AlUla, Wonder of Arabia

Recently I was lucky enough to be invited to present at a symposium in Beijing. The symposium focused the incredible active archaeological sites in AlUla, highlighting the work that is being carried out by researchers and their discoveries.

Beijing may seem very far away from AlUla (it is), but the The Royal Commission for AlUla and their French partners, the Agence fran?aise pour le dévelopment d’AlUla (Afalula), have developed an exhibition which is on display in the Forbidden City, Beijing. The exhibition, AlUla, Wonder of Arabia, is a beautiful display of the richness of the history of AlUla, going back around 250,000 years. Objects from the incredible archaeological sites here are on display, showing an impressive range of bones, stone statues, inscriptions, and more. It’s a story of ancient cultures, ancient civilizations, stories that are only recently being brought to the attention of the world.

You may be wondering what a geologist is doing at an archaeology symposium? This is where things get even more interesting. Because the story of AlUla doesn’t end 250,000 years ago with those delicately carved hand axes. The story goes back, way back, to almost a billion years ago. And it’s a story that has shaped the landscape we see around us today. A story that leads us to understand the very reason humans have settled here for thousands of years.

The landscape in AlUla varies depending on where you are. This is because of the different types of rocks. In the south of AlUla, there are dark grey rocks forming sharp peaks. These are the oldest rocks. Formed from the fire within our planet, these created some of the earliest land on Earth. There are other rocks here too. Metamorphic rocks, so intensely heated and crushed that their origins become almost enigmatic.

Here in the south, it appears to be a relatively harsh environment. The rocks are solid, fracturing into blocks (because igneous rocks are made up of crystals that are fused together). Without modern equipment such as drills and saws, they are very hard to work. There are archaeological sites scattered around, and some flint tools that have been found. But humans didn’t settle in this area for long periods of time. Nor did they build large cities here.

Moving northwards, the landscape changes, and more importantly for our story, this is because the rocks change.

As you take a few gulps of your water, and start to hike north, you notice the land around you change. From gravelly plains and jagged mountains, the rocks around you change to strikingly red: large steep-sided cliffs and ridges blushing in the early morning sun. (Geologists call a steep-sided ridge like this, a messa). These rocks are much younger than those in the south, and date to around 540 million years old. The redness comes from the tiny iron particles in the rock that have reacted with oxygen (similar to the rusting on your car).

Carry on your hike, and you notice the landscape change again. The rocks become more honey coloured; a light brown that dances as the sun moves overhead. You are walking back in time. You are now in a landscape where the rocks were formed from sediment deltas, enormous rivers, and shallow seas around 500 million years ago: a paradox in the heat of the desert sun where the only water you can see is in the bottle in your hand. Those huge messas surrounding you are gone. The landscape opens up to some stunning sandstone outcrops with open expanses.

The rocks show the history of how AlUla formed. But this landscape was not always here. The thin crust of the Earth is constantly moving. It is made up of several segments (known as plates) that are floating on hot molten magma. These plates are constantly moving: some are moving away from each other, others slide past each other, and some even smash into each other. This is plate tectonics: the very force that has literally shaped our planet. Around 500 million years ago, the continents as we know them today didn’t exist. AlUla as we know it today didn’t exist.

AlUla lay at the edge of an enormous continent called Gondwana, and masses of sediment were laid down from rivers, deltas and shallow seas. All of this sediment lay buried, hidden underground where heat and pressured squeezed it, compressed it, and crushed it, turning it to stone. This rock, all the layers that were formed from these long vanished environments, lay hidden just beneath the surface, as plate tectonics slowly pushed them eastwards.

After laying in slumber for over 400 million years, they were suddenly awakened. Around 30 million years ago, the opening of the Red Sea caused huge shifts in the nearby land. The sandstones that held little clues to their origins locked inside, were dramatically thrust up high above ground. If you stood here in AlUla around 30 million years ago, you wouldn’t be standing in a large valley, or in an expanse of sand, you would be standing on the top of a large sandstone plateau. Erosion, the unseen but unbelievably powerful force, slowly broke down a lot of this plateau, creating the landscape we see today.

And what does all this have to do with humans? It’s a good question, and the answer shows how the geology has shaped the where and the why humans settled in a place.

When all of that rock was thrust up with the opening of the Red Sea, it was filled with huge cracks. The force of the uplift shifted the rock, causing faults (some parts of the plateau were raised higher than other parts). These were weak points in the plateau. Weak points that the wind and the rain would attack first. Slowly, over a few million years, these faults and cracks were broken down creating valleys and plains. These valleys created easy routes for people to travel through. But there is something even more important: water.

Sandstone is not like those other rocks in the south of AlUla. As you know, igneous rocks are formed of all those minerals sealed together. Unless there are a few cracks, nothing will get inside. Sandstone on the other hand is made up of lots of grains of quartz. They are tightly packed together, but the grains are irregular, so they are not perfectly packed together. This means that there are tiny holes in the sandstone: holes that trap water. Underneath the ground in north AlUla is a vast aquifer: a huge underground water storage tank within the rocks. It is almost with a little bit of irony, that these little grains that were once tumbling around in water, were rehydrated once more as water seeped into the sandstone.

Those sandstones trace their origins back over half a billion years. And today, as they have been in the past, they are vital for all life: from humans to plants. This aquifer is mostly under the ground, but in AlUla, natural springs occur, and to dig into the aquifer, historically you didn’t have to dig very far. Water is the most precious thing in the desert, more precious than gold, or incense. And this is why people used AlUla as their route through Arabia, why many people settled here, and why cities were built.

The Kingdoms of Dadan and Hegra were built here because of the landscape. A landscape that was almost a billion years in the making. These civilizations carved into stone to build their tombs. They used the stone to build their houses and temples. The people found water here. The archaeology, just as the wildlife, is so rich because of the geological history. The formation of sandstones, the movement of the Earth’s plates, all created the ideal landscape for humans to not only pass through this region, but to settle here.

The lives of humans has been intricately linked to the geology for over 300,000 years. We can see this vividly in the stunning landscape of AlUla. And what’s more, this link isn’t just something of the past. The geology is a large part of peoples lives today.