We were told that visitors are sometimes shocked by seeing volunteers digging up untouched archaeological layers with a spade. The archaeologist said that this is the safest way to act. If a trowel were used, the shoe (and there are thousands of shoes being found) would almost certainly be damaged, whereas the method of taking a spade full of soil and then pulling it carefully to pieces by hand is likely to produce an undamaged artefact, or at the worst to divide it with a clean cut.