Ancient drawings survive in layers of ‘rock varnish’
From June 26, 2006:
As a newcomer from Germany, I am fascinated by Arizona's rich Native American history. Here is something I don't understand. How have the rock carvings survived for so long?
Germany, huh? Well, welcome. I hope you like it here. Kinda warm, ain't it? I used to know how to sing all the verses of “A Mighty Fortress is Our God” in German, a skill I quickly realized was going to be pretty much useless to me in making my way in the world. So I quickly forgot it.
But let us turn to the matter at hand: rock varnish, which is the stuff that has allowed the thousands of petroglyphs to be found around Arizona to survive for all these centuries.
Rock varnish forms on light-colored basalt rocks over a period of thousands of years.
It is a layer of iron oxidation — rust, more or less — mixed with a coating of dust or particles of clay. As the centuries roll by a black patina is created on top of that by the oxidation of magnesium that's in that dust and clay, plus organic material such as lichen or moss or bacteria.
The ancient ones created their petroglyphs by using rock tools to chisel through that black patina — rock varnish — to expose the lighter colored layers below.
As long as we are on the subject, there are three basic types of rock art.
There are the aforementioned petroglyphs, incised into the aforementioned rock varnish. Next come pictographs. These are designs or symbols painted on rocks with paint made of mineral or vegetable matter mixed with a binder such as fat or blood.
Since these are less durable than petroglyphs they tend to be found in sheltered areas such as caves or under rock overhangs or places like that.
Last, we have intaglios, also known as geoglyphs. These area huge animal or human figures on the ground made by removing pebbles and stones from the desert floor.