Geology is the story of our planet. From it’s formation 4 and a half billion years ago, to the features in the landscape today. It’s an incredible story. A powerful one. It not only tells us about how our planet formed, and how the land as we see it today came to be, but shows us how life was in the past.
There are some big concepts in geology; plate tectonics, evolution, deep time. These can be difficult to grasp: plates moving at the rate your finger nails grow thrusting up mountain ranges; or the unfathomable amount of time - a billion years. We are small and our planet, and it’s process, enormous, so it is often hard to visualise how rocks formed, or how the landscape was shaped through time.
But there are places that can help. All you need is to open up your mind. And some sturdy walking boots.
Sharaan is one of AlUla’s five nature reserves. Dominated by beautiful 500 million year old sandstone outcrops, slowly shaped by the wind, it is an environment that is full of life. Numerous species of plants flourish here. With the plants, the animals will follow. And follow they do. This amazing place is home to dozens of species of insects, birds, reptiles, and even large mammals such as gazelles, ibex, oryx and wolves. It is here in Sharaan, close to the edge of the nature reserve, that our 4x4 stops.
I wonder where our hike will begin as I look around.
The beginning of Harra Trail. A sandstone outcrop, around 100m high.
That crack of course is a wadi – a valley within the sandstone, that has been carved out from thousands of years of flash flooding. Originally, this valley would have been a small crack, as thin as a strand of hair. Each time it rains (once or twice a year), the water makes its way inside the crack, breaking down the grains that make the sandstone, slowly, slowly widening it. I look down at the dark boulders. Strikingly different to the surrounding rocks, these are not sedimentary rocks (like the sandstone around me), but harder, tougher, heavier. These are igneous rocks, rounded through countless millennia of being rolled and tumbled in those flash floods.
The climb isn’t too strenuous, almost like a steep staircase. We pass some large pebbles sticking out of the sandstone. Pebbles that were rolling around in a river 500 million years ago. All of this sandstone on either side of us was deposited by an enormous river. A river wider than anyone has ever seen, stretching thousands of kilometers long, and over a hundred kilometers wide. This was an enormous river. The layer of pebbles shows a time when a channel in this river was fast flowing. The layer above, made up of small quartz grains, shows a different channel, a slower channel.
Behind us is an expanse of light brown sandstone outcrops. Browned through the polishing of time. And in front of us, towering above like something made to keep King Kong inside, is an enormous mass of sandstone. At the very top is a thin, flat black layer: perhaps easily mistaken for a gigantic toupee resting on the sandstone beneath. A river of dark boulders meander through what appears to be a thin crack, giving the appearance of blood seeping out of a wound in the rock.
The sun is low, and our feet sink into the sand with each step, as we walk towards the enormous outcrop. And I realise, we are walking towards the dark boulders. It appears as though we are going to be trekking upstream. The crack opens up, and isn’t as terrifying as I imagined: the boulders provide good steps for our hike. We are going to be trekking up, up, and up. To the very top.
There are some familiar features too. Features that add a little more recent geological character to the landscape all across AlUla. There are holes scattering the sides, looking like Antonia Guadi has been creating another magnificent building to match those in Barcelona. However, the artist in this case was Mother Nature. These holes are known as tafoni, and take tens of thousands of years to form. After a pebble falls out from the sandstone, the gap left behind is full of looser grains. The wind, invisible, but powerful, gets trapped in these gaps, loosening grains more, and slowly, very slowly, making the hole larger. This process can create some spectacular natural forms, and this hike is the perfect open air display to see and appreciate them.
Another common feature looks almost as if someone has painted the rocks. Stripes. Splotches. Splatters. Again, the artist is the wonderful Mother Nature. There are tiny particles in the air and these are coated on the sandstones by wind, and rain (when it occasionally rains). They build up over time, giving the appearance of painted areas. This is desert varnish. You can see the desert varnish formed by water very clearly, stipes run down the rocks. The red colour comes from iron in the particles, while those stripes and splotches that are back are from more manganese. \It’s easy to get lost in these features. Just admiring their beauty and actual complexity. But we move on. Hard hats are now securely strapped to our heads, and we are near the top. This is what we have climbed for. What we have sweated for.
The rays of the sun break scatter through the rocks as the top comes into view. It is flat. Dark. Full of those dark basalt boulders. Here we are standing on top of something spectacular. It is an ancient lava flow, weathered over time, broken down but still strong. And up here there are some magnificent stories of our planets past. Stories which you can really see using the landscape around you.
Standing here, on this flat, almost empty landscape, you’re standing on a lava flow that happened sometime in the last 10 million years. Dozens of volcanoes erupted in AlUla from just 10 million years ago to half a million years ago. They were not explosive volcanoes, but more like fountains of lava spewing out hot liquid rock, which spread across the landscape. It would have been an incredible sight to see. Steam from the cooling lava in the day, glowing red at night.
This lava coated the sandstone beneath it. And this is the important thing we can see. Looking one way, we see large flat plateaus covering sandstone. Turn around, and you can see sandstone outcrops and expanses of sand. What we are seeing is how the landscape of AlUla formed.
Perhaps one of the most beautiful views you will see. Ancient, flat, black lava flows in the distance, and beautiful sandstone outcrops where the lava didn’t reach: these rocks had no protection from the hard basalt, so we are witnessing the effects of 10 million years of erosion.
All the sandstone rocks are formed from sediment that was in a river 500 million years ago. This sediment was deposited and buried deep underground. Here it lay for almost 400 million years, hidden from everything happening above, and slowly edging eastwards for thousands of kilometers. Sometime, around 40 million years ago, it was thrust up, high above ground, forming a massive sandstone plateau. Since then erosion has been slowly breaking it down to the sand beneath your feet.
And here’s the interesting thing you can see. Basalt is a tough, hard rock, so it takes a lot longer than sandstone to break down by the wind and rain. Those lava flows covered some of this sandstone plateau, but not all of it. The areas it covered were protected, and around the edges we see how the rest of the plateau has been turned to sand by the wind and rain. There is perhaps nowhere in AlUla that this can be seen in such a dramatic way.
The forces of our planet are truly incredible. But they are sometimes really difficult to visualise. Sandstone plateaus thrust up. Plate tectonics. Even 10 million years of erosion. But here, on the Harra Trail, not only do you feel a real sense of achievement from hiking up to the top of an amazing outcrop, you also feel the majesty of our planet. The views are spectacular. Perhaps what is more spectacular are the geological processes that formed them, and how your imagination begins to see.