Like you, I think these may be natural formations that happen to resemble a wall. Could just be an intrusion of harder material exposed by erosion? It would be nice to know if they actually enclose anything. i.e. turn corners, that might help the case for 'man-made'. If it is a wall, the next question is; how long would it take for the the "weathering" to have this effect. What type of rock is it anyway?
Also, it is hard to know if the the second pic. which looks very like a wall, is part of the same formation as the others. It just looks 'different' to me.