What does this symbol mean?

I was in the Badlands, South Dakota this past week storm chasing and we came up on this rock formation on the edge of the grassy pasture that immediately dropped of the cliffs there. the symbol was a large circle with an X through it and then there was a smaller circle with the same center as the big one? from a birds eye view it looked like a target with the outside circle quite large and the other circle as the bullseye. If anyone has any clue what this could be, that would be great. It looks like this except the lines dont extend past the outside circle and the inner circle was not filled it. http://thumb10.shutterstock.com/thumb_small/103246/103246,1271760028,5/stock-photo-sniper-target-scope-or-sight-isolated-on-white-background-51342220.jpg