Most deserts aren't sandy, but stony. Most theories explain this through geologic processes that blow away the sand or winnow out the rocks. But a recent theory turns them upside-down: the stones have always been on top while the fine dust and grit build up underneath them. In this article, About's Geology Guide, Andrew Alden, discusses the latest theories behind the creation of desert varnish, that artistic medium used by humans all over the world for creating geoglyphs. This image is a geoglyph placed into the Atacama desert site of Alto Ariquilda, one of the sites discussed in the Geoglyphs of the Atacama Desert, Luis Briones M. (c) 2005.
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