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|
Table 1. A simplified local late Neogene chronostratigraphy.
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).