The Earth is like a box of chocolates. You never know what you’re going to get. A long time ago, the local council put in new sewage pipes in the town where I grew up. The smell was overwhelming. Normally, a smell of coffee would permeate the town whenever the wind went northwest: the main employer was the local Douwe Egberts coffee factory. My father worked there on developing instant coffee. He never drank it himself. The taste of instant coffee did not compare to Douwe Egbert’s real coffee, and the process of making the coffee powder used far too much energy, in his opinion. The whole thing smelled wrong.
The smell of sewage that came from the deep hole lasted for several weeks. But whilst the nose strongly objected, the eyes were glued to the hole. The sandy soil turned out to be strictly layered, with thicker and thinner layers of sand and clay of different colours. One deep, thin layer was bright green, far more convincingly so than the green beach at the southern tip of Hawaii. The town was about a meter above sea level, which locally made it a hill; the sandy soil made it an anti-oasis of poor gardening. The ‘hill’ signified an ancient sand bank that had build up when this area was near the coast. Every time the land subsided or the sea rose, the area would again be flooded, and the sea would deposit a new layer of sediment and sand. The shape of the sea still shows up in the land, even though the elevation differences are no more than tens of centimeters. Trees also provide clues: small difference in the soils give large difference in the height of the mature trees. And now, the different layers in the hole showed how this land had been build up, in a battle with the fluctuating sea. I never found out how old the various layers were. Perhaps the Earth isn’t really like a box of chocolates. It is much more like a layer cake, where every layer looks and smells different.
The layers in our sewage hole were closely spaced and a well-dug hole could reveal all. Other places are not so lucky. In the US, where everything is supersized, it takes a hole of gigantic proportions to show the roots of the land. Luckily, nature has provided one.
Arizona’s grand hole
We had driven up from Tucson, through the desert landscapes of Arizona. After Flagstaff, the first volcanic cinder cones appeared, a reminder that this dry land was still very much alive. The last eruption may have been only a thousand years ago. As the altitude along the road increased, the shrub desert gave way to forest. The camp site appeared and we put up our tent amidst the trees. Nothing in the landscape gave any indication of the humongous hole nearby. Even a few hundred meters from the Grand Canyon, there was little to show the approaching end of the world; perhaps there was even a bit of a lack of scenery. It is notable that few photos of the Grand Canyon show the plateau around it.
At the rim, the first impression was earth shattering. The early June air had been clear during our drive up, but now there were some showers over the canyon. They gave the impression of a broken curtain hiding part of the view, but with enough clear gaps to show the far side of the canyon, a kind of window-gazing a world away. The colours were unreal: every rock seemed to have a different hue. It took a while to get a sense of the scale, to realize that what appeared to be a rock on the other side, was large enough to be a mountain in its own right. The scale of the abyss was unbelievable. The river wasn’t visible immediately below: you had to look along the canyon to glimpse it. These first few minutes at the edge left us speechless. What would it have felt like to the Spanish expedition who found the canyon, and knew that their quest for El Dorado was over? What was it like for the Indian populations who lived below? Did the views make the hardship worthwhile? You could easily believe it did.
The Grand Canyon is 1800 meters deep and 450 km long. Even so, it is neither the largest nor the deepest canyon in the world. In the US, Hell’s Canyon is deeper. In Peru, Colca Canyon is twice the depth of the Grand Canyon and in Nepal, the Kali Gandaki Gorge is more than thrice its depth. Yarlung Tsangpo Grand Canyon in China is 500 km long, and an unnamed canyon under the Greenland ice cap, only discovered 5 years ago, is 750 km long. Still, these canyons are invariably compared to the Grand Canyon and that already shows the public perception. None of the others are as accessible and none are as well known. And standing at the edge, it seemed hard to believe that anything else could possibly compete.
The layered canyon
Early the next morning, we drove back to the Canyon. The air was now completely clear, and it felt like you could reach out and touch the far side. The intricate detail made it seem like a picture frame. Everywhere we looked the rocks showed horizontal layers. They came in bunches. One bunch would create a vertical wall. The next bunch might create more of a slope. At the bottom, the river responsible for this mayhem stared back at us, as if to say, ‘who, me?’. The fact that the layers were all horizontal showed a lack of tectonic upheaval. This area has been calm for a long time. But it is not completely silent. There are occasional earthquakes here, about one every two years. There was an M4 in 2013, an M5.6 in 1992 and another M5.6 in 1959. The earthquakes tend to be around the volcanic field near Flagstaff, and north of the Grand Canyon near the border with Utah. You would not want to be in the canyon during an earthquake!
Oblivious to this risk, our goal for the day was to go down. We had picked the South Kaibab trail. This was back in the days when it was still possible to park at the trail head. Nowadays, there is a free bus service to get you there. Finding the trail was a matter of following the early-morning crowd. The path began at the rim and zig-zagged down a side gorge of the canyon. The day was not yet too hot, a good thing since shade seemed in short supply. The foot path seemed suicidal at times but was in fact remarkably well designed.
The hike became a kind of a geological Benjamin Button. Each step down took us further back in time. The lower the layer, the older the rock. The early zig-zags took us down a rock face of hard lime stone. These were the youngest rocks, deposited most recent. But in the Grand Canyon, ‘recent’ is a relative term, something like a young centenarian. These rocks were 260 million years old! In the time since they were laid down, continents had drifted, dinosaurs had evolved and become extinct, birds had evolved and survived, and a small tree-dwelling primate had become a primordial Eve and given rise to all of humanity. Of all that history, there was nothing here. It was as nothing had happened since the days of the first reptiles. Further north, in Bryce Canyon, much younger rocks exist, some even deposited within the last few million years. Once such rocks had been at the Grand Canyon too, but erosion had removed all trace. Only the lime stone had been hard enough to survive the erosion.
This already showed us one of the important aspects of the rocks of the Grand Canyon. There is time missing. Sometimes, the processes that deposit new layers had failed. New material can be brought in by water, by wind, or by volcanic eruptions. But when nothing happens, the book of time is never written. At other times, new rocks did form but erosion had later removed it again. And so, in the layer cake of the Grand Canyon, subsequent layers may come from different times, deposited in vastly different environments. The missing time is called a non-conformity.
The hard limestone was called the Kaibab, the same name as that of the trail itself. The word comes from a local Indian language and means ‘lying-down mountain’, and apt description of the Grand Canyon. The Kaibab were the rocks we had camped on. They dated from the Permian, when the world’s continents had come together in a supercontinent, called Pangea. The collision between Laurussia in the north and Gondwana in the south had created a long mountain chain, the Himalayas of their time. They still exist in the form of the Appalachians, now much eroded. North of the mountains, the land had subsided a bit and a shallow sea had formed. The sea was salty and tropical: the warm water allowed the limestone to form, complete with fossils of the corals of the days. At the other end of the shallow sea, these corals grew into the Capitan reef, another attraction that still exists today. The southwest US really is a geological treasure box.
The date brings to mind the Permian extinction, 251 million years ago, when life almost completely disappeared. The immediate cause of the mass extinction was the Siberian traps, some of the largest on-land volcanic eruptions the Earth had ever seen. The eruption filled the air with CO2, and the deadly heat killed over 90% of all life. But that extinction event was not seen here: the rocks of the Kaibab end just before this disaster.
The Kaibab was not a uniform block; it consisted of layers with slightly different composition, perhaps as the shallow sea changed. The layers were clearly visible in the rocks. But as we descended the rocks changed, and the slope became less vertiginous. These rocks were called the Toroweap, and had formed 270 million years ago. At that time, the sea was just reaching here and the Grand Canyon was near the sea shore. Sometimes the sea would cover the area and sometimes it would withdraw; the alteration gave a mixture of limestone, gypsum and sandstone.
We had reached the end of the gorge and the view opened up, showing the Grand Canyon vistas properly for the first time. But the rocks changed again, and the trail clung perilously to a vertical cliff. The cliff was the Coconino formation, a yellowish, hard sandstone. The layers were no longer strictly horizontal but showed angled lines. The cross bedding was the tell-tale sign of fossilised sand dunes. We were now in a time before the shallow sea, when the area was an arid sand desert. The sandstone was rock hard. There are several mnemonics available to remember the order of the names, Kaibab, Toroweak, Coconino, and so on. Richard Fortey tells of one that starts ‘kissing takes concentration’; the mnemonic for the following layers is not entirely suitable for a public blog. Less controversial ones can also be found. But the basic problem remains that geological names just do not roll of the tongue.
Now the trail followed a ridge, with views in all directions, and the rocks had almost instantly turned red. This was the Hermit Formation, eroded remnants of a softer rock. There is a sharp distinction between the Coconino and the Hermit Formation. The Hermit, a fine sandstone mixed with siltstone and mudstone, was deposited when this area was an ephemeral floodplain; it frequently dried out. Seasonal rivers would flood an otherwise quite dry environment. It was much like the Okavango, but without the elephants (although possibly with the mosquitos). Later, the rivers stopped flowing and the area became a desert: this river failure marked the boundary between the Hermit Formation and the sand dunes of the Coconino. The Hermit Formation formed some 280 million years ago.
The trail reached a solitary peak, and started to come down its steep side, into the canyon. The steep rock face consisted of blocks. This was the Supai formation. At times the rock face had really seemed impassable. The South Kaibab trail was made using dynamite: it was easy to see why! The trail bypassed a sheer face of red stone, appropriately called the Redwall formation. Looking harder, the red turned out to pretence. The rocks themselves were in fact white. The red colour was a varnish coming from the red Hermit above.
Below the Redwall, a shallower incline heralded the Muav Limestone. This was another marine formation, formed off-shore in relatively deep water, 510 million years ago. The rise of the sea was not unique to this region: it was seen worldwide. Typically, when a supercontinent has formed, the sea floor is mostly old. Sea floor is warm when it forms in a rift. It cools down as it ages, and this makes it contract and increase in density. For this reason, old sea floor is much deeper (5-6 kilometer) than young sea floor. During a supercontinent, more of the sea floor is old and deep, and therefore sea levels are low. When the continents break up, there is a lot of new, warm sea floor which is much less deep. Thus, sea levels rise and the low-lying parts of continents flood. This was the situation in the Cambrian, when the future Grand Canyon found itself near the shore, but in deep water.
The slope shallowed dramatically as we arrived on a large platform called the Tonto. This platform had been obvious from the top of the Grand Canyon. These rocks were called the Bright Angel Shale: we were still in an ancient sea. The rocks had a greenish colour, glauconite, which only forms in sea water, under conditions of low oxygen. On the platform, the trail picked its way between the fallen rocks.
Laurussia formed between 480 and 410 million years ago, when the core of old North America collided with the Baltic shield and with other floating bits of northern hemisphere. The collision with Gondwana was 300 million years ago. The Grand Canyon layer cake we had seen so far had all come from the long-drawn formation of Pangea.
We were not planning to descend to the river itself. That would be too much for a day return, especially this close to summer. Instead, here on the Tonto Platform we turned left, and followed a trail along the platform. Now we were alone. The number of people had been dwindling as we came down, but everyone else seemed to be be going down to the river. The trail curved along the slope. At times it was harder to keep balance on the slope, even though it was not particularly steep. We now walked between low bushes, and we quickly found out it was best to keep bare skin away from them, or risk having to leave some of it behind. We were getting tired and I had to sit down for a bit. Luckily we still had water left.
(Temperatures may have been high. A few days later, in Salt Lake City, we wandered around in a cool evening, to discover it was in fact 33C. Do not underestimate the heat in the Grand Canyon!)
The deepest part of the canyon that we missed was very different to the trail that we had seen. The top, just below the Tonto platform is another horizontal layer: it is a hard sandstone called Tapeat, which formed 510 million years ago. It is the remnant of a pebble beach, predating even Laurussia. There is sandstone here too, with cross bedding but at a steeper angle than the Coconino. This sand was deposited near the beach in sea water, swept in ridges by the currents and waves. In fact, it had formed very much like the fossil sand bank where I grew up, but dating from the time of the Cambrian explosion. Below this, the canyon cuts into heavily distorted rock. No horizontal bands here! In places, granite shows the heart of an ancient mountain from a time long before Laurussia. The continents had come together once before, about 1.2 billion years ago. This earliest known supercontinent is known as Rodinia (Russian for ‘homeland’). The collisions created a mountain chain which ran close to that of the Gondwana/Laurussia collision. There is little left of it, but in places you can still see the remnants. One such area is Central Park in Manhattan: the rocky outcrops come this collision – a Russian legacy in the heart of Trump-land.
But the base rocks of the Grand Canyon are even older than that. The Vishnu complex, the bottom of the Canyon, presents the true birth of Arizona. But its story is not easy to read. Everything here is meshed up. The oldest inclusions are mafic. There are remnant of volcanic dikes here, dated to 1.8 billion years, but the rocks that they intruded must be even older. Other rocks show pillow lavas and magmas, and remnants of turbidity deposits, mud flows into deep oceanic trenches. This part of the history has been dated to 1.75 billion years ago. About 20 to 30 million years later, the area had become a subduction zone and a volcanic arc had formed, similar to what Indonesia is now. The Gand Canyon rocks contains remnants of plutons which are typical for such arcs. Later, granite magmas intruded into the rocks, 25 kilometers deep below a mountain range. Continental fragments had collided here to form those mountains, but it is is hard to know what had collided with what, after all this time. Tellingly, both Canada and Northern Europe contain rocks of similar age.
Over the next half a billion years, the mountain range eroded away, bringing the granite to the surface. The region had drifted around, broken up again, and found other continental colleagues: this was the time Rodinia formed. It threw up a mountain range that too is now long gone. New sediments, perhaps the eroded Rodinia mountain, re-buried the surfaced rocks. When Rodinia broke up, it caused faulting. Bits of the surface were raised, whilst others sank. The raised bits eroded down to the already ancient rocks. The sunken bits did not. And finally, another half a billion years later, the region was buried again under the beach deposits of an encroaching Cambrian sea. Shortly after, Laurussia formed, and the recent history of the Grand Canyon had begun. Nowadays, where the Rodinian surface was eroded, the Tapeat lies directly on the Vishnu, hiding over a billion years of history. Elsewhere, the Rodinian surface still exists and lies in-between the two. But even here, at least 200 million years is missing, an unconformity of history. Whatever happened left no trace.
None of this we saw. Instead, we followed the horizontal Tonto trail, walking on stone left a trifling half a billion years ago. The track followed the contours of the land, moving back and forth but without much up and down. Finally, it crossed the main trail into the Canyon, the Bright Angel Trail. We turned left and began the long slog up. Unlike the South Kaibab, this trail followed an old fault line. Because of this it had captured a bit of water. There were some trees along the trail and the shade they gave was much appreciated. The water fountains along the trail were gratefully used: the South Kaibab trail had lacked any trace of water. We were now getting back into the tourist crowd, with everyone climbing back at their own pace. Sweat poured out of every pore and my red face seemed covered in white salt dripping into my eyes, a personal replay of how the Hermite had formed. Each laboured step up became a step forward in time. The continents drifted around us. Below us, Rodinia broke up and send its fragments across the world ocean. The north came back together as Laurussia. We saw Gondwana approach and collide, throwing up an ancient Himalayas to the south. We stepped through the shallow sea of the Muav, and saw the desert come and claim the land. We saw the land subside and the Permian sea advance from the Nevada shoreline, creating a coral extravagance. But the climb was too hard to spend much time looking back at the wonder of the rocks and scenery, or even at the sparse wild life staring suspiciously back at us.
As we came back up higher, it became harder to breath. The rim of the Grand Canyon is 2 kilometer, high enough that acclimatisation is helpful. When the rocks of the Grand Canyon were laid down all this was near sea level. Why was it so high now? The rise of the land came relatively recent. In fact that rise had been responsible for the Canyon. The Colorado River was already here; when the land rose up, the river had to cut down in order to keep flowing. The initial cut happened perhaps 5 million years ago. Erosion went faster during wet times. The river carried the stones that fell into it, and those stones helped further to cut the channel. Some layers eroded easily, and this allowed to Canyon to get so wide. But the ancient granite at the bottom was highly resistant: it had seen all of this before. Here the river could only manage a narrow, albeit deep gorge. The first person to traverse the river from one end of the Canyon to the other was John Powell. He later founded the USGS. So in a way, the river not only made its own bed, it also fathered the organisation now tasked with protecting its creation.
I mentioned that the youngest rocks are a staggering 250 million years old. That is not entirely true. Further along the canyon, there has been some volcanic activity even in the past few million years. The lava flows tumbled into the gorge and blocked the river, forming large lakes. Eventually, or perhaps quite soon, the dam was breached, and an enormous flood would come down the river. Erosion multiplied, and the river quickly removed most traces of the recent lava.
Having walked up a different trail, we ended up at the rim at Canyon Village, miles away from our car. One of us (not me) volunteered to get it. He came back amazingly quickly, having been given a ride by an Indian family in the back of a pick-up truck. Silently we drove our ancient green car back to the camp site. At the end of the day, all we could do was to crawl into our tents, exhausted and happy. And in the morning, I woke up to the sound of breakfast and the smell of coffee.
Albert, January 2018