Geology in the old walls of Essex

Materials seen in local churches and walls include:

Flint. 
​Flint frequently forms the majority material in Essex cobble walls. Flint is extremely variable in size, colour, texture and shape. By far the best way to ‘get your eye in’ is to view as many different verified flints as possible. Flints that have been quarried from Chalk are very often black inside with a white surface; they may be either whole or broken. Some flints were carefully broken or ‘knapped’. Skilled knapping produced beautifully shaped blocks, squares and special shapes in flint for some walls. Many flints from gravel and glacial till deposits are stained brown to red internally with iron minerals. Some are brown, grey or black on the outside or quite mottled. Many are in strange shapes; some have holes in or through them. Red and pink flints, usually crackled, have been heated in fires. Flint pebbles and cobbles were rounded on ancient seashores and incorporated into gravel deposits. If possible, note the proportions of flints used as cobbles (naturally-worn), ‘raw’ nodules (unworn, as they came from the Chalk), or ‘knapped’ (split or shaped artificially). Flint frequently forms the largest proportion of a cobble wall.
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Look very closely at the materials in the wall to distinguish the flints –
Many flints are broken: look very closely at any breaks and observe characteristic conchoidal (shell-like) and curved fracture surfaces, a bit like broken bottle glass. These are characteristic of flint.
Flint pebbles frequently show ‘chatter marks’ on their surfaces: fine, slightly curved indentations into the pebble surface which look as though a fingernail has been pressed in. These marks are characteristic of flint and help to distinguish flint from the other common materials in the walls.

Vein quartz.
White, occasionally mottled with pink or greenish minerals. Not sugary, but crystalline in texture. Often with small cavities that show a ’twinkle’ of tiny quartz crystals. Tough, sometimes ‘knobbly’, occasionally quite well rounded. Sometimes bright, pure white. Shine a torch into the cobble – if it looks quite translucent it may well be vein quartz.
Not usually as smooth looking as quartzite.
NOTE: if both quartzite and vein quartz appear in the same frequency category, assess which is the more abundant in one church or wall, as this might be geologically relevant. 

Quartzite.
Two categories may usefully be distinguished in Essex walls:

‘Bunter’ quartzite and other quartzites. 
The ‘Bunter’ stones are usually very smooth cobbles of fine-grained or sugary sandstone. Colour varies from white through pink to brown, often mottled. Other types of quartzite are dark brown or coarser in texture, usually these all look ‘sugary’ when seen close-up. Count these all simply as ‘quartzite’. ‘Bunter’ cobbles are commonly seen to have split along flat planes. Mostly they are less than about 6” or 15cm across. Opaque looking c.f. vein quartz. 
Sarsen quartzite. 
These are pale quartzites that are extremely tough. They are often larger than 6” or 15cm across, and are sometimes quite large. They are normally not rounded or wholly smooth; sometimes they look gnarled. They have a very fine ‘caster sugar’ texture. White to pale creamy brown in colour unless weathered and stained grey to dark brown.
A really close look at quartzite (use a torch) often reveals their fine, sandy or sugary texture – they are made of cemented sand grains.

Puddingstone.
Aka ‘Hertfordshire Puddingstone’, although much is found in Essex. Make sure you can distinguish this stone from ferricrete (described below). This extremely tough stone is the conglomerate equivalent of sarsen. It consists of rounded to broken flint pebbles set in a pale quartzite matrix. The flint pebbles are usually (but not always) brown or reddish with a black ‘rind’. Characteristically the cobbles of puddingstone have split straight across both matrix and pebbles, due to their equal degrees of toughness, providing a flattish plum pudding picture. Some surfaces may be shiny. Sometimes shards of flint are seen in matrix.

Ferricrete.
A brown to black conglomerate of flint pebbles and fragments set in a ‘rusty’ sandstone. The sand is much less tough than the flint, so the cobbles do not show breaks equally across flint and sandstone. Instead, the pebbles protrude like plums in brown cake. Flint pebbles may be whole or broken. Old ferricrete may look almost black and may contain cavities from which flints have dropped out. Ferricrete with no pebbles looks like quartzite or dark red, brown or blackish sandstone – these can be counted as ferricrete here. Iron cemented sandstone is often called ‘carstone’. (Also see clinker below – note that ferricrete has a sandy matrix between flints, unlike most clinker.)

Cementstone.
Usually in rounded lumps but occasionally in rectangular blocks. Pale grey when fresh, it turns to brownish or buff quite rapidly. Frequently weathered buff or deep yellow-brown with a spalled onion skin texture. Very often cementstones are deeply weathered and their powdery-looking spalled surfaces are either ‘dished’ outwards or inwards within the wall surface. Very occasionally round borings are seen – holes made by marine molluscs.
Cementstones sometimes have infilled cracks filled with white to brown or yellow calcite (‘septa’) which give these the name ’septarian nodules’ or ’septaria’; however, these are infrequently seen in walls.

Limestone.
Blocks of various kinds of limestone were commonly re-used within walls. Note these merely as ‘Limestone’ regardless of type and origin: do not attempt to categorise different limestones unless it is very obvious to you (e.g. perhaps Greensand and Purbeck). Limestone blocks exist in a variety of shapes due to former uses as quoins (cornerstones), flooring, steps, gravestones, etc. Crumbly, heavily-weathered pale whitish blocks may be of Reigate Stone, a limy-sandstone. Do not assess the blocks of limestone (or sandstone) used for quoins, arches, windows, ledges and copings: these might be noted (e.g. if made of tufa, chalk), but they do not form the most essential part of this wall survey because it can be very difficult to decide the origins of these stones. 

Chalk.
Although chalk is a limestone, it is distinctively pale and soft. Usually pure white, it can also be grey and less crumbly (‘Chalk Rock’). Sometimes cobbles of chalk from glacial till are seen in walls. They are very rounded, soft and powdery and are pure white.

Tufa.
Off-white stone with an ‘open’, spongy or cellular textured limestone. It is very local in occurrence, as it forms in limy deposits around plant fragments in certain river beds. It was cut into blocks or used in lumps. Some compact varieties of tufa used in buildings are called travertine. 

‘Cornish rock’.
Very tough, smooth pebbles, usually less than 4” or 10cm in size. They are often black-and-white, either black rock veined with white quartz or a ‘breccia’ of white or pale rock fragments set in a black matrix (ground mass). See pictures for distinctive textures. We use the term ‘Cornish’; however, these have been brought by rivers from anywhere in and around the area of SW England.

Brick and tile.
Although not natural materials, these are also included in the survey if they occur within the cobbled wall fabric, as these provide information on re-use of building materials. Note any Roman material, if you are able to decide. 

Clinker.
Usually these are melted or semi-melted masses of brick from brickworks. Many garden walls were made from these, notably up to the 1930s. Very occasionally they are seen in church walls. Usually they look frothy, bubbly or semi-molten and are black to very dark brown. Occasionally a brick end may be spotted within the mass. Not to be confused with the very similar-looking ferricrete. Usually clinker does not contain flints, but this has been seen, and can be confused with ferricrete. Clinker often looks melted in parts. 

‘Other’.
Occasionally you see a glacial ‘erratic’, or a stone transported by humans, that looks ‘different’. Some erratics are Scottish or Scandinavian igneous or metamorphic rocks and a few are truly beautiful and fascinating. Best take a photo and note its exact position in the wall, and then we can go to have a look. Similarly, Thames gravels brought exotic rocks to Essex, such as volcanic ignimbrite from North Wales and conglomerate from South Wales, as well as the ‘Cornish rock’, ‘Bunter’ cobbles and vein quartz. Rare ‘other’ rocks should be noted; they add to the geological story and can be significant despite their rarity. Take a photo and we shall try to identify these.