Jan Lindeman, of Taupo, asks :-

Since archaeological "finds" always seem deep under ground, how does the ground build up above them, where does this extra earth come from and does it cover the whole world?

Peter Sheppard, an archaeologist at the University of Auckland, responded.

This is a really great question because it helps us think about a very important idea which lies at the core of geology and biological evolution. This is the principle of Uniformitarianism which simply states that big effects like the creation of mountains or the development of complex organisms can be the result of the seemingly minor forces operating around us today.

Most archaeological sites or the remains of human activity have in fact been destroyed. However in some circumstances they are preserved. In fact much material still sits on the surface so on the top of ridges the wind, rain or gravity- the forces of erosion- can move dirt away from artefacts and leave them exposed. Hence a million year old stone tool may lie beside a coke can. The dirt can then go down hill and cover artefacts in the valley so the layer containing the million year old stone tools may be many metres under the ground while the coke cans are on the surface.

Under some circumstances changes in the forces of erosion or movements of the earth may expose these old layers for archaeologists. A good example is Olduvai Gorge in East Africa where erosion is exposing many very old layers millions of years old in which we can find the remains of human ancestors.

Of course other forces may not be so extreme so out in your flat horse paddock if you leave a stone on the surface in time it will be buried. How this happens was of great interest to Charles Darwin the father of evolutionary theory. He was interested in how slow processes we can see around us can have major impacts over time and for that reason he wrote a very popular book (The Formation of Vegetable Mould Through the Action of Worms, With Observations on Their Habits), on earthworms in 1881. In this book he reported how earthworms processed the soil and brought new dirt to the surface continually. He found this out partly by looking at archaeological remains of known ages in England and seeing how they had been buried by earthworm activity over time.