3. Mud

Our regional landscape has its origins in mud on the seabed, so at this point in our trudge through the stygian gloom of geological history, let us take a closer look at that mud, in all its various flavours, gloopy, crumbly and the various types of stone that it morphed into. 

Apologia. The passively interested reader might find this the most stultifyingly tedious post on this blog, with graphics that are awe-inspiringly uninspiring, even compared with the low standards I set. 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. What we see today is based on the compacted sediments of sand, gravel and biological detritus that slowly accumulated on the old sea floors. Under their own weight and the water above them, they turned into stone. 

The different types of stone are the result of 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 which started out in deeper waters 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 : Expand

The coloured blocks 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. For instance, if you added the thickness of a sheet of paper every year for 10m years, which is a geological eyeblink, you would have a pile as high as the tallest hill in the region. 

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 caps the low hills to the north of Aylesbury Vale around places like Quainton, Brill and the ridge between Waddesdon and Wichendon. Lots of it has been used to build those lovely honey-coloured, picture-postcard Cotswold villages. Useful stuff limestone. It is a 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, 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 

Chalk 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).

It is actually a softer and purer form of Limestone formed almost entirely from the remains of coccoliths, a type of plankton in deeper water, where there is less coastal debris and vegetable matter. At that stage, the area was around the latitude of what is now North Africa. 

Coccoliths are tiny. You could fit fifty or so into the width of a hair, yet under the Chilterns is a layer almost 300m thick, and whales live on these minute creatures simply by swallowing tons of them. It is only really possible to make sense of it all in the context of the time frames and volumes involved. It is estimated that over 60bn died and sank every year in every square metre of the ocean floor.   

Chalk gives our area a lot of its character, so I intend to reward it with its own post, the next one, rather than go into more detail here. 

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. It forms in many different ways and places. In London, the deep stuff tends to be blue/grey while the shallower and more recent layers are brown. That's the stuff that gardeners and park footballers hate. Try running with a kilo of it stuff stuck on your boots! 

'Gault' clay, which is the blue-grey stuff, 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 a 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 once embedded in it. You find a lot of it in the Chilterns in layers that are thicker than the paltry soil covering over 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. It formed on top of the chalk in the London Basin, late in the Cretaceous period, when it was an inlet of the sea. 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 really solid enough to support traditional foundations. 

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 are 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 flooding and swelling. As a result, it is cursed by farmers and householders alike, and it 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 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. 

Link: A Thousand Words on a Brick

Creative! 

Next Post :  The Chalk