The topography of Lincolnshire reflects its underlying geology (Fig. 4), in that the two principal escarpments, Lincoln Edge and the Lincolnshire Wolds, that traverse the county from north to south, correspond to the outcrops of relatively erosion-resistant formations: the Lincolnshire Limestone giving rise to Lincoln Edge; and the Chalk, together with Lower Cretaceous sands, to the Wolds. The low ground on softer, mainly clay formations, lying between and flanking these escarpments, is mostly drift-covered. To the west of Lincoln Edge the ground slopes gently westward to the river Trent; between the Edge and the Wolds is the mainly boulder clay-covered Lincoln Clay Vale, its southern half drained by the river Witham and its northern half northwards by the river Ancholme; while the Chalk outcrop east of the Wolds is covered by boulder clay (from the Last Glaciation), flanked along the coast by marsh deposits. The south-easternmost part of the county is fenland, and fen deposits extend up the Lincoln Clay Vale almost to Lincoln.
The strata at outcrop in Lincolnshire are listed in Table 1. A formation is a set of strata sufficiently uniform in rock-type to be mappable across country, and a set of related formations can be designated a group. The table shows how these lithostratigraphical units are related to the chronostratigraphical units (divisions of geological time) of Fig. 4.
STONE TYPES USED FOR THE SCULPTURES
Four sites in the north of Lincolnshire, at Crowle, Barton-upon-Humber, Thornton Curtis and Holton le Clay, have sculptures of Millstone Grit sandstone. The formation has no outcrop in Lincolnshire. The stone was, however, in common use for Anglo-Saxon sculpture in York (Senior 1991). Dr Senior examined the Lincolnshire sculptures in 1994, and affirms their Millstone Grit origin. He comments that there is no indication of their being of Bunter (i.e. Triassic) sandstone, and that any reddening (usually from hematite, but from limonite in less oxidised horizons) that might suggest this is derived from Permian strata overlying the Millstone Grit of the sculptures at its source, which was in the Nidd valley near Knaresborough, or elsewhere in Yorkshire.
a. Lincolnshire Limestone in general
The Lincolnshire Limestone formation includes much high-quality building stone, and was the main source of stone for Anglo-Saxon sculpture. The formation represents the greater part of the Inferior Oolite Group. It extends northwards from Kettering to the Humber. Its stratigraphy is complex (Fig. 5).
Ashton (1980) described all the available quarries and other exposures of the Lincolnshire Limestone between Lincoln and Stamford, and subdivided the formation into a number of members, grouped into lower, middle and upper Lincolnshire Limestone. Some limestones in the lower division are quartzose, classifiable as silty or sandy, while the remainder, and those in the middle division, are variably oolitic, some with a fine-grained matrix. His upper Lincolnshire Limestone consists throughout of cross-bedded, variably shelly oolitic limestones. In current British Geological Survey work in the Grantham area it has been found (Sumbler 1993) that for mapping purposes the only practicable scheme of subdivision is into Lower and Upper Lincolnshire Limestone, the former comprising Ashton's lower and middle divisions.
Sumbler, Lott and Berridge (1991) saw the varied lithologies in the formation as characteristic of deposition in a shallow marine setting, part of a barrier bar-lagoon complex. Their Lower Lincolnshire Limestone, dominated by variably oolitic limestones with a fine-grained matrix, represents lagoonal sediments to the west of a major north–south barrier bar, while the Upper Lincolnshire Limestone, with an erosional base and consisting of strongly cross-bedded oolites, represents bar sediments prograding westwards over the earlier lagoon. Near its southern limit, in Northamptonshire, the Upper Lincolnshire Limestone occupies channels cut into and locally through the Lower Lincolnshire Limestone (Taylor 1946).
In northern Lincolnshire the Lower Lincolnshire Limestone is represented by three members, in ascending order the Raventhorpe Beds (mainly sandstone and siltstone), the Santon Oolite, and the Kirton Cementstones (calcareous mudstones and thin limestones); and the Upper Lincolnshire Limestone by a single unit, the Hibaldstow Limestones (Gaunt, Fletcher and Wood 1992). The successive environments of deposition evidently corresponded to those south of Lincoln.
b. Lower Lincolnshire Limestone
While most building stone comes from the Upper Lincolnshire Limestone, some of local importance is provided by the lower part of the formation. Among Anglo-Saxon sculptures there are two types of stone than can only have come from the Lower Lincolnshire Limestone. One is a ferruginous sandy limestone as typified in the grave-cover, Hackthorn 1, and also seen in twelve of the sculptures from Lincoln St Mark. The silty limestone of Little Carlton 1 is similar. The lithology corresponds to that described by Evans (1952) as the 'Blue Bed' facies – brown-weathering, blue-hearted, ferruginous sandy limestone. Evans noted that while this rock-type is usually developed at the base of the Lower Lincolnshire Limestone, it might also be found at other levels in his Blue and Silver Beds division, below the Cementstones (the last-named being equivalent to the Kirton Cementstones of Gaunt and others (1992)). Ashton (1980) recorded sandy limestone both in his basal Sproxton Member of the Lower Lincolnshire Limestone and in the next overlying Greetwell Member. The Sproxton Member, best developed to the south of Grantham, passes southwards into the Collyweston Slate. It is more likely that the stone here described is sandy limestone from the Greetwell Member, occurring from Leadenham northwards beyond Lincoln. Ashton (1980, 210) considered that there is only one sandy bed in his Greetwell Member. He named it the Wragby bed, which he found to be 1.5m thick at the Greetwell Hollow quarry at Lincoln (TF 003721). It appears to match Evans's Blue Bed facies (1952).
The second distinctive type of stone resembles that used for Lincoln Cathedral. In Anglo-Saxon sculptures, the stone has a fine-grained micritic (or calcite mudstone) matrix, with sparse, irregularly scattered ooliths and pellets, some ferruginised, and is quite fossiliferous, containing mostly thin-shelled bivalves but also nerineid gastropods and occasionally corals. The facies indicates a 'low-energy', quiet-water environment of deposition, contrasting with the 'high-energy' current-swept environment in which Upper Lincolnshire Limestone oolites were deposited. A particular variety of this stone, used for Lincoln St Paul 1 and 2 and for Lincoln St Mark 11 and 20, is a pale grey, finely granular limestone with bivalves of the genus Modiolus.
The stone type was described by Evans (1952) as the Silver Bed facies – 'a good "freestone" which has been used extensively as a building stone, particularly in the construction of Lincoln Cathedral'. He considered that limestones of this type were interbedded with those of Blue Bed facies. Ashton (1980) distinguished a Cathedral Beds division, at a higher level than that indicated by Evans, which he described as 0.91m thick in the Dean and Chapter Quarry at Lincoln (SK 977734) and equivalent in horizon to part of the Kirton Cementstones of north Lincolnshire. Gaunt and others (1992) confirmed the presence of channel-infills of limestone of Ashton's Cathedral Beds facies in the Kirton Cementstones, below a persistent limestone bed known as the Scawby Limestone. For the sculptural stone here under consideration, an origin in the Lincoln vicinity seems likely, though probably not solely from the few strata designated by Ashton as the Cathedral Beds.
Limestone of the type in question is represented in three sculptured stones (15, 21 and 27) from Lincoln St Mark in addition to the two mentioned above, and also in Lincoln St Mary-le-Wigford 6, at Conisholme 1, Glentham 1, Lusby 1 and Marton 1 to 3 and possibly 4.
c. Upper Lincolnshire Limestone
Among Upper Lincolnshire Limestone sculptures, two distinctive varieties of limestone can be recognised, Barnack stone and Ancaster Freestone, while a remainder are of oolite that cannot with confidence be ascribed to a particular location of origin.
Barnack stone comprises pale yellowish grey (between 10YR 7/1 and 10YR 8/1–3) oolites of medium to coarse grain size, with ooliths generally 0.4 to 0.6mm in diameter closely set, with worn shell fragments, in a crystalline calcite matrix (see Hudson and Sutherland 1990). On weathered faces of the stone, ooliths may become dislodged from the harder calcite matrix, giving the surface a honey-combed appearance when viewed with a hand-lens. Blocks used for sculpture are mostly rectangular in section, with a greater width than depth, the latter rarely more than 0.3m. Bedding is commonly planar, with bedding planes marked by concentrations of shell fragments showing as straight lines on the sides of slabs. Near the village of Barnack in the Soke of Peterborough, stone of this type is known as Barnack Rag, to distinguish it from Barnack Freestone, a finer-grained, softer variety of stone which seems to have been used for building only in the Barnack vicinity. Barnack Rag was quarried from Roman times onwards and became worked out in the fourteenth century.
The second type of stone from the Upper Lincolnshire Limestone, here distinguished as Ancaster Freestone, is very different from Barnack stone. It is commonly of a more yellowish colour (10YR 8/3–4), finer-grained, and very evenly graded, its ooliths usually of 0.4mm diameter and rarely beyond the size-range 0.3 to 0.5mm. The ooliths are closely packed together (grainstone texture) and not broken across on surfaces of the stone, which thus has a 'millet-seed' appearance. Where shell fragments occur they are small and thinly scattered, though they do tend to show a common alignment. Bedding is not well-marked, and blocks as quarried are commonly square in cross-section, rather than having the slab-like form of much Barnack stone.
Stone much like that of Anglo-Saxon sculptures occurs in the present-day Gregory's Quarry at Ancaster (SK 989410) as the 2m-thick Hard White Bed (Berridge 1993). Ashton (1980, fig. 9), describing the quarry as the Lincoln Trust Quarry, indicated a thickness for the bed of more than 4m. Above it, and overlain in turn by Rutland Formation clays, is a coarse shelly oolite, the Weather Bed of the quarry, and elsewhere known as Ancaster Rag. It can be presumed that the Ancaster Freestone of the Anglo-Saxon period came from the quarries in the Ancaster area in beds of 'Hard White' type. Much of the Ancaster stone of late- and post-medieval buildings is less evenly graded than that here described as Ancaster Freestone, and includes shelly streaks cemented by crystalline calcite.
Apart from Barnack stone and Ancaster Freestone, various shelly and pelletal oolitic limestones from the Upper Lincolnshire Limestone are represented in Lincolnshire Anglo-Saxon sculptures. Some may have come from near Clipsham, or from Stamford (Fig. 4). The latter is perhaps a more likely source, for none of the sculptures seen by the writer are of stone that resembles undoubtedly the hard, durable Clipsham stone much in favour as a building stone in Oxford in the nineteenth and twentieth centuries; in texture this modern Clipsham stone is more pellety than oolitic, with grains of various sizes up to 2mm. In the north of Lincolnshire, according to Evans (1952), the Hibaldstow Limestones of the Upper Lincolnshire Limestone have not been quarried for freestone northwards from Leadenham to the same extent as limestones in the Ancaster district, owing to their variable lithological character.
A number of sculptures in the north of Lincolnshire, namely Beelsby 1 and 2, Cabourne 1, Hawerby 1, Nettleton 1, North Kelsey 1, Stallingborough 1 and Winterton 1, all of which except Winterton 1 (eleventh or early twelfth century) are from the Saxo-Norman period or are of uncertain date (Appendix A), are carved from an olive-grey to brownish-yellow weathered, silty to slightly sandy ferruginous limestone. The same stone was formerly used as a building stone for churches in that area, and where so used it shows a similar poor weather-resistance to the carvings, since it is subject to frost-flaking. The stone generally has a fine-grained calcareous matrix, containing rounded quartz grains of silt to fine sand grade (up to 0.2mm), pale brown goethite (iron oxide) ooliths of 0.3mm diameter, and occasionally shell fragments. Thin goethite veins are common.
In the Lower Cretaceous period a varied sequence of sandstones, limestones, ironstones and clays, grouped into the formations shown in Table 1, was deposited in Lincolnshire. The stone used for sculpture is presumed to be from the Tealby Limestone, which occurs in the Tealby Formation, with Lower Tealby Clay below and Upper Tealby Clay above it. The limestone beds, generally less than 4m thick, crop out along an escarpment northwards from Goulceby (TF 255790) to near Caistor, where the whole Lower Cretaceous sequence is overstepped by Chalk (Dr R. G. Thurrell, pers. comm., January 1994; Gaunt and others 1992, 71–5).
Dr Thurrell points out that beds of similar lithology to that in question occur also in the Claxby Ironstone and in the Roach Formation. The Claxby Ironstone is calcareous in its upper 3m or so (Gaunt and others 1992), but the overlying Lower Tealby Clay is reported as much landslipped; this could conceal the Claxby ironstone outcrop, and would in any case hinder working. In the southern part of the Wolds, strata equivalent to the Roach Formation are known as the Fulletby Beds; they include a central cemented member, the Roach Stone, described as a ferruginous sandstone 4m to 5m thick and locally a low-grade iron ore (Kent 1980, 84). Closer to the outcrop of the Tealby Limestone, however, the basal part of the Roach Formation northward of Tealby, as described by Gaunt and others (1992), is a 1.8m to 3m thick biomicrite (fine-grained limestone with shell fragments) containing degraded goethite ooliths and scattered quartz grains. This description would fit much of the stone under investigation. And since the thickness of Upper Tealby Clay in that area, separating the base of the Roach from the top of the Tealby Limestone, is given as less than 0.6m, quarries as likely as not might have taken stone from both formations together.
