Ancient artifacts - depth = age

Hello Mr Adams -
I read the article on “Why Ancient Cities are buried”, and it was informative, as far as it goes. However, there is still a related mystery - that is, why are inorganic artifacts (like pottery pieces) dated by how deep they are buried? I don’t know the exact figures, but for instance, if such pieces are buried 20 feet deep, they are accepted to be, say 200 years old, while 40 feet means 400 years? (rough estimates). This seems to be applied everywhere - on all continents. Thus, the implication is that the earth is expanding continuously. Could that be due to the
“10,000 tons of meteorite dust a year” that we pick up? Where is all this extra earth coming from, and why does the depth/age ratio seem to increase in tandem? If a piece is 100 feet below the surface, then it is 1000 years old?

Are you referring to this article? The earth isn’t expanding. Well, it does pick up new material but that’s not why cities get buried. It’s all part of the erosion process. Wind and rain bring in new material and seeds, plants live and die on the ruins which become soil. Then new plants grow on the new soil, and the cycle repeats. If you look at an abandoned parking lot and see plants growing through the cracks in the asphalt, that’s how it starts.

The depth to age ratio is meaningless, some areas get buried faster than others. Buildings in the Sahara can be buried in a matter of hours, whereas in the savannah they can last for decades before being buried.

They are not. The depth at which a piece of pottery, for instance, is found gives only a relative date: pottery fragments found deeper than this are older while fragments found from above it are younger. This does not give the age of the pottery. For this absolute dating methods such as radiocarbon dating (possibly from organic crust preserved on the pottery shards) or thermoluminescence dating (for burnt inorganics) are needed.

To make things complicated even relatively speaking, there are any number of exceptions to the “deeper is older” - principle. A pit dug into the ground and later filled may have young stuff in it lying deeper than older stuff deposited in the undisturbed soil next to the pit, for instance.

The rate of accumulation of soil varies wildly due to local conditions. In locations where plant growth is limited but the ground is covered by slow-growing plants preventing “shifting sands” (ie. the Low Arctic), 10 000-year-old artifacts may still be found close to the present ground surface. In others, equally ancient finds may lie under several feet of peat, sand, clay etc.

Moved from General Questions to Comments on Cecil’s Columns.

samclem, Moderator

Bioturbination also acts to move soil upwards around objects, effectively moving them deeper in the ground over time.

Another force putting artifacts underground is burial of ruined cities. When an older city was flooded, burned down or reduced to rubble by an earthquake, a practice used in many places was is to level out the remains, cover them with dirt and start over. So you might be able to salvage some materials but a large portion of the former city was just buried to provide a more habitable place to lay down the new roads and foundations. Ancient Troy is divided into nine successive cities, although I am not sure if each of these relate to complete raising and rebuilding level.

It ain’t anything like that simple. Generally, there is no overall attempt to flatten a city and start over. Rather, what happens is more of a jumble - people build on old foundations, level small plots, excavate others, re-use stones originally used for something else, drop all sorts of debris in the process of living there, dig cellars, etc. etc. Occasionally the whole place burns, but generally not to the ground - if the city is made of stone buildings …

What you get is, in the ME, known as a “tel” - a mound built of remains. It is often a very difficult task to say exactly which remains belong to what period. What helps is if there is an intact and datable floor - you can be reasonably sure that anything under it is older than the floor.

Stuff that helps with dating: pottery shards, coins, organic remains, relative positions. It is often a puzzle.


Thanks for the clarification.