Here's the image. This depression is in a boulder that's part of an outcropping in a ridge in New Jersey, I believe it's part of the New Jersey Highlands. You can see the Delaware Water Gap from there. A woman took me there when she learned I was interested in Indians and stones because she remembered partying there. (It's possible that teenagers may still be among the most likely people to lead us to interesting and special spots. They find them to be good places to get high!)
Anyway, this isn't the first of these depressions I've seen on high rock outcroppings, but it's the only with such an interesting shape, sort of paw-print or leaf shaped. The person who started me looking at stone work in PA said he believed pre-European peoples ground medicinal herbs in special places like this in order to include in them some of the natural power of the rocks and place.