What's that rock? Part 3

The sandstones of AlUla have colours. They have patterns. There are stripes, patches, and splotches of blacks and reds.

It sounds like something from a Dr Seuss book, but these colours in the rocks are as real as your Christmas socks. The patterns you see in the stone, are very curious indeed and quite unknown.

Although I could write this entire post with gimmicks, rhymes and even rap, it's time to learn about those geeky facts that aren’t on any App.

Whether you are enjoying a tour at Saudi Arabia’s first UNESCO World Heritage Site, Hegra, or seeing the beautiful tombs carved into the side of the mesa at Dadan, or even just a little hike, you will notice the patterns in the rocks. The sandstone has colouring to it. Sometimes there are dark, black patches, and sometimes you see red stripes running down the rock. It’s almost as if someone has painted the rocks.

This isn’t paint. This is desert varnish.

It’s an unusual feature, but a common one. And it’s formation is pretty amazing.

There are tiny, microscopic particles in the air all around us. In arid environments, these are blown around the landscape (in wetter environments, these particles are picked up by the rain droplets, and fall on the ground or in the soil).

When the wind blows in the desert, it splatters these little particles against the exposed rock. 'Splatters' would be the right word if we were shrunk down to be smaller than the full stop at the end of this sentence. Gently kisses the rocks is probably a better phrase. The wind gently kisses the rocks. It isn’t lipstick on the collar it leaves behind, but these tiny clay particles. Slowly, incredibly slowly, this builds up. The width of the hair on your head is around 1 micrometer: it can take over 1000 years for desert varnish to reach this thickness.

The wind kisses, and with each kiss, the clay particles build up, creating patches of black on the sandstone.

The black colour is because of the manganese element attached to the clay particles. But in some places you will see red colours, and these are mostly when you see stripes down the rocks.

These stripes form in a different way to the black patches. Whereas the patches are formed by gentle kisses, those stripes are formed by sweat. Well, not literally. But sweat is a good way to describe how they form.

Remember we said earlier how in wetter environments, the rain droplets pick up the clay particles as they fall? When it rains in AlUla (not often, only about twice a year), the rain droplets pick up these clay particles. When they hit the rock, they run down the side, leaving behind the clay particles, a little like sweat. Our sweat is basically water and salt. If we don’t wipe the sweat off our face, it will trickle down our cheeks. When it dries, it leaves a little trail of salt: the water has evaporated, leaving the salt minerals behind. The same thing happens to create these stripes. The water runs down the side of the rock face, leaving behind those little clay particles.

The stripes are red, because those clay particles that contain manganese, also contain iron. The iron has reacted to the water and oxidized, making it red (just like the rust in your car).

There's more to this story. And something rather surprising. Some researchers have argued that there may even be a biological component to those desert varnish layers. While the dust, those microscopic clay particles, are from the wind, there may be bacterial colonies that ‘glue’ them together. These microscopic single-celled organisms, may live in-between the minute layers, hardening the clay particles, cementing it together. While some DNA has been discovered with some desert varnish layers, these have been in less arid conditions than AlUla. For bacterial layers to form, and in such abundance in this very hot, dry environment, is probably very unlikely. But, this is a pretty interesting area to look into a little more, as we do know that there are many species of single-celled organisms that live in bizarre and extreme environments (I even discovered recently, that some live deep, deep down in the rock under our feet in tiny cracks!).

Sometimes, when you have a little walk through AlUla, the rocks look bonkers. Almost as if someone has poured caramel over them. This is desert vanish showing just how tough it is. The solid layer of desert varnish, is harder than the sandstone it is covering. Where the sandstone is not covered by desert varnish, it is exposed to the elements. The little rain there is, gets in there and breaks down the glue holding the grains together. The wind batters those little grains, removing them bit by bit. The result? Some incredibly unusual shapes, and some wonderful natural formations.

The desert varnish story doesn’t end there. For generations, humans have used this smooth, hard surface as a canvas. Rock art showing dozens of different species, as well as hunting scenes, or just messages as people travelled through AlUla, can be found in the rocks all over the area. The black, smooth surface, is perfect for carefully carving out images or writing. And what is quite nice is the desert varnish is removed, showing the light coloured sandstone beneath, so the rock art really stands out.

Desert varnish is something we all see, every day in AlUla. It is a part of the landscape. A part of our everyday lives here. How it forms, how it paints the rocks, is a story better than any Dr Seuss book.

For me, it’s just one more of the myriad of stories that truly are romancing the stone.