Age dating desert varnish

Rock varnish is much more than just chemicals that leached out of material above and ran down than surface of a rock leaving a stain.Rock varnish is a layer (sometimes hundreds of layers ) of mostly inorganic material that builds up on the surfaces of rocks that are found in environments where very little erosion occurs.

Dramatic sandstone escarpment faces of the Colorado Plateau in such places as Monument Valley are frequently coated by a reddish-brown silica glaze formed inside the unopened joint fracture and then exposed by block wasting. Rock coatings change the appearance of bare rock landforms.I expect that most people simply look at these dark streaks and think they are simply stains like a stain on a cement driveway or that line of rust color running down a wall from a rusty nail above. But what if you were shown a picture of the same rock taken 50 years earlier and the stains looked identical to you?You might start to wonder just how old those stains are on those rocks.This petroglyph is in Indian Petroglyph , New Mexico, USA.Desert varnish is commonly seen coating rocks in deserts.It is made up of clays and iron and manganese oxides.


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