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As sediment builds up over the earth, it is compacted into layers of rock, which can be seen in the geological record. However, urban areas typically have good maintenance which washes away any opportunity for sediment to settle.

If we continue to maintain our urban areas, would there not come a point where the land around us starts to tower over the rest?

The "sinking city" problem helps illustrate the question.

In every city I can think of, the entire area is essentially a plateau of concrete and stone, my imagination conjures up dystopic-esque images of towns surrounded by walls of natural landmass - Which I think is pretty cool but probably not all that accurate. Jakarta and Lagos are the best (and only) examples I can think of, yet these are both coastal areas. Since geology is slow, is it safe to just assume that by the time this becomes a problem for us, we'll have already built over the pre-existing cities - circumnavigating the problem completely? What are your thoughts?

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    $\begingroup$ Can you explain why this is a worldbuilding question and not a landscape engineering discussion hint? $\endgroup$ – L.Dutch - Reinstate Monica Aug 14 '18 at 15:33
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    $\begingroup$ "In every city I can think of, the entire area is essentially a plateau of concrete and stone". New Orleans is built on sediment, and is sinking. In fact, since the Mississippi River has been contained within levees for flood control, the whole delta is sinking since it's not being refreshed with more silt. $\endgroup$ – RonJohn Aug 14 '18 at 15:34
  • $\begingroup$ And how does the fact that Jakarta "sits on swampy land" jive with "every city I can think of, the entire area is essentially a plateau of concrete and stone". Your question is very confusing. $\endgroup$ – RonJohn Aug 14 '18 at 15:36
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    $\begingroup$ Have a look at this article. $\endgroup$ – Michael Vehrs Aug 14 '18 at 15:43
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    $\begingroup$ @MichaelVehrs this is exactly what I was looking for. Appologies for such a poorly worded question. I wasn't too sure how to go about it since I was unable to research the topic. $\endgroup$ – Zac Walton Aug 14 '18 at 15:54
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Cities build their own "sediment": new roads layers get paved over old ones, new houses are build upon the debris of the old ones. You can literally trace history of the city though these cultural soil layers.

Raising ground level is very common in construction projects: sand or gravel or dirt are very cheap and usually available close by. Obviously, human construction happens much faster than a geological processes like sediment build-up.

Jakarta is an exception, it is sinking b/c of extremely swampy soil, lack of sand or gravel, as well as rising seas.

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Classically, you build the city on the city.

"I thought your man said they filled up the old street," said Boddony.

One of the dwarfs had stepped through with a lantern. "They're like... tunnels," he said.

"The old pavements [AmE: sidewalks]," said Sacharissa. "It's like this all round this area. After the big floods they built up the sides of the road with timber and filled it in. [...] And then it was simpler to just put heavy slabs over the old pavements. And so there's these - well, forgotten spaces."

[...] The dwarfs were already creeping along the tunnel at the other end of the buried street. By the light of their torches she saw old frontages, bricked-up doors, windows filled with rubble.

-- Terry Pratchett, The Truth

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