One interesting tidbit is that Dr. Camille Paglia has devoted a lot of time into researching these sites.
There are a wide variety of them.
“ There are many sites around the USA where ancient rock glyphs exist, but their exact locations are kept secret because of vandals, souvenir hunters, and eBay hustlers.”
Nevada is covered with petroglyphs everywhere. Shockingly, I was 4 wheeling about just outside of Reno one day and came across a locked gate in the middle of a tight canyon. Annoyed, I was about to turn around when I spotted a small sign beside the gate. The sign said it was a petroglyph area. I walked in and from the wash bottom to the top of the cliffs in the ravine were literally thousands of glyphs of varying complexity. The canyon was where water eroded down through a very substantial lava flow. According to the interwebs, some of the glyphs were over 20000 years old, and it is less than 10 miles from downtown. It is one of the largest and oldest such sites in the US. Its existence isn’t technically a secret, but neither is it advertised and you either have to know where it is or stumble into it.