November 7 2006. During the visit to Broomfield Church in Essex last Friday, it was noticed that one cluster of London Clay concretions in the nave contained borings by the modern marine worm Poydora ciliata Johnstone not seen elsewhere at convenient elevations in the walls and associated in one 130 mm by 80 mm stone with 6 mm diameter bivalve borings and insitu shells. In eastern North America, this type of bivalve or clam are given the name Angel Wing, more by comparison with the six and elongated wing development reviewed in Isaiah 6 v.2 than more speculative Christmas card illustrations with two spread wings like a bird. The bivalves in the wall building stone had bored in when it was on the lower or middle foreshore of Essex or North Kent, with rough radial ribs extending to a diameter of 6 mm at the working face and a delicate concentric ornament of growth ridges extending towards the entrance for the shell length of at least 15 mm before the stone was collected in Roman, Saxon or earliest Norman times. A complete and fully investigated shell would probably have an additional pair of small shelly plates filling the gape between the valves at the working face, or alternatively a fused similar folded structure there. It was not clear to me which species was present as juvenile and partly hidden shells in the church. Bearing in mind the age of the wall and the lack of evidence of repairs at that spot it is unlikely that the American species Petricola pholadiformis Lamarck [False Angel Wing] needs to be carefully separated from the unrelated homeomorphic English Native species Barnea candida (L.) [White Piddock] allied to the American B. truncata say [Small Angel Wing]. Petricola was first noticed in the River Crouch Estuary of Essex in July 1890 (see N. Tebble British Bivalve Seashells (1966) p. 126). However there is also a smaller English species, which although looking wider than those in the wall are are, is known from the Kent coast and provides an alternative identification. This is Barnea parva (Pennant) [Little Piddock]. There are also larger species Pholoas dactylus L. [Common Piddock] and Zirfaea crispata (L.) which also bore into rocks in southern England but which look somewhat different to the church shells.
The host rock would once have been a large concretion embedded in the London Clay below and probably partly covered by modern mud or sand above. It was then split along natural joints and a few thin septarian calcite veins either by the sea, during collection or transport or more likely just before building of the nave. One can see that the wall consists of a regular size and shape of the stones, with similar-looking fragments of probably the same original concretion placed next to each other, and the Polydora bored surfaces concentrated at the one spot in the whole wall. Unlike many other Essex walls there are no large intact concretions, and no thick-veined or open-veined concretions present. Elsewhere in the nave and older part of the Chancel there are concretions split along sepatarian calcite veins with a half-thickness of up to 4 mm and more iron-stained prisms at the edge nearest to the calcite claystone matrix. But they are not common enough to be seen with the bored material and this suggests a source in the division B, or middle London Clay of Kent or Essex, where septarian veins are thin and sparse. However, since both veined and unveined concretions are termed septaria in archaeological accounts of Essex it is reasonable to use that name for London Clay concretions regardless of vein frequency. This stratigraphical deduction implies that the concretions were not gathered from the subsequently suitable habitat for boring bivalves in the Crouch estuary or indeed the Roach and on the Thames at Southend. These are all upper London Clay foreshore sites, termed division C to E, in which even a small sample of concretions soon shows some thicker and or more open septarian veins. At Southend one can certainly find Polydora borings but often they and other marine animal traces are so common as to make the original surface features of the concretion hard to observed. This is particularly the case with museum specimens of division B dredged-up from subtidal environments at Clacton and Whitstable in early Victorian times. In all probability this mode of collection was not known to people in Roman or Saxon times and the resulting building stones at Broomfield look as if they came from a muddy foreshore in which the sedimentation rate was fast enough to limit the growth of the oysters sometimes seen on concretions in churches.
A more direct method of working on the provenance of the bored concretions is to record the colours of the pre-collection weathering zones by direct comparison with the Geological Society of America Rock-Color chart held up in the same illumination. Holes cut in a grey cardboard (N7) are placed in front of each spot on the stones. Weathering in the wall mainly has the effect of exfoliating the outer layers and increasing the lightness value. Probably this weathering was limited by a covering of limewash mortar until the Victorian era. The stones display bright colours developed in Pleistocene subsoils and later to greys modified in deoxygenated foreshore muds.
Much time at Broomfield was spent on measuring 70 individual coloured areas on typical concretions at the S.W. corner of the nave another 47 in and directly around the bored concretions. Using the conventions of the Munsell Color Company these two sets of observations have slightly different averages equal to 0.42 Y 6.314/3.286 in the S.W. corner and to 9.57 YR 6.149/2.660 in the unusual area including borings. In the rough terminology of American words the typical Broomfield concretions average as a dusky yellow and the abnormal area of the wall with borings as pale yellowish brown. At the times I noticed that one set of Plydora borings were in a greyish orange (10YR 7/4) surface, around an unusually dark core of moderate brown (5YR 4/4) which also formed a hard cortex to an adjacent unbored ellipsoidal concretion with a soft pale orange interior (10 YR 8/6). The adjacent fragment with the bivalves in it showed a similar pale orange exterior (10 YR 7/4) which had exfoliated in the 5 mm thick cortex of the Polydora borings to show the more original colour to be greyish orange pink (5 YR 7/2). Previous experience suggests that this layer was once composed of hydrogen sulphide developed in foreshore muds and around empty Ploydora borings, and has presumably become lighter by oxidation in the dry wall. Inside this cortex the bivalve concretion had a very pale orange (10 YR 8/2) exfoliated matrix. Elsewhere in the wall darker brown matrix and locally red stained joints seen in the abnormal area were less conspicuous and this is reflected in the average colourations subsequently calculated.
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