Archaeological layers build up over time as people build and rebuild in the same place. Archaeology in London is usually found within the top nine metres below ground level. Most of the train tunnels are much deeper than this, often 30 to 40 metres below ground.
The archaeology was therefore mainly found during the building of new stations which are not as deep, where other forms of surface work was taking place, where grout shafts were excavated to help stabilise the surrounding land and at the portals, where the larger tunnels surfaced.