chunk: The amount of earth brought to the surface by worms can be estimated by the rate at which objects on the surface are buried and by weighing the earth brought up in a given time. Information from farmers on marl, cinders etc. sinking into the ground allowed Darwin to make calculations. He conducted a 29-year experiment on chalk at a field near his house. Objects of all sorts "work themselves downards" as farmers say. Large stones sink because worms fill up any hollows with castings, then eject them beyond the perimeter and the ground around them starts to rise. He visited Stonehenge and found some outer stones partly buried, the turf sloping up to meet them (see figure 7). Darwin weighed castings and had friends do so in other countries. He also weighed castings per unit area per year, then worked out how thick a layer castings would make, compared with rates of sinking. Additionally, he worked out casting weight per worm per year.