Bivalve Borings at Rayleigh Church

December 4 2006. Visited Holy trinity Church in Rayleigh, Essex to buy Christmas cards sold to fund restoration of the building by registered charity 1069853. Also determined more colours on the London Clay Formation calcite concretions there, giving 46 determinations from the relatively old South Aisle (averaging the yellowish grey 1.74Y 5.76/2.96 in Munsell system) and the younger walls of the early 16th Century porch, buttress and flint diapers, plus a north-west extension which is presumably of 18th or 19th Century age. These younger wall concretions average another yellowish grey 0.12Y 6.18/3.02 and look like a less exfoliated version of heterogeneous concretions seen in the South Aisle. Perhaps this is because parts of the old walls from inside the church were reused in these relatively small extensions. Both parts show poorly veined and numerically dominant concretions bored by the modern marine worm Polydora Ciliata Johnston (as at Broomfield Church reviewed earlier). Polydora is more conspicuous in the relatively unexfoliated concretions between the early 16th Century red bricks of the porch and buttresses. A South Aisle concretion, fallen from a decayed part of the wall not yet renovated, showed Polydora borings of 1.5 mm U-tube diameter and 12 mm length extending from a poorly veined joint which was once open on a foreshore or subtidal wavecut platform. What appeared at first sight to be holes left by missing pebbles proved to be modern bivalve borings of 18 mm length and at least 45 mm length. These showed concentric ridges transverse to their axis. These are not seen when pebbles are removed from a soft rock or concrete. In addition, there were a modern bryozoa, presumably Membanipora, grown inside of the bivalve cavity. The sequence of events evidently started with the exposure of a wavecut platform of the London Clay Formation with a band of the subhorizontal calcite concretions worn into a pitted surface rather than the raised Eocene silt burrow fillings of radial Glockeria (seen in porch) and 1.5 mm diameter shafts and polygons (seen in all later walls). The septarian cracks, which were probably not thickly veined in the bored concretions at Rayleigh, became open and calcite-free surfaces extending as an escarpment of the platform with both Polydora worms and probably Burnea bivalves boring horizontally into the stone with a generally uniform weathered colour determined dry as the typical greyish orange 10YR 7/4 seen in churches. the similarity of the colour on the horizontal exterior, previously protected by mortar in the wall, contrasts with a local region of the bored vertical crack showing evidence of reduction of oxygen in the form of a 4 mm thick layer of pale blue 5 PB 7/2 showing no deflection around the Polydora borings passing through it. Since the bivalves had died and had their homes slightly encrusted by bryozoa before the concretion was collected, it is likely that it was displaced by the sea into a muddy foreshore before it was picked up. Probably the original site was a lower foreshore channel with dangerous riptides and the collection site was an adjacent muddy beach. This setting would match The Street at Whitstable today but there may have been similar environments in Essex in Roman, Saxon or perhaps later times before the South Aisle was built (presumably in 13th Century) but from an older church materials still being used for the buttresses etc. in c 1510 and later extensions. This brief reconstruction does not refer to the other type of London Clay concretion present in the South Aisle and new N.E. corner, except as part of the average colours. These clearly came from the Upper London clay, most probably in division D at Southend, rather than division E in the Rayleigh brick and tile excavations of post-1400 vintage. They show thicker sepatarian veins with coarse prisms of ferroan calcite including a dark yellowish orange (10YR 6/6) layer flanked by less chroma, and forming a rough surface in the middle of the cracks that in one case shows small rostettes of barite (barium sulphate) of the size seen at beach level where the coast bends between Westcliff and Chalkwell. What is less clear is whether these true septaria if the upper London Clay came from the same source as the bored concretions at Rayleigh described above. There is no actual proof of middle London Clay fossils in Rayleigh, although there is at Prittlewell and Rochford Churches in the less thickly varied material. Glockeria is seen both in division D at Chalkwell and division B in Essex, and they apparently thin-veined bored concretion noted above may just have come from the tip of a large tabular one in which the full thickness of the calcite veins is not developed. Certainly many of the Rayleigh concretions with a drab and smooth exteriors are just parts of the thickly veined ones, with a coarse silt matrix more typical of burrow-fillings in division D or E than the Glockeria and crinoid concretions of division B. The extent to which the average colour reflects a mixture of two different provinances is unclear in this more complex church building.