Church Walling Stones
The walls of most churches in Hertfordshire are built of flint, or what Nicholas Pevsner often described as ‘rubblestone’. The flints obviously come from the Chalk, while the ‘rubble-stones’ are the tougher constituents of river gravels and ‘Ice Age’ Boulder Clay and Glacial Outwash Gravels.
Flint
RAW FLINT
Flints are found as nodules and occasionally flat sheets within the Chalk. When newly eroded or quarried out of the chalk these ‘raw’ flints are covered in a tough white ‘cortex’. When this is worn away the grey-black interior is exposed.
Flint cobbles can range in size from well rounded large pebbles to substantial sub-rounded smoothed off cobbles. They are often bleached or patchily iron (brown) stained. The surface is often pitted with small crescentic marks, called ‘chatter- marks’ indicating their water-worn state.
KNAPPED FLINT
These are flints which have been carefully broken to expose the internal surface of glassy black pure silica. They are used to give an even weather proof surface to the church wall.
Generally speaking when a wall is made up of ‘raw’ flints it is older that when a wall is made of well-knapped flints. But many medieval churches have had their rough walls covered in knapped flints during their ‘Victorian’ restoration.
Clunch
This can generally be seen as crumbly white blocks in walls, although it was most often used for quoins, window frames and doorways – see later under ‘Ashlar and Internal Structural Stone’ .
Calc Tufa
Calc Tufa can look a bit like Clunch but has a rougher texture and looks porous, often with a network of interconnecting holes running through the whole block. It is soft when quarried but when it has dried out it becomes very tough. It is thought by some geologists that the Calc Tufa blocks and slabs found in church walls were recycled from Roman villas during late Saxon times. It is sometimes called Travertine.
Hertfordshire Puddingstone (HPS)
This iconic Hertfordshire stone is made up of smoothly rounded flint pebbles from the Reading Beds cemented by silica during a period known as the Palaeocene-Eocene Thermal Maximum. Although superficially resembling concrete it breaks cleanly across the pebbles and the silica matrix because they are of exactly the same hardness. The pebbles are generally dark grey to black, often with a black outer rim, and are set in a light grey matrix. In some examples iron staining has produce a variety of brownish colours to the pebbles and even a dark cherry red. Large blocks can sometimes be seen in the foundations of a church but it is most often to be found as isolated pieces mixed in with flint walling stone.
False Puddingstone
False Puddingstone, alternatively called Gravel-stone or Ferricrete, can resemble a heavily iron stained Hertfordshire Puddingstone but the pebbles always stand proud of the cementing matrix of iron oxides because the pebbles are much tougher than the cement. The pebbles are more mixed that in Hertfordshire puddingstone, reflecting their origin from an iron pan formed in Glacial or River Gravels.
Sandy Ironstone and Iron-rich siltstone.
In the north of the county some churches have blocks of a tough dark brown gritty rock with a distinctly rough feel. It often shows regular bedding planes and ever false or current bedding at a shallow angle across the block. The blocks are derived from the Lower Greensand ridge of Bedfordshire. In the extreme south around North Mimms there is a similar looking rock, but made from a much finer sand and with abundant hollows called vugs. This is a mottled dark to mid- brown rock often with lighter patches and soft powdery silt in the vugs. It is from a very localised iron pan formed where iron rich springs emerge from the Reading Beds in the Mimmshall valley.
Bunter cobbles
These well rounded, very large ‘pebbles’ are of a tough quartzite brought into Hertfordshire from the Midlands during the Anglian Ice Age. They are well rounded and smooth and vary in colour from light tan to a deep rich red-brown. Some have traces of the original horizontal bedding and even cross-cutting veins of quartz. They are often found mixed up with Flints in old church walls in the west of the county and can be used to indicate the extent of the Anglian ice sheet.
Vein Quartz
Large rounded lumps of very tough white vein quartz can stand out prominently in a church wall. On close examination the fracture surfaces of the crystalline structure can be made out which can give the ‘cobble’ a jagged internal appearance. They have accompanied the ‘Bunter Cobbles’ down from the Midland during the Anglian Ice Age.
Millstone Grit
A very distinctive tough, coarse grained gritstone often of a light to mid fawn colour. The Millstone Grit strata were originally deposited as a large delta on which the coal forests of the Carboniferous period became established. It was brought from the Pennines or Northumberland coast by glacial action. Boulders and cobbles turn up in the formerly glaciated area of East Hertfordshire. A prominent glacial erattic of Millstone Grit is Rossia’s Stone in the middle of Royston.
Dolerite, etc.
Occasional very black cobbles of the igneous rock Dolerite can be seen in the walls of some churches mainly in the west of the county. These are glacial erratics brought by the Anglian Ice from the Whin Sill of North-East England. The rock has a very even texture of interlocking crystals which are difficult to see individually. If you can make out lighter rectangular crystals then it is most likely to be from even further afield, and may even be a Rhomb Porphyry brought from Norway by the Scandanavia Ice Sheet. A similar looking rock, but full of holes called vesicles, is a lava brought from Germany for use as millstones
Roman bricks, tiles and cement
A variety of bricks occur in churches. Modern churches and modern repairs to older churches are of typical ‘Fletton’ bricks while Tudor churches and additions to medieval churches, such as chapels and porches can be made of much thinner ‘Tudor’ bricks. Thinner still are Roman bricks which often have a distinctive dark grey inner layer in the middle of the otherwise red brick. There are a variety of styles, which an expert can pick out, depending on whether the original was a bonding tile, hypocaust tile, flooring tile or roofing tile, etc. Very occasionally pieces of flooring cement call Opus signinum can be recognised by the aggregate of fragmented red tiles giving the block a distinctive speckled look.
More recent building stone
Other building stones have come into the county, particularly since the coming of the canals and railways. The building of Victorian and later churches have utilised Kentish Ragstone, a tough grey sandy limestone of lower Cretaceous age quarried near Maidstone, as a walling stone and even as roughly cut quoins. There is even a church in Hertford built entirely of red Mansfield Sandstone transported from Nottinghamshire by rail.
If you are interested in walling stones, our neighbours, the Essex Rock & Mineral Society, have also produced an interesting guide to local church walls.