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?
It seems likely that some forms of rock varnish can be Can desert varnish be used to estimate the age of rock surfaces?
The answer: not very precisely but it can be a useful tool when combined with other forms of dating.
But is all varnish the result of bacterial deposition or could there be abiotic (non-living) chemical reactions that also result in desert varnish?
As I said before, there is considerable debate about the role of bacteria in the creation of rock varnish.
Rocks on the surface of the ground sometimes can form something called desert pavement and the surfaces of those rocks may also become covered with desert varnish.