They are merely craters raised above the level of the surrounding country by the gradual accretion of the soft oily mud, which overflows at frequent intervals whenever a discharge of gas occurs.
Its seaward accretion is estimated at 6 ft.
The opening of railway communication with London in 1906 resulted in a considerable accretion of residential population.
It was the final stage in a process of accretion dating back to the beginnings of the Christian reconquest in the 8th century.
There is a gunnery establishment in the harbour on Whale Island, the area of which has been increased to nearly 90 acres by the accretion of material excavated from the dockyard extension works, and various barracks including those of the royal marine artillery at Eastney, beyond Southsea.