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.
|Fossils at Green Park Tube Station
|The odd Royal Tribute stone on the
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
|The clay beneath your feet