Michel wrote:
[div style="font-style: italic;"]Desert varnish take minimally thousands of years to build, so it is a good indication of the relative age of the rock art in a location, no matter what the orientation relative the dominant wind is. As far as I know, I never heard of any rock art from the same period in the same site that show different shades. It's impossible. If you have different shade in one location, it means hundred if not thousands of years of difference from one petroglyph to the other.[/div]
I am not a desert varnish expert but i had in my mind that wind and sun were critical to the development of desert varnish - I had observed two rocks of similar composition similar area and one had several mm of varnish and another had nearly none ...
I went ot wikipedia ... it states ...
Originally scientists thought that the varnish was made from substances drawn out of the rocks it coats.[sup id="cite_ref-1" class="reference"][a href="vny!://en.wikipedia.org/wiki/Desert_varnish#cite_note-1" title=""][span][[/span]2[span]][/span][/a][/sup] Microscopic and microchemical observations, however, show that a major part of varnish is clay (which could only arrive by wind).[sup id="cite_ref-2" class="reference"][a href="vny!://en.wikipedia.org/wiki/Desert_varnish#cite_note-2" title=""][span][[/span]3[span]][/span][/a][/sup] Clay, then, acts as a [a href="vny!://en.wiktionary.org/wiki/substrate" class="extiw" title="wiktionary:substrate"]substrate[/a] to catch additional substances that chemically react together when the rock reaches high temperatures in the [a href="vny!://en.wikipedia.org/wiki/Desert" title="Desert"]desert[/a] sun. Wetting by [a href="vny!://en.wikipedia.org/wiki/Dew" title="Dew"]dew[/a] is also important in the process.
So I am thinking that the orientation of the rock (i.e. direct sun vs shade) wind direction (lee or stoss) would dramatically affect the amount of desert varnish that would develp in a period of time.
