Sounds like you're exercising a wise level of caution there Mr L ;)

Which could come in handy with this:
http://www.stonehengeusa.com/
and definitely would with this:
http://www.roadsideamerica.com/set/OVERhenges.html
But for faces on rocks, the old world can't beat this:
http://www.nps.gov/moru/
Gawds only know what future archaeologists will make of it.
In truth, the rock carvings in the SW US are more to my taste. And though there's definitely prehistory in north america, the landscape in places is snazzy enough to not need manmade monuments.