Submitted by inexister t3_11dyv29 in askscience
It might sound obvious but ancient buildings that were once above ground are in some instances several meters below ground now. So where does all the dirt accumulation come from? Could a plot of land theoretically be maintained and kept clear of debris for thousands of years? Why do many cities inevitably get buried under themselves?
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Depends on the city but many ancient cities were purposefully buried as part of expansion.
Some cities in the US are built on top of their predecessors. There are parts of downtown Seattle that are underground but accessible. You can even take a tour. Pick up "Four Lost Cities: A Secret History." It's a light history of four ancient cities that were abamdoned.
Imagine living in a city with no sewer system. A lot of waste is disposed on the streets regularly. I am thinking oof examples in European Antiquity here. So, in addition to chamber pots and kitchen rubbish, there would be various animal droppings. If roads are not paved, when it's the rainy season, streets become very muddy. So some more earth or gravel is put on the streets to make them less muddy. Because of This, the street level slowly rises up. So maybe what once was your ground level entrance now lies one step under the streetlevel.
When looking at the history of citis, you often find recordings of fires throughout parts of the city. Moreover, there might be destruction through earthquakes, flooding or war. When you have a lot of destroyed buildings, it is often easier to flatten the rubble out and build your new homes on top. Sometimes, the structures on the ground floor survive, but as the accumulations in the streets rose, what was once a ground floor now becomes a cellar.
Building activities without technical or mechanical help are much more difficult,
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Where cities are built on wetlands or above shallow aquifers (which historically accounts for a LOT of cities, for ease of access to water), the answer is subsidence. Soil acts like quicksand to buildings over long periods of time, if it is saturated with water. Chicago is a good modern example of this.
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Yeah, other post above are right with the water way thing, most cities were built near the ocean or river systems on account of transportation. London goes down deeper than I ever imagined. I went 250m below street on one visit. Just fascinating what's down there.
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We are building/living on a sea of dead debris.
If it goes far back enough you’ll have to start digging through a few thousand years of dust and dirt and so on.
I assume that you’d have to keep digging out the buildings assuming a society lived in the same buildings for those thousands of years.
Geology and specifically formations under the earths surface can provide far more detail than I.
For cities that have been inhabited for a long period of time, a lot of it is literally just people dropping litter, or similar. For most of the history of human cities, getting rid of waste was not something anyone really made happen, they just relied on rain to wash stuff away, and for heavier solid waste, it just sat there. If you pulled an old building down to put a new one up in its place, you would clear out the bigger bits of rubble, but a lot would just sit there, or get pushed into the street.
For a while I lived in a city in the UK with a ~1000 year old church. You had to go down four or five steps from modern street level to get to the churchyard. The churchyard and church itself have been in continual use and kept maintained over that period. That land is an example of somewhere in a built up area that has indeed been kept free from junk and debris for 1000 years.
The dirt is just a collection of debris over the years.
It's certainly more common with dirt roads outside, rather than it being paved and somewhere to clean back to.
Here's a good vid showing doors raised over time as the street level rises
https://www.youtube.com/watch?v=fz4ZdXpri04
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You know how a sidewalk, driveway, etc will get grass and trees in the cracks that you have to clip? And how the edge of the grass can creep over the edge of the concrete, meaning the sidewalk occasionally has to be edged to keep it clear? Same thing, but with a lot more time.
Worth adding that cultures would often knock down old buildings and just level the remnants, then put the replacement building right on top of the older one just a foot or two higher than the predecessor. Many of these are the big mounds you see that are listed as archeological sites.
Combine these two for generation upon generation, and you end up with ruins buried anywhere from a few centimeters up to several meters under what we now call ground-level.
Of course, wind and water can move dirt in -- and they can remove or erode it as well.
Terraforming events such as earthquakes, volcanos, floods, asteroids etc. We’re talking big things here, events capable of erasing all traces of human life, and burying it deep under rubble, which soon becomes part of earth itself.
By extension, does this mean that we might be able to uncover, for example, an Ancient Greek papyri cache with many of our lost plays and poems?
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Seattle moved the whole city one floor higher because the toilets flooded back up with the incoming tide. Apparently, the first camps and some building had the water at their back door as they were on the tidal plain. Cool tour, highly recommend!
Yes and this sort of thing does happen, however it grows more and more unlikely as we find and excavate what remains of sites that could hold such relics.
We might recover more, but it’s much less likely that paper and other organic materials survive in areas of high moisture, like wetlands or aquifers, unless they are low oxygen environments, like bogs.
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That's a great example of natural disasters like hurricanes adding to the debris. If people like that youtuber were around throughout history nothing would get buried!
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That's insane deep woah. Where at?
So many little contributors just adding up over time!
That's a great example! Do you recall the name of the church? And how weird that stuff just accumulates around it regardless.
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I will add to the voices with no stated expertise.
When I visited Rome and asked about it, I was told at the Basilica di San Clemente al Laterano that the primary cause was frequent flooding on the Tiber, with some historical floods dumping as much as a meter of sediment in places.
I knew someone would ask me that, I'll have to get back to you with the name of the no longer used station that we went in at street level. It's right near Holborn I'll post it when I can, but it ts not a name I had ever heard of. Been shut since WWII apparently.
Another good addition to the myriad of causes being mentioned. I can see how that might cause someone to abandon their humble abode.
Thank you. While I understand that rubble accumulates, the definition of a 'tell' really hones in on the sort of answer I'm looking for. "A tell can only be formed if natural and man-produced material accumulates faster than it is removed by erosion and human-caused truncation,[6] which explains the limited geographical area they occur in."
I think that's the same for any human settlement, not just limited to a small area, but whole modern cities. It's a question of rate of accumulation vs deterioration. Natural disasters just add to the effects of constant deposition.
“Could a plot of land theoretically be maintained and and kept free of debris for thousands of years?”
The Pantheon in Rome is an example of this actually happening. Since this great pagan temple was converted to a church, it was maintained. If I understand correctly, it was originally on the top a hill. Now you feel sort of like you’re walking down into a hole when you go to see it.
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There are several different contributing factors. One is that cities tend to get built on what are called depositional zones. These are places where sediments accumulate (as opposed to places sediment erodes from). People like to build cities near rivers and near the mouths of rivers....just the sort of places where rivers flood and dump a bunch of sediment. This naturally buries things over time.
The second is that, especially historically, rubbish built up in cities. There were no trucks to haul away rubble on a large scale. If a building collapsed, you just sort of knocked down the rubble and built a new house on top. Especially if your houses are made of stone or mud brick. This results in a layer on layer buildup of debris that can actually leave an artificial hill called a tell.
Also, there's a flat-earth levels of crazy conspiracy theory about how the whole world was flooded with mud about 100 years ago, wiping out evidence of some advanced globe spanning civilization and burying the lower levels of cities. It's not the truth, but you may run in to people talking about it so I figured I would mention it.
Those sorts of events are really extraordinarily rare compared to simply normal everyday gradual subsidence, regular annual floods changing geography and depositing sediment, constructing new buildings on top of the rubble of the old ones…
redhousebythebog t1_jac9vkk wrote
Even today, some people have lost their driveway and walkways to overgrowth, and fallen leaves. https://youtu.be/FEI6mUmOTaI
That stuff turns to soil. Sooner or later, your Machu Picchu is covered into jungle.
Good location (waterways, trade routes) and some laziness will have builders building on top of what was once there