The first deposit of the Thames Group is the Harwich Formation. It is very thin in
West Sussex and only exposed at Copperas Point in Chichester Harbour. It is a shallow
water silty clay deposited over the Reading Formation by rising sea level. Fossils
particularly include the small tube-like shells of a worm called Ditrupa plana. Pyritised
fossil twigs are common and careful searching may turn up the occasional fossil shark
tooth at this site.
Sea levels continued to rise and the London Clay was deposited in a fully marine
environment. Occasional beds of sandstone indicate periods of shallower water and
higher energy deposition. Fossils are abundant throughout much of the London Clay,
with fossil cockle shells, Glycymeris brevirostris, being common in the Bognor Rocks,
illustrated here at low tide.
Once known as the Aldwick Beds, the London Clay exposed on the foreshore in this
area of Bognor Regis is known for its fossil insects, one bed in particular being
called the Beetle Bed. The sediments here include large quantities of fossil wood
(lignite), fruits and seeds that indicate a period when large quantities of tropical
forest debris were being swept into the sea from rivers draining upland that lay
to the west.
In Chichester Harbour, the highest level of the London Clay is exposed on the beach
at Ella Nore, to the north of West Wittering. The clay here contains large numbers
of fossil Turritella, similar to those found at Bracklesham (but a different species).