What Lasts Ten Thousand Years?

You can walk around in monuments two thousand years old in Egypt. Archaeologists dig up mounds with relics from five thousand years ago. If we wanted to know if there was an advanced civilization somewhere on Earth ten or twenty thousand years ago, what might be found that could tell us at least where one of its cities were and even better, tell us a little about it?

If the ancients wanted to just let everybody in the future know they had been here on Earth, they might just make a huge block of tungsten, and set it up somewhere so that any following civilization that could identify metal would know that this was not a natural object, and could not have been made by some set of hunter-gatherers. Finding and refining out a cubic meter or ten of tungsten requires some serious metallurgy.

If we assume that the ancient civilization had some smart people, and for whatever reason, they wanted to leave a mark on the planet that they had been there, they would certainly have tried to think through all the possible events that might happen between the time they build their marker, and the time that another civilization would get to be sufficiently advanced to know what it was they were looking at. They wouldn't have thought of building the marker, or cared about it at all, if they didn't have the premonition, or more than a premonition, that their civilization wasn't going to survive much longer. They would have recognized the threat and deduced it was unavoidable. It is hard to imagine an entire civilization disappearing, and we don't have any clue as to what might do that, just some concepts that are perhaps possible. The ancients would be thinking of how to build a marker which wouldn't be engulfed by whatever was going to do their civilization in, and they also had to think about other possible catastrophes that could occur in the inter-civilization period.

Perhaps thinking of an example would make things clearer. Suppose the ancients were good at astronomy, and had noticed that there was an asteroid, far out in space when first discovered, that was going to make a direct hit on Earth, and it was big enough to annihilate almost everything. They had the telescopes to detect these, and had been detecting them for long enough to predict orbits very exactly. This means they might have been at least a couple of hundred years older, as measured from the advent to telescopic astronomy, that we are now on Earth.

Asteroid strikes have happened many times before to Earth and a prominent theory of why the dinosaurs stopped ruling the Earth and gave way to mammals was that a large asteroid hit the planet, landing in the Yucutan or just offshore, producing such a chaos of heat and dust and shock waves and tsunamis and earthquakes and vulcanism and lots more that dinosaurs couldn't survive. Some tiny mammals figured out how to, and they led to us, after another 66 million years of evolution. The asteroid for our example couldn't be this big, as there would be a geological record, but it couldn't be too small either. If it hit the deep ocean, there might not be any crater to find and no clues like the iridium layer that Alvarez found as a signature of an impact event for the Yucutan strike. An ocean impact would flood all coastal terrain, and create a huge amount of hot water vapor in the atmosphere, which would probably mean rain for a long, extended period, almost everywhere. The temperature would rise and stay up for a long time, as the Earth slowly returned to its pre-impact situation, less most life.

Orbits of asteroids vary in their periods, and some of the larger ones go far beyond the gas giants, and take a hundred years or so to cycle back to the near planets. So the ancients might have one or more centuries to plan how to make their marker, and could think long and deep about its preservation for millennia.

One initial question would be: where to put the marker? It couldn't be anywhere near the coast, or inland as far as the tsumanis would reach. It couldn't be inland anywhere that would be washed away by huge rains, which could flow in existing rivers but also might find other paths to the ocean. It couldn't be anywhere where the crust was thin, as there could be volcanos caused by a rupture in the crust from the impact.

Then there is the problem of ten thousand years of dust falling down on it, perhaps burying it. If it was put on a pedestal, that couldn't be too high and thin, as it might be tipped over. Maybe it should be huge, so the erosion of time would still leave something recognizable. If it was huge, it couldn't be made out of a single metal, like tungsten. Maybe there could be a cap of tungsten at the highest point. Rock easily stays around for ten thousand years, but it can't be too ordinary or the follow-on civilization might think it was from some so-far-unexplained natural phenomena. Before they became sophisticated enough to appreciate what the marker was, they might just think it was another opportunity for quarrying. Then the marker would wind up in parts in places that needed defensive walls, or temples of rock, or anything else extremely solid. Many archeological sites have been victimized by humans, in recent centuries, who had no interest whatsoever in preserving the past but a great interest in finding things that could be sold or used for their own purposes. Incan sites have been especially victimized by those bent on re-use of good materials.

One way to prevent re-use of the marker monument rocks would be to make them too big for a second civilization to use in its earlier period, before they became sophisticated enough to appreciate the preservation of ancient structures. They could also make them into something that could be re-used itself, or added to for re=use, rather than disassembled and carted off. Perhaps they would try to make their monument impressive, with giant statues of solid rock, so that the new civilization would think twice about abusing it. A new civilization might then try to make use of the monument for some purpose, like a temple or a royal palace or something else, and the new monarchs might even claim they had produced it, rather than found it after millennia of being ignored and abandoned. Perhaps a block of tungsten or titanium is the wrong approach, and something that caters to the likely situation in the new civilization's early years would be better. Writing on the monument might be fruitless, as the new civilization will need a long period of development before writing is established and they recognize what those markings are. So, a simple monument, made from large whole rocks that were of the hardest kind that could be used, perhaps with some statues, might be the final choice of the ancient civilization as they faced their doom.

Maybe there might be some remnant of their cities, or vacation spots, or ports or something else which survived the catastrophe and the following millennia, while the few remaining humans went through a return to prehistoric living conditions and gradually re-invented civiization. Sounds like some excellent archaeology needs to be done, and some careful scrutiny to make sure a misclassification does not occur.