The cliff at Monument Valley is not all the same color. The accepted
explanation is as follows:
- Over time, rock surfaces exposed to the environment may become
coated with a thin natural coating, termed rock varnish
or desert varnish.
- Varnish is composed of iron and manganese oxides and some
clays, and becomes darker with time if the rock surface remains
- Old, well-developed varnish makes a rock look much darker
than it truly is.
- Many rock faces have varying degrees of varnish development,
because not all surfaces have been exposed the same amount of
- Some darkly varnished faces have been undisturbed for tens
of thousands of years, whereas less varnished ones were exposed
for the first time more recently, such as by a landslide.
- Native Americans selectively scraped off desert varnish to
produce petroglyphs, a type of rock art.
- Varnish also occurs on clasts resting on the desert floor,
if the land surface has not been disturbed for thousands of years.
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