3. Mud

This is a pause in our trudge through the stygian gloom of geological history, to take a closer look at mud, mud, glorious mud, in all its various flavours, gloopy, crumbly and hard in the form of rock. The stuff of Golems. 

Apologia. The passively interested reader might find this the most stultifyingly tedious post on this blog, with graphics that are awe-inspiringly uninspiring. And I am aware that there is some competition. It is here for the sake of completeness and because, worryingly, I think that mud has a story to tell. But you might find it helps you to doze off.  

While tootling around the planet on its way toward its current position on the globe, South East England often found itself in a liminal zone between land and sea, sometimes one and sometimes the other, depending on the sea levels. The foundations of our landscape are the compacted sediments of sand, gravel and biological detritus that slowly accumulated on the old sea floors. In short, they began as as mud, which over aeons was compressed into clay and stone under their own weight and the water above them.  

Different types of stone resulted from different mixtures of materials, deposited at different times and in different places. As a heroic generalisation, the mud which became the limestone of the Cotswolds started life on the floor of shallow seas in the Jurassic period. It was followed by some of the sandstones such as those that form the 'Greensand Ridge' together with some earlier clays and mudstones. Then came the chalk of the Downs and Chilterns and maybe a little later still, the clay of the Thames Valley. In places the topping was supplied by the detritus left in the wake of the glaciers of the ice ages. 

The reality is more complicated. Not only do the sediments vary and overlap in time and space depending on when and where they were paid down, but the distinctions between them are rather arbitrary. For instance, chalk is just a purer form of limestone and the distinction between mudstone and clay is really just one of degree; the particles that make up clay are smaller. In many places other materials have been added and then comprehensively mangled in nature's blender. 

I cribbed the diagram below from the Buckinghamshire Geology Group and it shows how it all worked out there. I know it is a bit detailed for a phone so if you can't expand it easily enough here is the original. Link : Bucks Geology

Geology of Bucks: Cross Section 

The stuff in the bottom layer of the diagram are the hidden 'basement' rocks from the earlier periods in Earth's history. I touched upon them in earlier posts.   

You might wonder how there was ever enough compressed ooze to create these thick layers of rock. The answer, once again, lies in the unimaginably long timescales involved. It might take a decade to create a layer of sediment the thickness of a slice of ham, but left uncompacted, after a few tens of millions of years you would have a pile of ham the height of Ben Nevis. That is an eyeblink in geological terms. 

Limestone first. It dates back to the Jurassic period, over 150m tears ago, forming in shallow coastal waters from the usual sand and debris found on a sea floor in coastal waters together with calcium carbonate (calcite) from the shells and skeletons of marine life. Seawater is saturated with it. The sand is just the eroded remains of even older stones, often bits of minerals like quartz and feldspar. The planet is an assiduous recycler even if its inhabitants are not. 

The Cotswold Hills are classic limestone country. You can also see it, formed from coral debris, in the 'Golden Ridge' which runs east from Oxford, and capping the low hills to the north of Aylesbury Vale around places like Quainton, Brill and the ridge between Waddesdon and Wichendon. Lots of it have been used to build those lovely honey-coloured, picture-postcard Cotswold villages. Useful stuff limestone. It is good fertiliser for acidic soils and, crushed and baked, it makes cement. 

Limestone Buildings in Charlbury

Those seas wouldn't have been much fun for scuba divers, but part of their legacy is the fossilised remains of forests of sea sponges to myriads of small creatures and monsters of the deep.  In London it is easy to find examples of these smaller fossils because they are entombed in the ubiquitous pale Portland Stone used for building. 

Some of the best examples are on the rear wall of the park-side entrance to Green Park Tube Station (Link) Jurassic Fossils at Green Park . A lot of my work on this blog is done in the British Library where you can see the imprint of those sponges in the limestone paviours in the courtyard. They have even made a guide to the stones they used in building the Library and the fossils embedded in them. (Link) Geology of the British Library

Fossils at Green Park Tube Station

In comparison, you don't get as many fossils in the older hills in the rest of Britain where they rarely survived the more turbulent geology of crashing tectonic plates and volcanoes.

Chalk is a actually a softer form of Limestone formed almost entirely from the remains of coccoliths, a type of plankton. It is estimated that over 60bn died and sank every year in every square metre of the ocean floor in deeper water, where there is less coastal debris and vegetable matter. At that stage the area was around the latitude of what us now North Africa. Again, it is only really possible to make sense of this in the context of the time frames involved. 

Coccoliths, massively magnified

It gave its name to a new geological era, the Cretaceous period and is omnipresent in the Downs and Chilterns and lies beneath the clay of the Thames Basin. If you take a tablet for indigestion, like Rennies, you are basically eating chalk. There is probably some in your toothpaste too. (Oddly, blackboard chalk is not usually chalk, but gypsum).

The creation of rock beds isn't usually a continuous process but fluctuates as the climate changes and the seas advance and retreat. In the pic below, taken near Beachy Head, you can see the result. The chalk was deposited in layers and the result is something like tree rings. In the Chilterns and Downs you might hear references to 'white' or 'upper' and 'grey' or 'lower' chalk which is older and sometimes called Marl. 

Chalk layers in cliffs near Beachy Head 

The 'white chalk' is often riddled with flints, sometimes in bands and beloved by stone-age man. These are the crystallised quartz remains of some larger forms of silicon-based sea life. Erosion eats away the chalk and leaves the flints behind, sometimes covered in a white layer that is not chalk but another layer of quartz and quite hard to rub off. I am told that flints can contain fossils, but I haven't had much luck, most of those that I have found have been small and ill-defined like the small ammonites below. (You can easily see whoppers in the West Country!) 

My tiny ammonite fossil in flint

Someone else's ammonite fossil 

Sandstone, as the name suggests, is formed from compressed sand and mud, but doesn't have the same concentrations of skeletal calcite as limestone and chalk. Imagine those sandy bays and lake beds! It can support fertile, well drained soils. 

In the South East its outcrops originated in different places and times, so it comes in a variety of colours and flavours. For instance Bath Stone or 'freestone' can be cut in any direction and is easy to work, while some more local and granular sandstones are not as friendly to the mason.  The 'Greensand' of the Greensand Ridge which runs SW to NE through the local Counties, mostly originated before the chalk and after the Limestone. 

The odd Royal Tribute stone on the
Greensand Way. 

The often indiscernible 'green' tint, comes from the mineralised remnants of plants and (I gather) sealife poo in the ooze. See if you can find it in the rocks around Heath & Reach in Bucks. It does tend to hide its charms so the pic below comes from Lulworth Cove in Dorset. You can see the contrast between the greensand on the right and the old grey chalk on the left.

Mudstones are yet another product of compressed sea and lake floor mud. At this point definitions and distinctions become blurred. Maybe a rough rule of thumb is that mudstones feel crumbly, clay feels malleable, and the other stones are hard and rough. The complicated history of the early landscape means that you can find mudstones alongside, under or over the other rocks mentioned here. There is a long strip of it right across the region, muddling in with the gault clay, below the scarps of the Downs and Chilterns. If you want to stew your brain in the complications, take a look at this! Link: Atlas of Bucks Building Stones

Frankly, not being any sort of expert, I find it difficult to distinguish between these sometimes. This nice pic from Wikipedia just about illustrates the difference which mainly relates to texture. Both stones come in a variety of unappealing colours! 

Shades of Grey! 

Clay. If you pick up pieces of mudstone and clay you can sense the difference. A geologist will tell you that clay is a stone, but plasticky, so you can attempt to mould it with your fingers. It forms in many different ways and places. In London, the deep stuff tends to be blue/grey and the shallower and more recent layers are brown. That's the stuff that gardeners and park footballers hate. Try running with a kilo of the stuff stuck on your boots! 

'Gault' clay, which is the blue-grey stuff which turns brown when exposed to air. It formed before most of the chalk, where large rivers flowed into the sea, and is also found in the Vale of Aylesbury and Vale of the White Horse. When it lies on the surface it doesn't drain well and can be difficult to plough but in many places there are more user-friendly surface deposits on top of it. Often good for fossils! 

What an large ammonite looked like.

'Boulder' clay is (in our area at least) relatively recent. It's the stuff left behind by retreating glaciers and their outwash streams. Boulders can mean anything from small pebbles to (yes!) boulders, with a lot of additional sand and gravel mixed in. You might also hear it referred to as 'drift' or 'till'. It is fairly common on higher ground such as the eastern Chilterns. 

Boulder Clay 

'Clay with Flints' is found where the exposed surface chalk layer has been eroded away by wind and water, leaving behind the harder flints that were one embedded in it. You find a lot of it in the Chilterns in layers that are thicker than the paltry soil covering on most chalk downland and where it provides quite decent farmland.

Flints in a field on chalk

In contrast 'London' clay is a massively thick layer of the stuff, formed on top of the chalk in the London Basin, when it was an inlet of the sea late in the Cretaceous period. If you stare into deep holes created during building works in London, you might see it, because it is the first layer you reach that is solid enough to build traditional foundations on. 

In common usage (as opposed to the alchemic argot of geologists) clay comes in lots of other different flavours, depending on where and when it formed. It is frequently found near rivers, often mixed with gravel, where it can be referred to as alluvium and provides sought after farming land.  There are lots of clays found in and around London that arose from later inundations. Many aren't solid enough to carry traditional building foundations but on the plus side, as the world heats up and the hop crop suffers, can be ideal for growing vines to make good red wines. I am told that Essex Pinot Noir's are better than tolerable. 

The clay beneath your feet  

There is a few other things you might want to know. 

To over-generalise as usual, limestone is permeable. Water sinks through its cracks and fissures, which can grow significantly over time because rainwater is mildly acidic. Hence the caves. In contrast chalk is more porous, it can simply absorb water or let it seep through it. Sandstone allows both. The result is that you get more surface water in limestone country and little on chalk. Mudstones and clays both tend to absorb water and then retain it which can cause so  flooding and swelling. As a result it is cursed by farmers and householders alike, and explains why you might need subsidence insurance for your house!  

Wookey Hole in the Limestone Cotswolds.
Water goes in but not through! 

If you want to know which materiel is most resistant to weathering, look to see if it is used for building. Clay is only really useful once baked into bricks. Chalk isn't much better, it's very soft and soaks up water so you might see it for boundary walls but not often in the buildings themselves. Limestone is tougher and some sandstones tougher still. Witness, the northern Cotswolds, which withstood the chilly embrace of the glaciers quite well, and some of the hilly outcrops north of the Aylesbury Vale which owe their existence to limestone caps. 

I confess to being geeky about building materials and stone can be beautiful. But don't ignore bricks! Their colour tells you a lot about where they come from. So the mottled yellow of London Stock bricks reflects the sand content in Thames Clay while iron impurities give Reading bricks a red hue. Elsewhere light grey bricks often have a high lime content. I love those London Stock bricks so they have a post all of their own. 


Next Post :  Chilling Out


  1. I thought you might like to see some of my collection of mudflats and intertidal zones (From wandering around sea defences and coastlines):


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