I found five mortars in a cursory look. There well could be more. They were presumably created by local Native Americans. I feel they must have been used for ceremonial purposes. This location is out of the way, and far from a good place to grind acorns. And it has marvelous feng shui -- a wonderful confluence of energies. Hawks, vultures, and pumas frequent the neighborhood. (There are also depressions in the vertical of the entrance sandstone, from climbers in the 1970s.)
Here are images of each of the five mortars I found, singly and in groups. More images from the Sacred Space and beyond are in another nearby collection.