure, it makes me wonder if that implies a burial site in the area.