Desert Varnish...Grand Canyon
Dark staining seen on some vertical rock faces in desert areas. What the varnish consists of is fairly well established. A fine windblown clay substrate hosts or traps dark pigmentation that is rich in iron and manganese oxides. The process by which the varnish forms is less well known.
See, for example,
Getting this close to a wall of Kaibab Limestone that has been heavily stained requires doing a little walking along the rim trail, away from the popular overlooks at the shuttle stops. This example lies between Hopi Point and Mohave Point.