Large areas of Somerset, known as the Levels, are at an altitude approximating to that of present High Water Mark. Below this surface lie buried valleys, filled by considerable thick-nesses of Quaternary sands, gravels, clays and peats. These are shown to be mainly of Holocene age, deposited in estuarine or coastal environments, over the last 9 000 years, at heights determined by the progression of the Holocene sea-level rise.
Older Quaternary estuarine sediments are also present, at heights above that reached by the Holocene sea, being related to trangressions probably of Ipswichian age. These interglacial deposits show evidence of reworking, around their margins, by the Holocene sea.