The London Clay is a stiff bluish clay which becomes brown when weathered and oxidized. Nodular lumps of pyrite are frequently found in the clay layers. Pyrite was produced by microbial activity (sulfate reducing bacteria) during clay sedimentation.

As with other clay soils, it is ‘heavy’ and not naturally good for agriculture, but two characteristics are very important for London: it is easy to be tunnelled and it makes very good bricks.