Carboniferous fossils protecting the coastline at Barton on Sea

David N Lewis (UK) and Stephen K Donovan (The Netherlands)

Many people regard fossils, quite rightly, as rare and exotic objects. Yet how often do people come into contact with palaeontological remains without appreciating it? Probably the easiest example to cite is that of quarried stone, either appearing as facing stones or, in a less aesthetically pleasing setting, when ground down or crushed for concrete or road ballast. Often, quarried stone is utilised a large distance from its source. For example there are no exposures of Carboniferous Limestone in the Netherlands, yet this rock is common in Dutch towns and cities where it is found as facing and decorative stones, far from its origins in Belgium and elsewhere. Obviously such uses of rock are to be admired visually but not hammered; yet this is not necessarily always the case. In this article we introduce you to exotic blocks of Carboniferous Limestone which are so situated that they are actively worn down by the elements, exposing the treasures contained within.

The cliffs of the famous fossil collecting area of Barton on Sea are part of the (often slumped) sea cliffs of Christchurch Bay in Hampshire and Dorset, extending, in the west, from Friars Cliff, near Christchurch, to Milford-on-Sea, near Lymington in the east (Fig.1). These are composed of Eocene clays and sandstones, overlain by Pleistocene plateau gravels (Fig. 2) and have been systematically eroded over long periods of time. Percolating groundwater acts as a lubricant and at certain levels causes the beds to slide one over the other, resulting in rotational slipping and collapses along the whole length of the cliffs. The sea removes the toe-load of the falls and so the process continues. Higher up on the terraces, pools of water accumulate, some quite deep and extensive (Fig. 2C).

Fig. 2. A View of the cliffs to the east of Chewton Bunny, looking eastwards. Erosion of the clays and sandstones produces extensive slips and mudflows over the cliffs, to beach level.

Fig. 2. C. View of part of a clay terrace with a pool of accumulated water.

Fig. 2. B Mudflows and rotational slipping of the Barton cliffs at beach level. The plateau gravels are the yellowish coloured beds at the top of the cliffs and the clays of the mudflows are grey.

Ernest St. John Burton was a local amateur palaeontologist and avid fossil collector who, with careful bed-by-bed collecting, described the stratigraphy and compiled a faunal list of impressive proportions for the Eocene ‘Barton Beds’ of Christchurch Bay, in his classic descriptions of the cliffs (Burton, 1929, 1933). His collection of fossils is now housed within the Department of Palaeontology at the Natural History Museum in London and his faunal list has had additions made to it over the years as more species have been discovered. Interestingly, Burton also recorded the rate of erosion in 1933 to be some three feet (about a metre) every year, approximately the same rate as today.

Currently, there are unprotected stretches of cliffs extending as far as the westernmost Barton defences and eastwards from the easternmost Barton defences, towards Milford-on-Sea. This includes the area beneath Naish Holiday Village, just east of Chewton Bunny (‘Bunny’ is the local name for a small river valley (Fig. 3) and, in this instance, the valley corresponds to the boundary between the counties of Dorset and Hampshire). For the collector of fossils, it is highly satisfactory that new exposures are constantly appearing since this area continues to be eroded, but, for the stability of the cliffs and the land above, this is a very serious problem.

Fig. 3. B. View of the Portland Stone blocks that make up the strongpoints of the defences at Chewton Bunny. Hengistbury Head is in the background.

Fig. 3. A. View from the west side of Chewton Bunny of the undefended cliffs beneath Naish Holiday Village, looking eastwards to Barton. The defences in the foreground are of Portland Stone.

Attempts to stabilise the cliffs elsewhere have partly succeeded where extensive – and expensive – coastal protection works have been built and repaired over the last 40 years or so. These have included not only the typical seaside groynes erected at right angles to the line of the cliffs, but also steel inter-locked shuttering pile-driven into the ground, parallel with the cliff face and positioned at various levels down the terraces. Large blocks of Portland Stone and Carboniferous Limestone have also been used (fig. 4), together with smaller blocks down to railway ballast size, to create a toe-load and to make walkways for the use of visitors.

Fig. 4. B. View from the undefended cliffs at the easternmost end of the Barton defences, showing the strongpoints of Carboniferous Limestone, the height of the cliffs and their erosion. The cliffs here are composed of clays overlain by relatively soft sands, capped with plateau gravels.

Fig. 4A. Blocks of Carboniferous Limestone of the Barton defences at sea level. Hengistbury Head is in the far distance.

On the Dorset side, Portland Stone is used from the west of Chewton Bunny towards Friars Cliff and Mudeford, ending just beneath the cliffs at Highcliffe Castle. On the Hampshire side, eastwards along the cliffs, and directly beneath Barton-on-Sea itself, both Portland Stone and Carboniferous Limestone are used.

It is not just the cliffs and foreshore at Barton that yield interesting fossils.  So do the weathered surfaces of large blocks of Carboniferous Limestone protecting the cliffs here. Fossils found in these include echinoids (sea urchins (Fig. 5B)), which are uncommon in the Carboniferous Limestone of Britain, and crinoids or sea lilies (Fig. 5A), which are very common. Sometimes, limestones composed almost entirely of crinoid stems are found.

These are referred to as crinoidal or encrinital limestones. Corals, brachiopods, gastropods, bryozoans and rare trilobites are also present in the blocks at Barton, as well as trace fossil burrows made by unknown animals. The Carboniferous Limestone blocks are very tough and hammers wielded by collectors bounce straight off their surfaces, releasing a sulphurous smell. Some echinoderm specimens from the blocks at this location are the subjects of a scientific paper (Lewis et al. 2003) and have been added to the collections of the Natural History Museum (Fig. 5).

The presence of large blocks of stone to protect the cliffs around our coastline suggests interesting possibilities for study, of the fossils and minerals they contain. The limestone at Barton is known to have come from the Mendip Hills in Somerset – from the large Foster Yeoman ‘Torr Works’ Quarry at Merehead, near Shepton Mallet. It was brought by road and laid in place by crane. These  rocks are relatively local if compared with, for example, blocks of larvikite protecting Hurst Castle Spit, that come all the way from quarries near Oslo in Norway.

Coastal protection schemes use stone from various quarries, some of which may no longer be in production or existence. Other quarries may be unsuitable for study for a variety of reasons, for example, because they may be deemed too dangerous, are working quarries, are filled with water, or they may be landfill sites. Therefore, their only available geological record may be the defence works created with rock extracted from them.

By asking the local authorities and other bodies responsible for coastal protection works, it is possible to determine and record the origin of blocks, together with the methods used for transport and emplacement. The build-up of such information will be useful to later generations of geologists and industrial archaeologists.

David Lewis works in the Department of Palaeontology at the Natural History Museum in London and Stephen Donovan works in the Department of Geology at the Nationaal Natuurhistorisch Museum in Leiden, The Netherlands.

References

Burton, E. St. J. 1929. The horizons of Bryozoa (Polyzoa) in the Upper Eocene of Hampshire. Quarterly Journal of the Geological Society of London, 85, 223-239.

Burton, E. St. J. 1933. Faunal horizons of the Barton Beds in Hampshire. Proceedings of the Geologists’ Association, 44, 131-167.

Lewis, D. N., Donovan, S. K. & Sawford, P. 2003. Fossil echinoderms from the Carboniferous Limestone sea defence blocks at Barton-on-Sea, Hampshire, southern England. Proceedings of the Geologists’ Association, 114, 307-317.