Large septarian concretions, produced by microbial activity (oxidation of organic matter) in the ancient seafloor during clay early diagenesis, are also common. These have been used in the past for making cement. They were once dug for this purpose at Sheppey, near Sittingbourne, and at Harwich, and also dredged, off the Hampshire coast. The clay is still used commercially for making bricks, tiles, and coarse pottery in places such as Michelmersh in Hampshire.
