Boxstones: In search of Miocene Suffolk
Tim Holt-Wilson (UK)
The date is 24 May 2014 and I am browsing across East Lane Beach at Bawdsey in southeast Suffolk. A brown lump of sandstone with a white fossil shell impression catches my eye. A boxstone. This is the first one I have ever found with a fossil in it. Looking closely, I see that the sea has abraded the shell’s outlines, although the margins have survived better than the rest, so it should be possible to identify the specimen (Fig. 1).
Boxstones are fragments of a vanished world. They are all that remains of a lost geological stratum in Suffolk called the ‘Trimley Sands’ (Balson, 1990), although deposits of similar age are still present across the sea in Belgium and other parts of Europe. Boxstones are lumps or concretions of brown sandstone, which may contain shell fossils and – if you are extremely lucky – bones and teeth. They are beach-rolled and rounded, and typically measure between 5 and 15cm in diameter. The sand is mostly quartzose, with a rich assemblage of secondary minerals, and is cemented with carbonate-fluorapatite (a phosphate mineral) and calcite (Mathers and Smith, 2002).
Boxstones can be found scattered sparsely across the shingle beaches at Bawdsey and Felixstowe Ferry (Fig. 2), and in situ as a common component of the basement beds (nodule beds) at the base of the Coralline Crag and Red Crag formations of southeast Suffolk (Fig. 3). They are eroding out of the cliffs and probably also from some offshore location. In former times, they could also be found in the small phosphorite quarries that dotted that local landscape:
With one exception all the masses of sandstone I have seen thus picked out from the diggings have been spherical, oblong or irregular masses about the size of the fist, or an average, or sometimes of an elongated cylindrical form. The exception was in a pit at Trimley, near Ipswich, where I found four blocks of a flagstone shape about a foot and a half square, which contained casts of shells, and seemed to be identical in origin with the box-stones” (Mathers and Smith, 2002).
Boxstones are an enigma that has attracted the attention of geologists and palaeontologists since the nineteenth century; and molluscan fossils have played a key role in unravelling the mystery. Unfortunately, preservation of the fossils is not good, because those with aragonite shells have dissolved away, leaving only mould and cast impressions (Fig. 4), while those with calcite shells (a far fewer proportion) show only patchy preservation (Fig. 5).
The cavities may then be infilled with quartz (Fig. 8), iron oxide or calcium carbonate (Mathers and Smith, 2002). Writing in 1870, Prof ER Lankester was able to identify a fauna of some 25 molluscan genera. Over the years, the list grew. Frederick Harmer (1902) listed 17 species and, by 1917, Alfred Bell had recorded over 100 (Bell, 1917; Figs. 6 and 7). The fossils are not easy to reach because the sandstone matrix is so tough – luckily my own specimen was exposed due to marine action.
Also, not all boxstones contain fossils. There are varying estimates of the proportion of the stones that do: the Rev H Canham suggested 75%, but Prof PGH Boswell believed that it was less than 10% (Boswell, 1928). However, in my experience, Boswell’s figure is the more correct. Very occasionally, other types of fossil are found, for example, wood, crustaceans (crabs and lobsters), coronulids (whale barnacles) and sharks’ teeth (Fig. 9), with a notable example of the latter being Carcharocles megalodon, figured by Reid (1890, figure 1). The exterior of boxstones may be encrusted with epifauna, most commonly barnacles (Fig. 10). These are evidence of submarine exposure before burial, most likely during the Pliocene.
Dating the boxstones
Lankester was able to use his molluscan fossils to correlate the boxstone fauna with a better-preserved geological sequence in Belgium. He suggested it was of Diestian age (Lankester, 1870) in the Pliocene – an interval now attributed to the Tortonian to Messinian stages of the late Miocene (Laga et al, 2001). More recently, Peter Balson has correlated the fauna with the Syltian/Messinian stages of northwest Europe (Balson, ibid). Thus, it seems likely that the boxstone fauna is of late Miocene age, somewhere between 5.4 and 7 million years old (Laursen, 1999; Table 1).
|Epoch||Million yrs BP||Stages||Suffolk||Belgium||Netherlands|
|2||Gelasian||Red Crag Formation|
|Pliocene||4||Zanclean||Coralline Crag Fm||Lillo Formation|
|5||Zanclean||Kattendijk Formation||Breda Formation|
|Miocene||6||Messinian||Trimley Sands?||Diest Formation||Breda Formation|
|7||Messinian||Trimley Sands?||Diest Formation||Breda Formation|
|8||Tortonian||Diest Formation||Breda Formation|
|9||Tortonian||Diest Formation||Breda Formation|
|10||Tortonian||Diest Formation||Breda Formation|
Let us therefore imagine the history of the boxstones. The ‘Trimley Sands’ and their shelly remains were deposited in the late Miocene, then eroded during the following Pliocene and some of their fossiliferous debris ended up in the basement bed of the Coralline Crag (perhaps 4.4 million years ago). The Trimley Sands continued to be eroded during the Pliocene and further debris (plus that derived from the Coralline Crag) was incorporated into the basement bed of the Red Crag (perhaps 2.8 million years ago). Today, both beds are being eroded with the clasts ending up on the beach (Fig. 11).
As an aside, it is important not to confuse boxstones with ‘coprolites’, which are also hard, brown stones found in the Crag basement beds and on local beaches. Unlike the sandy-looking boxstones, the ‘coprolites’ are much smoother and more fine-grained: their exterior has a polished look (Figs. 11 and 12). They are not true coprolites, but a kind of highly phosphatic mudstone (phosphorite) concretion, rolled and smoothed by marine action in the past. A wide variety of phosphatised fossils has also been found in these beds, including Mesozoic brachiopods and ammonites, Eocene sharks’ teeth and seed pods from the London Clay, along with Pliocene mammals, such as rhinoceros, deer, dolphin, whale and mastodon (Reid, 1890).
The coprolite-rich ‘nodule beds’ were quarried locally in the nineteenth century to make mineral fertiliser: the Fison company started this way, excavating the nodules from the Crag basement beds and then crushing them and treating them with acid to make superphosphate (‘Fisons’ – online at https://en.wikipedia.org/wiki/Fisons).
The battered boxstone in my hand is a portkey for researching and imagining what Suffolk was like in late Miocene times. In fact, as Mathers and Smith (2002) say: “Strata of Miocene age are very rare in Britain and none are preserved in the district“. Resurrecting this period will be like piecing together a jigsaw picture where almost all of the pieces are missing.
A few Miocene deposits have survived in pockets and cavities. Sands and clays have been found at the bottom of karstic sink holes at Brassington in the Peak District of Derbyshire (Pound et al, 2012). Blocks of fossiliferous sandstone have been found in solution pipes in Chalk bedrock at Lenham in Kent (Balson, 1999). Further afield, fossil-rich deposits in hollows in limestone at Hollymount in Ireland have been tentatively dated to the Miocene or early Pliocene (Coxon and Coxon, 1987). So little has survived because this period was one of crustal uplift in Britain, and erosion was active, while the North Sea was a subsiding basin area (Mathers et al, 2007).
Parts of southern England are thought to have remained above sea level; some ancient duricrusted land surfaces may have persisted here from as far back as the Eocene (Jones, 1999). (Duricrust is a hard layer on or near the surface of soil.) The Straits of Dover did not exist then – instead, there were chalk hills stretching between the Weald and the Artois region of France, while the early rivers, Thames and Rhine, drained northwards into a wide southern North Sea Bight. Much of Suffolk is thought to have lain beneath the waves (Tertiary Rivers: Neogene (Miocene and Pliocene) – online at https://www.qpg.geog.cam.ac.uk/research/projects/tertiaryrivers/neog.html, accessed October 2017)
Given its gritty sandstone matrix, my boxstone mollusc’s habitat was evidently a sandy one. Some of this sand must have been washed into the sea from rivers streaming off the English landmass, although mineral studies suggest that some of the sediment may have come from metamorphic rocks in the Ardennes region of Belgium (Boswell, 1928) or from reworked earlier strata containing this mineral suite.
The sandstone then became solidified beneath the seabed, along with its enclosed fossil. Deposits of a slightly earlier age have survived at Deurne near Antwerp in Belgium, where sediments of the Diest Formation lie close to the surface. They include sands rich in the grey-green mineral glauconite and containing fossil shells, sharks and Plesiocetus whale bones (Bosselaers et al, 2004; Figs. 13 and 14). (Glauconite is an iron-potassium silicate mineral related to mica formed in low-oxygen, shelf sea environments. Presumably oxidised glauconite has contributed to the brown colour of the boxstones.) Similar deposits have survived in Holland, where there is a thicker, more continuous Miocene sequence. This is because it was closer to the depositional centre of the North Sea basin, where downwarping and subsidence allowed a greater quantity of sediment to accumulate.
The Dutch equivalent of the Diest Formation is represented by the Breda Formation, which has a total thickness of over 700m (The Breda Formation – online at https://en.wikipedia.org/wiki/Breda_Formation, accessed November 2017). Glauconitic sandstones with shell fragments and phosphorite pebbles have been found at depth in the Colijnsplaat Borehole, with fossil mollusca and foraminifera indicating water depths of about 50m and warm temperate to sub-tropical climatic conditions (Slupik et al, 2007).
These continental formations give us a glimpse into the late Miocene of the southern North Sea Bight. We may assume that Suffolk seas were similar, perhaps a little shallower, and that they would have shared the marine fauna, including toothed whales, baleen whales and seals (Post and Reumer, 2016). Lankester considered that a phosphatised partial skull of Choneziphius (a small toothed whale), from a crag pit near Felixstowe, was an indigenous member of the boxstone fauna (Lankester, 1870). Other marine mammal fossils from boxstones include the extinct sea cow, Miosiren canhami, the beaked whale, Mesoplodon, and the sperm whales, Hoplocetus and Scaldicetus (Spencer, 1970).
What was life like onshore in late Miocene times? The Brassington deposits contain an assemblage of plant fossils that can be used to reconstruct the climate and flora. They suggest that the mean annual temperature was about 16ºC; by way of comparison, Suffolk today has a mean of 10ºC, while Madrid has 15ºC. So, we are talking about a period of warmer climate, before the general cooling trend that took place after two million years ago, which ushered in the successive ice ages of the Pleistocene.
The list of species seems to have more in common with an Asian forest garden than English woodland. Exotic taxa include Cedrus (cedar), Tsuga (hemlock), Liquidambar (sweetgum; Fig. 15), Sciadopitys (Japanese umbrella-pine), Symplocos (sweetleaf) and Cryptomeria (Japanese cedar). The forests of Derbyshire must have been richly aromatic places. There are also more familiar British trees such as Alnus (alder), Picea (spruce) and Corylus (hazel). The ground flora includes mosses, ferns, herbs and grasses. The salt-tolerant herbs, Armeria (sea thrift) and Limonium (sea lavender), suggest the sea may not have been far away. Given that the Brassington site is now some 330m above sea level, this indicates how much crustal uplift has taken place there over the last five million years.
The boxstones provide meagre evidence for life onshore in Suffolk. Only a handful of terrestrial fossils have been found, originating from remains washed out to sea. These include the elephant-like Mastodon (Fig. 16), the ancestral pig, Sus palaeochoerus, and Pannonictis, a mustelid carnivore related to mink, otter and wolverine (Spencer, 1970).
Other Miocene taxa have recorded from the Red Crag basement bed, for example, the panda (Ailurus), tapir Tapirus, three-toed horse (Hipparion), bear (Agriotherium) and hornless rhino (Aceratherium). However, some fossils could theoretically have been derived from the ‘Trimley Sands’ (see discussion in Spencer, 1970. The ‘nodule bed’ taxa are listed in Newton, 1891).
If we want to expand this evidence, we can turn to the fascinating Dorn-Dürkheim site in Germany, dating from about eight million years ago (Franzen et al, 2013). Fossils have been recovered from mud in an abandoned meander of the early Rhine. A wealth of over 80 mammal species have been identified and, looking through the list of them, I have a sense of a modern zoo fauna seen through the distorting mirror of time and change (Fig. 17). Aceratherium (a rhinoceros) and Hippotherium (a horse) roamed the forest, along with the mastodon, Anancus, dirk-toothed cat, Machairodus, bear, Ursavus, and deer, Procapreolus.
These are all extinct genera, but some Miocene mammal types have survived pretty much unchanged to the present day. I even have one of them visiting my garden, the muntjac deer, Muntjacus, which has same the long canine teeth and prong-like horns of its relations from Dorn-Dürkheim. It is a native of Asia, which has been introduced here and has unwittingly recovered its ancestral ground in Europe.
Other Miocene descendants are living today in warmer parts of the world, where they found refuge during the cold phases of the Pleistocene or remained living in secure habitats. An example is the white rhinoceros, Cerathotherium simum, from Africa, closely related to the Miocene species C. neumayri, from subtropical Greece (Agusti and Anton, 2002). Other examples are the raccoon dog, Nyctereutes (a native of the Far East) and the tapir Tapirus (southeast Asia and South America).
These animals remind me of the currents of evolution that flow through our present-day wildlife, and which have their sources in the exotic, warmer world of the late Neogene period of Earth history. As for us, are we not the descendants of Miocene apes? The recent discovery of a hominin-like tooth dated at about 9.7 million years old from Eppelsheim in the Rhine valley has refreshed public interest in primate evolution in late Miocene Europe (ResearchGate Blog, 20-10-2017, online at: https://www.researchgate.net/blog/post/9-7-million-year-old-teeth-fossils-raise-questions-about-human-origin).
Some concluding remarks
As for the identification of my humble boxstone fossil, I think it is an example of either the extinct cockle, Laevicardium decorticatum or more likely the extinct clam, Glycymeris obovata ringelei. It lived and died on the seabed of the North Sea, in a moment of time well beyond human memory, even before the dawn of human awareness. Perhaps, we may treat the Miocene as a metaphor for the mythical Eden-like world before the Fall of Man. If so, the present Anthropocene epoch is a bitter confirmation of that Fall, as human beings para-consciously consume and abuse ever more of the world’s resources.
Exploring this fossil and discovering its world has put me in touch with the evolving stream of living things from the Miocene to the present. It has reminded me of the biodiverse richness of the tropical parts of our planet, now threatened as never before; also of the tides of continuity and change operating on million-year timescales, transforming species and environments. It has also reminded me of the fragile nature of my own animal awareness and genetic identity which is present – for a moment in time – on the surface of the Earth, and yet part of an ancient continuum of being. In this, I am no different from a mollusc that was alive seven million years ago – or one living today.
Mark Bosselaers – molluscan identification and help with contacts.
David Fenwick – Glycymeris photo.
Kathryn Riddington / Colchester & Ipswich Museums – help with photographing boxstones.
Dr Sinje Weber – help with sourcing artwork.
Wolfgang Weber – Dorn-Dürkheim artwork.
David Whatley – collecting and splitting boxstones.
Dieter Wuyts – whale photos.
Places to visit
The best public display of boxstones is in the geology gallery at Ipswich Museum (two cabinets visible in 2017) – The High Street, Ipswich, Suffolk IP1 3QH; tel. 01473-433-551.
The best places to find boxstones are the following coastal sites, paying due regard to tides, weather and unstable cliffs:
- East Lane Beach, Bawdsey, Suffolk – OS grid ref TM355394.
- The beach and spit, Bawdsey, Suffolk – TM332375.
- The beach, Felixstowe Ferry, Suffolk – TM328372.
Identifying Miocene fossils
The following online resources may be useful for identifying boxstone mollusca:
- European Caenozoic Contact Group – http://users.telenet.be/cr42644/Caenozoic_fossils/index/index.html.
- Natural History Museum Rotterdam online molluscan database – http://www.nmr-pics.nl/.
If you find interesting specimens please share information about your finds with the scientific community:
- Tertiary Research Group – http://www.trg.org/.
About the author
I am fortunate enough to have done geology at ‘O’, ‘A’ and university levels. I am active in geoconservation in East Anglia: a member of the Geological Society of Norfolk (President, 2015); a founder member of the GeoSuffolk Group; former Co-ordinator of Geo-East; interested in the links between past environments, wildlife and early humans, and in communicating them.
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