Early occupants of semi-arid areas found a cultural use for what is one of geology’s greatest annoyances: desert varnish. Annoying because once developed it leaves an extremely durable brownish to black, shiny coating over rock surfaces: be they dunite, marble or quartzite, sandstone or granite, desert outcrops all look very much the same. You have to bash them unmercifully to see the true texture and mineralogy, and, except on images of thermally emitted infrared, remote sensing doesn’t help as the varnish has the same reflectance whatever the wavelength of radiation. Yet to the former inhabitants of dry lands – and latter day ‘taggers’ – desert varnish has been irresistible for millennia. Lightly peck away with a sharp pebble – and some ability to depict your thoughts – and you can leave an almost indelible sign that you and your ideas were at that very rock face: a petroglyph, picked out for all time in the manner of chalk on a blackboard. Even more spectacular, given an oversight of a varnished cobbly plain and it is possible to magnify your tag, or whatever petroglyphs once signified, a hundredfold or more. That happened on the famous Nazca Plain of Peru and continues to do so in especially dry places in the south-western US, as around Lake Havasu City in Arizona. Varnish forms only on the exposed face of cobbles, the downward side remaining more or less the original rock’s colour; generally lighter. Turn over the cobbles in an organised way, with a degree of persistence as well as talent and you too can make your mark on Google Earth! (Do not pass this on to Banksy – it doesn’t hurt the ecosystem, but will annoy the authorities immensely).
For all this period of artistic endeavour, stretching back in some places to the Palaeolithic, it now seems that desert varnish also records how environments have changed as well as the religiosity, humour or downright egotism of its inhabitants (Dickerson, R. 2011. Desert varnish – nature’s smallest sedimentary formation. Geology Today, v. 27 (November-December issue), p. 216-219). As well as reviewing how the varnish forms (see also Desert varnish in EPN May 2008, in Subjects: GIS and Remote Sensing)., Dickerson flags-up the little-known fact that the minute layers produced as varnish imperceptibly develops record changes in environmental conditions – wet, dry and middling – and, moreover they can be dated precisely despite being extremely thin (e.g. Liu, T. & Broeker, W.S. 2008. Rock varnish microlamination dating of late Quaternary geomorphic features in the dry lands of wester USA. Geomorphology, v. 93, p. 501-523). Liu and Broeker were able to match variations in the colour of varnish layers with important climatic episodes of the Northern Hemisphere, such as the Younger Dryas and other warming-cooling, dry-wet shifts as far back as the Last Glacial Maximum. Their approach offers a chance of dating petroglyphs and thereby cultural changes during critical stages in the history of modern human migrations, occupations and abandonments, even when no artefacts or bones remain. That is because once made, petroglyphs gradually become varnished themselves.