I have come across something very similar to this in Arizona, but this information may not be applicable to your site.
These "slabs" like this can sometimes be maps, or can have maps, on the back-side of it, where the slab "rests" up against the outcropping. There is one way to tell if this slab has a map on its backside, and that is by looking at this backside, where it "meets" the outcropping, and look for little pebbles between the slab and the outcropping.
These types of maps, when put into place against the outcopping, are very suseptible to the effects of cold and heat, in other words, they naturally move. The slab, being a smaller mass than the outcropping, will move more than the outcropping, which will cause the two "faces" to rub each other, which can erase, or alter, the map on this backside. Hence, the need for these little pebbles, which prevent these two faces from coming into contact with each other.
In the case of the "slab map" in Arizona, the slab was about six feet tall, and the very top two feet or so of this slab had been busted off from the main slab, and then set back on top of the slab map once the main body had been set into place. If you have this at your site, or if any of you find a similar slab map to the one I am desrcibing, then know this: that you have to remove this broken off top piece before you attempt to "pull" the main slab forward. These types of maps have very important information on them, important enough to where these broken off top pieces, if not removed first, will slide down between the slab and the outcropping (as you attempt to "pull" this slab forward to read what's behind it), effectively erasing/altering the map to a point of being totally useless.
After "they" carved the map onto the slab and were ready to put it into place against the outcropping, these little pebbles were stuck onto the slab, or stuck onto the outcropping, with a little bit of "glue" (mud, etc.), and then the slab was put into position.
It's very interesting, the color contrast in this case. The color red, in itself, gives a "direction" to look to the "west". It would be of great help to actually see a photo instead of a drawing. You're a great artist, but I can guarantee that there are some very valuable pieces of information that can only be seen with a camera or with the eye. If you do take a photo(s) try to do it at several different times throughout the day...a sunny day. (every hour or so)