What's that rock? Part 1

AlUla, in northwest Saudi Arabia, is a place sculpted by time. Large sandstone formations create some of the most spectacular landscape on the planet. Every hike, every explore, and you discover new natural formations, new views that leave you speechless. It is a humbling reminder that we live on a planet. A planet that is dynamic, active, and quite simply beautiful.

We see an enormous range of landscapes, a myriad of shapes, a plethora of features. But what are they? How did they form? And, what do we call them?

The wait is over. I know your eyes are sparkling with anticipation, and you will not be disappointed. Each week I’m going to name a different rock formation, and give you a little background into how it formed. Now you will know your messa from your pinnacle. And you will be able to dazzle your friends with stories from the wonders of our planet.

But first, a little background into the iconic sandstones in AlUla.

Those sandstones were not always here. They began their life as small grains of quartz, slowly tumbling at the end of a river around 540 million years ago. Closer to the rivers source over a thousand kilometers inland, were large fragments, big blocks. But as these were eventually broken down by the continuing flow of water, sometimes smashing boulders into each other, breaking them down a little more, What we end up with at the end of this very long river, are the hardest minerals, the quartz.

They eventually settle, forming the river bed. More and more grains fall out of the water, and the layers build up, and they get buried deeper and deeper underground. Over time, rivers gave way to deltas and deltas gave way to shallow seas. You and I see small changes in a week, a month, a year: these changes happened over tens of millions of years, as the land sunk and sea levels began to move higher. These water environments left an awful lot of sediment. Sediment which was buried deep underground. It was squashed, heated, and crushed, squeezing out any water, compressing the sediment into solid rock.

Hidden underground, it slowly drifted northwards, as the silent but powerful force of plate tectonics creeped along, unnoticed, like the inevitable aging of our skin. Life above continued as it always had and always will. Dinosaurs evolved and became extinct. An ocean was born, while another disappeared. All the while, the rocks beneath slept. They were jolted from their dreams a little less romantically than Sleeping Beauty was (although, for a geologist, the story of the rocks being awoken is pretty romantic). Rifting began between African and the Arabian Peninsula around 30 million years ago, as the Red Sea began to open, pushing the Arabian landmass east. This squashed the landmass, and all those rocks that were below the ground, sleeping, were suddenly thrust up, exposed for the first time in over 400 million years. The power of our planet can be read as a love story: what we see today is the result of random forces over hundreds of millions of years, just like all those random events that led to the destiny writing inside of meeting of that one special person.

And here is where the landscape we see today begin. Wind and rain has slowly broken down what was an enormous sandstone plateau, removing those little grains of quartz, breaking through weak parts in the rocks. Erosion is a slow beast. We may see little change in the landscape in our life time, but if we came back to the exact same spot in one million years, we wouldn’t recognize it.

Today we see a snapshot in time. A moment. As if erosion has halted so we can enjoy the landscape: Mother Nature allowing us to see her beautiful marvels. But it hasn’t stopped: it will continue on until the end of time, as it has done since the beginning of time.

And so, the stories of these myriad formations we see all around us in AlUla, are an intricate tale of forces both past and present. Forces almost as powerful as love itself.

Mesa

Our first formation is one which every visitor will see when they arrive in AlUla. We see them surround us on both sides as we move through central AlUla. I can see them from my office window as I type. Large masses of rocks, dark red tower above us. Flat tops like tables from ancient giants.

I’ve often been asked what these are. Are they ridges? Mountains? Canyons?

Technically, these are what we call mesas. A mesa is a large ridge, with a flat top, very steep sides, separated by valleys or flat plains. Some mesa may have the last parts of what was once on top of it: lighter brown sandstone, curved from the wind. Soon these will be gone.

There are a few places where we like to hike up to the top of a mesa. It’s a little bit of a steep climb, zig-zagging along old trodden Ottoman and shepherd trails. We pass a few sparse bushes, and follow a beautifully shimmering golden green jewel beetle, larger than my thumb, before it lands on an acacia tree. Looking up at these large mesas from below, you may think there is nothing but rock, but there is life here.

We reach the top, and we can see a little more how they form. Large broken plates scatter the surface, a couple of inches thick, as if someone has shattered a layer of caramel. This is an iron bed, hard and compacted, cemented with tiny particles of iron, making it very solid and much slower to break down than the sandstone beneath.

The ancient city of Dadan is build into a mesa, where around 3000 years ago the people dug tombs directly into the side. A mesa is actually a great place to build a city. The rock behind protects the people and the houses from wind and from invaders. There is a free and constant supply of material to build homes, shops, and temples. And high up, people can spot visitors, or herds of animals from far away.

There are many mesas in central AlUla. From the air, you can see a huge number of them. This large concentration of mesas formed because of giant cracks in the sandstone, called faulting. Remember the opening of the Red Sea? This pushed Arabia eastwards, and thrust all that rock up. When the sandstone was buried, it was baked and squashed, and all of this pressure put thousands of cracks in the solid rock, and you can still see cracks in the sandstones all across AlUla. When it was pushed up, the compression of the movement caused rock on either side to in different directions: and these weaker parts eroded much quicker that the solid rock.

Here’s the interesting thing. If you take two cookies and place them side by side next to each other so they are touching. You are holding two solid cookies. Now if you keep holding the cookies just like this and move your hands up and down, the parts that are touching begin to crumble.

On a much less tasty scale, the same thing happened to the sandstone. As it was pushed up, the compression rubbed parts of the rocks against each other. Wind and rain attack these faults a lot faster than the surrounding rock, and we can see these in the wadis around AlUla today. Erosion along the faults created valleys, separating the mesa from one another. Central AlUla itself is one huge valley that has formed between two mesas after hundreds of thousands of years of erosion. So, really a mesa is just a smaller part of what was once a much larger sandstone plateau.

On our hikes, we can be walking across the top of the mesa in some places where it is tens of meters wide, and then reach thinner parts near their edges, which are a leg-wobbling half a meter wide. Up here, you feel like you are somewhere else. There are no sand dunes. There are no cars. Rarely will you even see people. It’s quiet, calm, and beautiful. Completely paradoxical to the very forces that created it.

A mesa is a snapshot in time. A place where we can see the beautiful landscape of our world.