The Bracklesham Group consists of four geological formations: Wittering, Earnley, Marsh Farm and Selsey. They outcrop on the coastline of Bracklesham Bay where exposures are dependent on low tides and removal of sand cover. The sediments are mostly sands and silts and are rapidly eroded when exposed to tidal action.

The deposits reflect deposition in shallow marine or coastal marsh environments. The better known sediments are the marine sequences of the Earnley and Marsh Farm Formations, which are rich in fossils.

The shell beds just south-east of Bracklesham are a spectacular site when fully exposed. Many people find it difficult to believe that these are shells really 45 million years old.

At Selsey, the marine sediments are often less commonly exposed. One bed forms the off-shore rock reefs known as The Hounds (or Houndgate Rocks) and The Mixon, the latter about 1km south of the tip of Selsey Bill.

The Mixon was formerly worked for a building stone and the older parts of Selsey are mostly built of it. The stone was also used by the Romans, who favoured Mixon for well lining. It can also be see at Fishbourne Roman Palace

The Selsey coastline was once the most rapidly eroding coastline in Britain. Even today, the sea defences cannot protect against the most severe storms when failure and over-topping of the shingle bank has caused massive flooding of the area.

The Environment Agency have decided that continued repair of the defences is no longer practical and a managed realignment of the coastline is planned. For details, see the Environment Agency web site.