Ray Chapman (UK)
The cliff exposure of the Barton Beds between Highcliffe in Dorset and Barton on Sea in Hampshire are the type section of the Bartonian age and are highly fossiliferous. They are Middle Eocene in age and were deposited between 41.3 and 37Ma. They extend to Southampton in the east, Wareham in the west and Fordingbridge in the north with some other minor exposures in Southeast England.
The beds are marine clays, silts and sands deposited in a generally shallow sea that stretched to the southeast of the present shoreline and across the Hampshire-Dieppe Basin. Terrestrial input was from the west and northwest. The environment was sub-tropical partly because the average global climate was higher than today and partly because Britain was about 100 further south of its current position. The beds are alleged to contain some 600 species of molluscs, marine vertebrates, reptiles and other taxa.
Christchurch Bay, between Milford on Sea and Bournemouth, has developed over the last 10,000 years. Previously, the ‘proto-River Solent’ ran eastwards from the rivers Frome, Piddle, Stour, Avon and other small rivers. It ran behind what is now the Isle of Wight along what is now the Solent and joined the large ‘Channel River’ flowing westwards from the Rhine, Rhone and Seine. At the end of the last glacial period, the chalk ridge to the south, which joined what is now the Needles on the Isle of Wight and Handfast Point on Studland, was breached as sea levels rose. This allowed the erosion of the soft clays and sands that continues to this day at about one metre a year.
It is probable that fossils from this area have been collected since time immemorial. However, Gustavus Brander, a Swedish merchant and ambassador who lived at Highcliffe, made the first known collection. In 1776, he presented his collection to the British Museum. This formed the basis of the beautifully illustrated work entitled Fossilia Hantoniensia. Since then, there has been continual collection and research into the beds and the fossils.
In 1933, Ernest St John Burton classified 14 beds in the succession. These are based on lithology and palaeontology and are set out immediately below. The beds dip to the east and, as a result, the older beds are exposed to the west. The beds were laid down as a sequence of regressive and transgressive marine cycles with Beds K and L containing brackish-water molluscs.
Further east, the Headon Beds are lagoonal and estuarine. At Highcliffe, the basal Pebble Bed is considered to be the start of the marine incursion with a featureless clay band occurring above and before the described beds start. The lower beds are rarely accessible today: sea defences, slipping and a higher beach profile mean that Beds A1 and A2 are rarely seen. However, an opportunity arose in the middle of 2005 when some work was carried out on the beach path below Highcliffe Castle.
Although no clear exposures were made, material from deep postholes enabled a number of samples to be taken, sieved and the lithology and fossils checked against the comments of the early authors. The pebble bed was not reached. However, extrapolating from exposures further west, it was established that Beds A1 and A2 were as described and the Nummulites prestwichianus band, described by Gardner, Keeping and Monkton (1888), Burton (1933) and Curry (1937), was located.
At least three different levels of slipped and slumped clays can be seen with the River Terrace Gravel capping at the top. Slumping is caused by ground water percolating through the permeable sands and clays and lubricating a layer at the top of an impermeable bed. This causes a rotational slump. The shearing and slumping occur at different levels in the strata. One level is in the upper part of Bed F with gravel and brickearth colluvium. This falls onto the shear near the base of bed D and mixes with clay from Beds E and F.
The final level is a shear at the base of Bed A3 that collects colluvium from all of the upper beds. This level extends to beach level and is partly covered by beach sand and shingle. Bed A3 is mainly covered by this deposit and by beach sand and shingle with Bed A2 below it. The slumps tend to mean that, in many places, the terrain is a melange of the various beds and, in winter, the clays can be treacherous and trap the unwary.
All of the beds are fossiliferous, some more so than others. Species are not limited to a particular bed and can miss several to reappear in a later bed. Most beds contain wood fragments, some quite large. Many beds are pyritic and glauconitic, and contain small selenite crystals. The following is a brief description of the fossils that can be found in each bed:
|Barton Bed Formation|
|• Beds A1 and A2 have a fairly limited range of small molluscs and it is claimed that fir cones (Pityostrobus dixoni) have been found.|
|• Bed A3 has a wide range of species including Nummulites prestwichianus (see picture), echinoderm fragments, many molluscs and fish remains, shark’s teeth, vertebrae, ray dentition and otoliths (which are bones found in the inner ear, in this case, of fish).|
|• Beds B and C have less mollusc species but have a considerable amount of fish and turtle remains. Bed C is identified by the septarian nodules at the top and bottom of the bed. The lower nodules are often obscured by slipped material.|
|• Bed D has a limited range of species, many of which are decalcified or crushed. Perfectly preserved examples of the large gastropod, Clavilithes macrospira (see picture) can be found.|
|• Bed E contains most of the large molluscs in good condition. It has a single row of septarian nodules at the top.|
|• Bed F contains the large gastropod, Athleta luctator (see picture) near its base and many of the smaller gastropod, Turritella sp higher up. It has a number of drifted, winnowed shell layers, which when washed and sieved, contain a wide variety of small molluscs, bryozoans, corals, otoliths, echinoid fragments and small fish and shark’s teeth|
|• Bed G is a winnowed shell bed that is iron-cemented.|
|• Bed H is a sandy clay with a wide range of species including the bivalve, Chama squamosa (see picture).|
|• Bed I is a quartz sand with no fossils except some indistinguishable clasts.|
|• Beds J and K contain brackish water molluscs and, occasionally, crocodile teeth. Bed J is a grey, sandy clay and Bed K is white sand both of which contain the gastropod olivella branderi. Bed K contains the gastropod, Batillaria concava.|
|• Bed L is a black, friable clay with lignite. The Headon Beds lie further to the east and are named after Headon Hill on the Isle of Wight. These were laid down in lagoonal and estuarine conditions as the sea retreated. They contain freshwater molluscs, crocodile and mammal remains, but that is another story.|
Brander, Gustavus 1766, ‘Fossilia Hantoniensia collecta, et in Musaeo Britannico deposita’; Burton E St.J 1933, ‘Faunal horizons of the Barton Beds in Hampshire’. Proc. Geol. assoc. 44, 131-67.
Curry, D. 1937, ‘The English Bartonian nummulites’. Proceedings of the Geologists’ Association, 48, 229-246
Gardner JS, Keeping H and Monkton HW 1888, ‘The Upper Eocene comprising the Barton and Upper Bagshot Formations’. Q.J. geol. Soc. Lond. 44, 578-635.