Erratic rocks in fields and beaches of the Isle of Wight
Stephen K Donovan (The Netherlands)
The Isle of Wight is a marvellous place for the geologist on holiday, but there must be a suspicion that it has all been done before. When I first visited the island in 1999, my late wife Trina said that, of course, I would want to geologise at some point. She was surprised at my immediate and emphatic reply of ‘no’, until I explained that every square inch of the island was already ‘claimed’ by so many geologists and groups of geologists that I could not possibly get involved without starting a priority war. I was there to relax, not fight.
Today, I have a different approach. The family Donovan goes to the Isle for their summer holidays most years and I still go to the island to relax, not fight. But I am now working on a range of projects on the Island that are unlikely to impinge on other peoples’ research, while informing my own interests. These have included identifying borings in fossil wood from the Cretaceous greensands that have been misnamed since the nineteenth century (Donovan and Isted, 2014) and exploring closed railway lines using a geological field guide published a hundred years ago (Donovan, 2015).
To reminisce again, I probably first took an interest in erratic rocks in fields in 1981 when I was a research student at the University of Liverpool. I spent a week in a tent in a farmer’s field near Craven Arms in Shropshire, collecting fossil crinoids from the Upper Ordovician. The lady of the farmhouse showed me a collection of fossils and minerals accumulated on a mantelpiece, all picked up from the fields thereabouts. Thirty-five years later, I still recall in my mind’s eye an orthoconic nautiloid, larger and better preserved than any specimen that I had found in situ up to that time. Perhaps as a result, I now take a much more active interest in the rocks in the fields than ever before.
Through farmers’ fields by footpath
This year, I paid particular attention to the erratics of the Isle of Wight. Erratics have long interested me, particularly fossils reworked into the modern nearshore environment and attacked by modern borers (for example, Donovan and Lewis, 2010; Donovan et al., 2014). On the Isle of Wight, I have been impressed by the sheer number of stones that occur in some farmers’ fields. This first captivated me in 2014 when I spent some time walking past fields between Blackwater and Merstone, south and southeast of Newport (Donovan, 2015, fig. 5B; and Fig. 1 in this article).
No collections were made then, nor when I returned in July 2016. These fields are rich in cobbles and pebbles of flint and chalk (that is, Upper Cretaceous rocks), although the bedrock geology of this area is the Ferruginous Sands Formation (Lower Cretaceous; British Geological Survey, 2013). Rare fossils should be expected in flint and chalk erratics, but to accumulate a worthwhile collection would require luck and perseverance that I did not invest – I knew I would find richer material elsewhere. The shapes of some of these flints would have been worthy of identification of eoliths in former times (Ellen and Muthana, 2010).
Flint and chalk cobbles and pebbles are widespread as loose clasts in fields over much of southeast England and will be well known to many readers. More exotic and a specialty of the Isle of Wight are cobbles of mid-Cenozoic limestones containing the moulds of freshwater gastropods. When my family and I were visiting the Chessell Pottery in west Wight (off the B3401; Fig. 1), I set off north across the fields towards Lucketts Farm (A3054). On the way, I spent time examining cobbles of limestone in corn fields (Fig. 2B). The specimen in Figure 3A came from NGR SZ 3920 8675 (Site 1 in Fig. 1) in a field west of Churchills Farm and north of Shalcombe Manor, in the southwest angle of a ‘cross roads’ between a N-S bridleway and an E-W footpath. At this point, there was a local concentration where someone had been clearing stones from the field (Fig. 2A).
The figured specimen is particularly rich in mouldic gastropods, referred tentatively as Lymnaea sp. sensu lato (Fig. 3D). Cleaning of this and other specimens mentioned in this article involved simple washing in tap water or, at most, an overnight soaking in a bath of dilute household bleach to remove organics. Site 1 is within the area of outcrop of the Oligocene Bembridge Limestone Formation (British Geological Survey, 2013). That these are limestone hills was otherwise confirmed by the presence of common marbled white butterflies, which is “… especially common on the chalk downs of the south-west … usually affect broken ground, rough fields … or even sunny banks on the edges of cornfields” (Stokoe, 1982, p. 63).
Of course, man with his mechanical diggers and trucks may make artificial concentrations of erratics. In the above example, a small concentration of cobbles was most easily explained by being cleared into a corner of a field by hand (Fig. 2A). Near Westover Park Farm, south-west of Calbourne [NGR SZ 4195 8590] (Fig. 1, Site 2), there are two large dumps of chalk and included flint by the side of the track that trends SSE-NNW (Fig. 2D). These rocks are probably used for road maintenance and most likely were removed from the disused pit to the SSE. The commonest macrofossils were crushed irregular echinoids, mostly Echinocorys scutata Leske (Fig. 3C). Yet this dump of chalk is within the area of outcrop of the Reading Formation (Paleocene), albeit close to the contact with the Upper Chalk (British Geological Survey, 2013).
On Queen Victoria’s beach
The island’s beaches are also rich collecting grounds for erratics (Donovan and Isted, 2014, 2016). One of the most serene is Queen Victoria’s bathing beach at Osborne House, near East Cowes [NGR SZ 5250 9540] (Fig. 1, Site 3). Apart from being a good spot for the family, with deckchairs, shade and ice cream, it is also a fine beach for shell collecting (Fig. 2C). In particular, disarticulated oyster valves and other shells contain an interesting array of borings (Donovan, 2014).
In amongst the shells, there are at least some pebbles and cobbles of fossiliferous Oligocene limestones and rarer cherts containing disarticulated valves. Two of these clasts are particularly interesting. The specimen in Figure 3B is a flat slab of what superficially appears to be a fossiliferous limestone, but which is actually a chert, hence the superior preservation. The gastropods that are common on the surface are at least close to Nystia duchasteli (Nyst) (Anon, 1971, pl. 17, fig. 12), which is only known from the Oligocene Hamsted Member of the Bouldnor Formation, exposed over much of the northern half of the Isle. A second chert clast is more massive (RGM 792 525), about 45 x 40 x 24mm, its shape possibly defined in part by bed thickness (40mm) and joint spacing (24mm). When wet, it is seen to contain abundant valves of small bivalves, preserved parallel to the bedding.
Just as there is an awful lot of coffee in Brazil, so there are plenty of fossils in the Isle of Wight, but they are not always to be found where you might expect them. Erratics may not be in situ, but those described above have not travelled far. The cobbles of Oligocene limestones in gently undulating fields (Site 1) are apparently within their area of outcrop – they are probably little travelled.
Those fossiliferous slabs found on the coast (Site 3) are likely to have been moved laterally by longshore drift. Gastropods in the figured clast are freshwater and Oligocene; and the second, thicker chert clast could be Cretaceous, but this is unlikely as rocks of this age are exposed on the other side of the island. The preservation of bivalves is quite unlike anything I have seen in flint and they are probably also Oligocene. Mounds of chalk were man-made (Site 2) and had not come far – the nearest chalk pit is less than one kilometre away. All occurrences can be understood and correlated using the available information – lithology, palaeontology, topography and the geological map of the island – to make the observations of these (and other) erratics interpretable and informative.
Anon. 1971. British Caenozoic Fossils. Fourth edition. Trustees of the British Museum (Natural History), London, x+132 pp.
British Geological Survey. 2013. Isle of Wight. England and Wales Special Sheet incorporating parts of sheets 330, 331, 344 and 345. Bedrock and Superficial Deposits. 1:50,000. British Geological Survey, Keyworth, Nottingham.
Donovan, S.K. 2014. Bored oysters and other organism-substrate interactions on two beaches on the Isle of Wight. Wight Studies: Proceedings of the Isle of Wight Natural History & Archaeological Society, 28: 59-74.
Donovan, S.K. 2015. Geological rambles on closed railways in the Isle of Wight. Wight Studies: Proceedings of the Isle of Wight Natural History & Archaeological Society, 29: 81-88.
Donovan, S.K., Harper, D.A.T., Portell, R.W. & Renema, W. 2014. Neoichnology and implications for stratigraphy of reworked Upper Oligocene oysters, Antigua, West Indies. Proceedings of the Geologists’ Association, 125: 99-106.
Donovan, S.K. & Isted, J. 2014. Teredolites Leymerie in the Lower Greensand Group (Cretaceous) of the Isle of Wight and the problematic ichnology of reworked clasts. Proceedings of the Geologists’ Association, 125: 252-254.
Donovan, S.K. & Isted, J. 2016. Borings ancient and modern: reworked oysters from the Ferruginous Sands Formation (Lower Cretaceous) of the Isle of Wight. Wight Studies: Proceedings of the Isle of Wight Natural History & Archaeological Society, 30: 153-157.
Donovan, S.K. & Lewis, D.N. 2010. Notes on a Chalk pebble from Overstrand: ancient and modern sponge borings meet on a Norfolk beach. Bulletin of the Geological Society of Norfolk, 59 (for 2009): 3-9.
Ellen, R. & Muthana, A. 2010. Classifying ‘eoliths’: how cultural cognition featured in arguments surrounding claims for the earliest human artefacts as these developed between 1880 and 1900. Journal of Cognition and Culture, 10: 341-375.
Stokoe, W.J. 1982 [first published 1937]. The Observer’s Book of Butterflies. Warne, London, 192 pp.