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How do things get buried in dirt over thousands of years?
March 22, 2013 2:55 AM   Subscribe

It doesn't rain dirt from the sky, so how do ancient buildings get covered in dirt only to be dug up by archaeologists centuries later?
posted by almostwitty to Science & Nature (20 answers total) 23 users marked this as a favorite
 


Well, firstly, we build over them more often than not, and secondly, it does in fact, rain dirt from the sky. Well, it rains dust, and seeds that will grow into plants, that will die and form dirt. It rains water, that will break down the rocks. And much more here about soil formation.
posted by smoke at 3:10 AM on March 22, 2013 [2 favorites]


In addition to the above there's simply that it's hard for anything above ground to survive, so most of what we find is buried.
posted by michaelh at 3:18 AM on March 22, 2013 [1 favorite]


The Straight Dope has it, but the tl;dr is that until roads get paved, cities literally rise. That's because people bring things into them at great effort -- mostly food -- but, in the absence of cheap trucking, most of the waste gets dumped more or less in the city. Oyster shells get tossed out of the tent. Chamberpots get emptied out the window. Slowly, the accumulation of waste and junk raises the streets above the old buildings, and ground floors become basements. There are mounds throughout the Middle East that are nothing but layer upon layer of city waste, and of course there are mounds in North America that are hundreds of years of discarded oyster shells.
posted by musofire at 3:19 AM on March 22, 2013 [5 favorites]


In many cases, what gets dug up is the part of the building that got buried. Often it's only the foundations that are found - enough to reconstruct a floor-plan and make all sorts of inferences about use. Which is why places like Pompeii are so special. Any abandoned structure that's above the ground will tend to be scavenged for building materials.

(on reloady-thing, what michaelh says)
posted by pipeski at 3:19 AM on March 22, 2013


The Straight Dope's writeup is great, and there are a few cities that come to mind (like Thessaloniki, Greece), where you can see the churches quite clearly at a lower grade than street level.

But another great demonstration of this I saw was when I worked next to a building that was being torn down. The construction workers tore down the building and then spent at least a week or two carting off rubble until all they were left with was an empty lot, which was only possible because they had a lot of machinery and trucks available. Without the benefit of all that equipment, a fair amount of rubble and debris just gets left in place, and people build on top of it. Continue this process for 1000 years, and that's the result. It takes a lot of effort to keep new construction at street level.
posted by deanc at 4:09 AM on March 22, 2013 [1 favorite]


You can see this in Cambridge. Both the Gate Tower in Christ's College and St. Benets Church. In the latter case, steps from the street lead down to the church and its garden. This is mainly for the reasons musofire lists above.
posted by vacapinta at 4:24 AM on March 22, 2013 [1 favorite]


In addition, over hundreds to thousands of years, the earth accretes material as fields and forests go through annual cycles and leave another layer of... stuff that eventually becomes dirt, soil, etc. Some of it is blown around by the wind, or eroded by rivers and streams, so that it is stripped from some areas and builds up deeper in others.

Outside a city, if you stand on the ground, what was "ground level" a thousand years ago might be ten feet or fifty feet below you.
posted by megatherium at 4:30 AM on March 22, 2013




A more complete explanation is that "dirt" is moving around the Earth constantly by both nature (wind, rain, water, gravity mostly) and humans. The places where it gets deposited do a good job of sealing archaeological deposits, and so that is where sites are found and investigated. Places where earth has been removed destroys archaeological sites (or messes them up), so those are not investigated. In southern Kansas, there are places where natural forces scoured all of the earth down to bedrock because of drought, and the oldest soil only dates to 6,000-9,000 years old. There are places adjacent to the Mississippi river that were obliterated when the river has changed course, which it does a lot. Those sites have been obliterated.

As a result, most of the important sites archaeologists study, are by definition, places where "dirt" has accumulated. As many have said above, humans do a nice job at urban sites of moving in materials which has the effect of burying older settlements which is good. In other places, nature can do an effective job scouring places.
posted by Tallguy at 4:44 AM on March 22, 2013 [1 favorite]


Also volcanic eruptions, forest fires, dust storms and the like do indeed cause particles to come out of the sky and those things have to land and accumulate somewhere.
posted by mmascolino at 6:10 AM on March 22, 2013 [1 favorite]


most of the waste gets dumped more or less in the city
AKA middens.
posted by beagle at 6:28 AM on March 22, 2013


In addition to the excellent answers above also be aware that unless it's bedrock, the ground is a fluid, not a solid. I have a small brick patio in my backyard that is slowly being swallowed by the soil around it.
posted by Slap*Happy at 6:45 AM on March 22, 2013 [3 favorites]


The Seattle Underground used to be street level, and that's only a little over 100 years.
posted by Cool Papa Bell at 7:26 AM on March 22, 2013 [3 favorites]


Also, a lot of places are buried by changes in coastlines -- water rising, rivers changing course, floods. See: places flattened by Sandy.
posted by acm at 7:58 AM on March 22, 2013


It doesn't rain dirt from the sky

The sky begs to differ.
posted by flabdablet at 8:22 AM on March 22, 2013


Actually, it DOES rain dirt -- just take a look at my car, after it rains.

Without dust, there'd be no rain, because each raindrop condenses around a dust particle.
posted by Rash at 9:43 AM on March 22, 2013 [2 favorites]


As everyone has said, dirt get moved around, but there's another variable in this equation: The problem of specific preservation. Things that get buried get preserved. Things that become exposed are destroyed and scattered by erosion or rot or wind or what have you.

So when you look at where archaeologists are digging and asking where all this dirt is coming from, you are extrapolating from an incomplete sample set. To use my local terrain and prehistory as an example, there were probably huge numbers of Archaic sites along the edge of the river bluffs that you'll never see an archaeologist digging because they long ago eroded away to bury Woodland and Mississippian sites. Those sites, if they were still there, would be hovering in thin air as much as 50 feet out from the edge of the ridge and 100 feet off the ground. That's a lot of dirt that isn't there any more.
posted by Kid Charlemagne at 12:56 PM on March 22, 2013 [2 favorites]


There's also ground subsidence and flooding. Much of Rome's ancient city was buried after people abandoned the town. The river flooded annually, stuff got silted up and there were few people there to clear it away. Over time much of the place disappeared. Many cities now are on flood plains, they are only standing because people maintain them constantly.
posted by freya_lamb at 1:48 PM on March 22, 2013 [1 favorite]


You can see this in Cambridge. Both the Gate Tower in Christ's College and St. Benets Church.

Actually, the case of Bene't's is both weirder and more interesting. The churchyard is, indeed, about a metre below grade. This is very unusual indeed, as English churchyards are usually about a meter above street level due to their use as burial grounds. So the implication is that the street has somehow been raised by as much as two meters.

Close inspection of the surrounding land bears out this observation. The church is surrounded on two sides by Corpus Christi College and, in fact, used to do double duty as the college chapel (for many centuries, Corpus was popularly known as Bene't's College). The Old Court of Corpus is itself a truly ancient building and the lawn in the middle has basically never had anything built on it, ever. In other words, it's the same level as the medieval field that used to be there before the college was built around it. Sure enough, this ancient land-grade is significantly below modern street level; you have to go up stairs to get out into the real world.

So the other two sides of the church are defined by Bene't Street to the north and Free School Lane to the east. Bene't Street has been dug up many times and is basically full of dirt. Over the years, the denizens of Cambridge trucked fen mud up into the town and laid it down on the streets in order to raise the level of the town and prevent flooding.

But Free School Lane is different. In fact, Free School Lane is deeply cool. This is the place where the old Cavendish Labs were, the place they discovered the electron and where Rutherford did many of his famous experiments. Free School Lane also has, on the Corpus side of the road, weird bricked up windows which are remnants of the time the college was fortified during the Wars of the Roses.

Anyway, under the old medieval drainage scheme, Free School Lane had, in addition to an actual Free School, a drain running down the middle of it. It was a place where water could be shed from the higher ground of the town and down into the water-filled ditch which was supposed to protect the place from marauding bands of nogoodniks (until the town ditch was used as a plague pit then filled in with rubbish). So there was a very good reason not to spend effort filling Free School Lane with dirt from the river. Why is it so much higher than the medieval ground level then?

Back in the seventies some roadworks opened the ground to inspection and the mystery was solved. It turns out that Free School Lane sits on a metre-thick layer of mutton bones. Centuries upon centuries of Corpus cooks simply tossed their garbage out the back windows of the college, until there was such a thick layer of kitchen waste that it literally raised the level of the road and botched up the city drainage scheme forever.
posted by Dreadnought at 8:50 PM on March 22, 2013 [4 favorites]


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