Not all archaeological sites are deeply buried. Often they’re just under the surface. However, the well-preserved remains tend to be deeply buried as these are less affected by the processes that disturb the soil (for example cultivation, plant roots, burrowing animals, getting churned up by freeze-thaw cycles).
There are a number of processes by which sites get buried. None of them are mutually exclusive. A lot depends on the local soils, topography, and environmental conditions. The main ones are:
- Colluvial deposition. Over time water washes down sediment that’s deposited on the site downhill. This affects sites at the bottoms of slopes
- Aeolian deposition- windblown sediment is deposited on the site. Tends to be in places with light sandy / silty soils and exposure to the wind
- Fluvial deposition. Sediments carried by rivers. Sites on flood plains.
- Organic deposition. This happens in places where soil conditions mean organic matter doesn’t decompose and peat forms. Tends to happen in wet climates.
- People. A good place to live tends to be a good place to live over time. People will knock down buildings and deposit soil to level the ground to build new buildings. This is most striking in urban sites, where you can have buildings built on top of each other over millennia. In agricultural contexts centuries of manuring and adding material to the soil can lead to significant deposition.
- Volcanic ash or tephra can deposit a lot of sediment very quickly. See Pompei or many sites in Iceland.
- Marine sediment. Storms and tsunamis wash up material.
- Glacial deposition. Only really relevant for very old stone age sites where moraine and outwash cover things up.
An important aspect of this is that these processes are often accelerated by human land use. For example, deforestation will lead to an increase in erosion and thus sediment deposition.