The formation consists mainly of clays with some intercalated sands which were sedimented along a coastal plain in lagoonal and lacustrine/palustrine facies judging by the enclosed freshwater, brackish and marine biota.

A very diversified and well preserved biota can be found within the Bouldnor Formation comprising molluscs, vertebrates (especially mammals), charophytes and vascular plants.

In the middle section (i.e. in the Hamstead Member) the effects of the Grande Coupure on the biota are clearly noticeable and follow immediately after the negative oxygen excursion Oi-1 at the beginning of the Oligocene.

The Bembridge Marls Member was mainly sedimented in fresh or brackish water as indicated by cirripedia and gastropods like Terebia.

Relatively short-lived marine inraids are recognizable in horizons like the Bembridge Oyster Bed 1.5 metres above the base and a limestone band with bivalves like Corbicula and Nucula.

and other amiids have been found The fossil contents of the Bembridge Marls Member are quite varied, with freshwater species like Lymnaea and Unio and marine taxa like Melanopsis, Meretrix and Ostrea.

Amongst the finds are coleoptera, diptera, hymenoptera and arthropods as for instance Aeschnophlebia andeasi, Oligoaeschna anglica and Vectaraneus yulei.

Amongst the mammals are Anoplotherium latipes, Bransatoglis bahloi, Choeropotamus parisiensis, Ectropomys exiguus, Gesneropithex sp., Glamys devoogdi, Haplomeryx zitteli, Heterohyus, Microchoerus edwardsi, Palaeotherium medium, Paroxacron sp., Peratherium, Plagiolophus major, Plagiolophus minor, Saturninia gracilis, Suevosciurus ehingensis, Tarnomys schmidtkittleri, Theridomys bonduelli and Treposciurus.

The log bed is clearly a freshwater deposit as it contains besides the tree trunks plenty of washed-up seeds of the species Potamogeton and Stratiodes, and also the leaves of monocotyledon and dicotyledon plants.

This bed is almost 1 meter thick and is characterized by chocolate-brown ripple marks enclosed in alternating clays and sands.

The following mammal taxa were found in the Lower Hamstead Member: Amphidozotherium cayluxi, Amphiperaterium exile, Anoplotherium latipes, Bransatoglis planus, Butselia biveri, Cryptopithecus, Eotalpa anglica, Glamys fordi, Palaeotherium curtum, Palaeotherium muehlbergi, Paradoxonycteris tobieni, Pseudoltinomys cuvieri, Ronzotherium sp., Stehlinia minor, Suevosciurus ehingensis, Suevosciurus fraasi, Theridomys bonduelli and Xiphodon gracilis.

Roughly 10 metres above the base follows the Eomys bed and immediately above it the White Band also containing Polymesoda shells.

They belong to the following species: Amphicynodon sp., Amphiperatherium exile, Amphiperaterium minutum, Asteneofiber, Atavocricetodon atavus, Bothriodon velaunus, Butseloglis micio, Cryptopithecus, Elomeryx porcinus, Entelodon magnus, Eomys, Glamys fordi, Hyaenodon dubius, Isoptychus margaritae, Leptadapis sp., Myxomygale antiqua, Paradoxonycteris tobieni, Pecora, Peratherium perriense, Pseudoltinomys gaillardi, Ronzotherium romani, Stehlinia gracilis, Tapirulus hyracinus and Tetracus.

The marine character is also underlined by the gastropods Hydrobia sp., Pusillina turbinata, Sandbergeria vectiana, Strebloceras cornuides, Syrnola sp.

The log bed was formed during retreating sea levels and are part of a falling stage systems tract (FSST).

[5] The Grande Coupure within the Bouldnor Formation can be characterized by the enclosed biota as follows: In the Upper Hamstead Member 16 new taxa appear for the first time and 11 disappear.

Within the Bembridge Limestone Formation 47 taxa were present, whereas within the Upper Hamstead Member the number of species had reduced to 28.