Purbeck Marble (Lead Author: Pete Bath)
The Purbeck Group comprises the Lulworth Formation (three Members) overlain by the Durlston Formation (two Members). The Purbeck Marble occurs within the upper member of the Durlston Fm, the Peveril Point Member (Early Cretaceous, Berriasian), a little above the Broken Shell Limestone (qv). The Purbeck Group was deposited over a period of some 6 million years (146-140my).
Purbeck Marble is a brackish/freshwater Viviparus gastropod limestone, quarried since Roman Times between Peveril Point, Swanage and Worbarrow Bay from the Upper Purbeck Durlston Formation. The thin beds of this stone, are usually of less than a foot .3 m in thickness but rarely as much as 1.00 m or more and these beds are often varied in dominant colour upwards from green to yellow, red and to blue. Bed thickness alone has limited how this stone could be used but given its shallow depth along its considerable outcrop accessibility has not been too problematic. Although adits have been used southwards, from Roman times this stone was normally shallow quarried from open pits that have been back filled and returned to grazing land.
Famous as a decorative facing and ornamental limestone since Medieval times because like a ’true’ marble it takes a fine polish; it weathers badly, so has rarely been actually used as a building stone. It is not at all porous but composed of articulate 3D casts of unbroken Viviparus snail shells set in a matrix of weakly cemented broken bivalve and gastropod shell. Rare examples of its use in Dorset buildings are the ‘marble manors’ along the line of the outcrop in the valley out of Swanage and in which, the weathering is clear to see.
In addition to the easily recognisable articulate/unbroken Viviparus there are also a small number of minor or occasional low salinity biogenic components common to much of the Purbeck stratigraphy and better seen in thin section.
The Purbeck Marble beds, above the Unio Member, can contain the Unio bivalve which in its youth attaches itself to feed on fish. Two articulate examples are seen close together in this polished altar slab and can be described as ‘nesting’. It appears that both died and their empty shell was filled with the broken shell slightly muddy matrix surrounding them. The muddy brown and darker mixed matrix patches shows this rock to have been well bioturbated by other organisms. This portion of altar stone is yellowish, so could likely be from the green bed, Clements 239 for which he actually includes the Unio.
In this section the matrix is all of fine micritic mud which has fully filled this mid-length transverse Viviparus thin section and surrounding shell detritus. The absence of opaque black areas between the bioclasts tells us that in the absence of such vugs/cavities this rock is not porous. The retention of original microfossil shapes tells us that this rock has not been compressed during diagenesis. It is a biomicrudite because the Viviparus in it are more than 1mm in diameter.
The best introduction to this stone is probably seen at Salisbury Cathedral where 16,000 tons were used between 1220 & 1258 - notably in the main structural pillars of the nave, decorative facings, shafts and also for facings and pillars of the cloisters outside, where weathering is still well seen despite much replacement up to the present day. Attempts have been made to protect Purbeck Marble used inside from decay using lacquers and polish, especially in early Victorian times before leaking roofs and walls of decaying churches were restored and updated in great numbers. Medieval village churches invariably have good examples of this stone used as facings, tombstones, altars, pillars, fonts, and effigies. Re-cycled pieces are commonly seen as window sills and even in external re-build/repairs of monastic properties where they are immediately evident from their quite unique texture if badly weathered.
Also admired for decorative use, very occasionally used together outside Dorset and easy to confuse with Purbeck Viviparus Marble, is the Sussex Paludina Marble of the Early Cretaceous, Wealden. The Victorian names Large and Small Paludina have long survived in the ornamental stone trade to distinguish these two fossil shell limestones, both post Jurassic, so younger and evolved Viviparus fresh-water snail species.
Although there has been compression during the Purbeck diagenesis these photomicrographs of Purbeck marble reveal the original bioclastic content well and much of it unbroken. In the stained section, the original calcitic or chitinous carapace of ostracods has been unchanged. The dyes have stained the ostracod shells pink and the later ferroan calcite blue. The loose carapace valves are shed as the crustacean grows.
Ostracods are a common crustacean feature of semi-freshwater environments and the Cypridea setina setina, length 0.96mm, is common in the Upper Purbeck beds - and in unstained section, the carapace is revealed in a sharp but darker outline.
Where empty snail shells don’t fully fill with surrounding sediment and retain unfilled cavities, the remaining spaces commonly fill with calcium rich fluids during diagenesis and new sparite crystals will grow together to fill this space. This calcite is known as secondary calcite and is not original to the bioclastic structures. (Gravity ensures that mud fill, matching the rest of the matrix will settle matchingly in the lower parts of adjacent gastropod shells and the bright transparent secondary sparite fills all or most of space above the in-shell mud.) This is known as Geopetal structure in thin sections and tells the observer the 'right way up' to interpret vertically sawn thin sections. Many single shell molluscs have their newest/youngest sections filled with sparite including Viviparus and ammonites.
Text and photos by PJB with additional photos by JT and WGT.
'An engineering perspective on the Archaeology of the Purbeck Stone Industrial Industry''. 1978 Geoffrey Norris Ph.D. thesis Bournemouth University.
Link to document
'An investigation into the use of Purbeck Marble in medieval England'. Harrison & Son, Hartlepool 1978 R. Leach (out of print)