Plate Tectonics and the Mendips

[ST - 7/7/17]

This presentation was illustrated with slides and rock samples.

With this being the first presentation I have taken into account that the group’s knowledge of Geology can range from very little to a reasonable understanding with large or small gaps. So it is our job to go along gradually, filling in the gaps where we can.

I have not taken a specific Geological area or subject to concentrate on, but rather a general overview from the top of the Mendips southwards around the Wells and Shepton Mallet region, albeit with Tectonics as the connecting thread.

As this is the starting point I have stripped the details down to basics with little elaboration. This may give some discontinuity but all will come clear with time. I have laid it out in short paragraphs so that should you have questions please interrupt between breaths.

Once again answers may be pared down as later presentations will cover things fully. As with all science subjects, you can take them as deep as your interest leads you.

At first glance you would think that Plate Tectonics are not associated with the Mendip Hills and Somerset. We are nowhere near the European Plate Boundaries or any of the Volcanic and Fault line activities that are associated with Tectonic movements.

But Tectonic activities are responsible for the Somerset Landscape either by direct or secondary action.

The primary Direct action that was responsible for the formation of the Mendips was a long-distance operation and spread over a long geological time-line.

Raising the Mendips
Firstly, the now-North American and European Plates joined together and were then joined by the Siberian Plate. This union was named Laurussia or Laurasia. Off to the south was another plate formation called Gondwana; this contained South America, Africa, India, Australia and Antarctica. Then like a magnetic attraction they both decided to crash into each other to form the Pangaea Landmass.

It was this coming together that raised the Mendips into a Mountain range, but we were even then a long way from the boundary.

This Tectonic event has been named the Variscan Orogeny and in some publications it can still be found as the Hercynian or Amorican Orogeny but now research has found that it was all one event.

An Orogeny is the Geological term to describe a Mountain Building period. This particular event was felt over a wide area from the Harz Mountains in Germany, the Ardennes in Belgium, Brittany in France, mountainous regions in Portugal and Spain and in Britain the whole South West and South Wales. For North America the same event created the Appalachian Mountains but they like to call it the Alleghenian Orogeny.

Devonian Sandstones
So what happened to Somerset? At first we were in a dry climate where lots of sand accumulated. This came from mountains around what is now the Lake District as they were eroded. These deposits are now called the Devonian Sandstones and the time period has been named the Devonian Period (410 – 360 mya) and it followed the Caledonian Orogeny. These sandstone rocks are only found locally on the top of Mendip where the covering Limestone has been removed. However this group is more relevant in West Somerset where the Old /New Red Sandstone is the predominant rock.

Carboniferous Limestones
Following on came the Carboniferous period ( 359 – 299 mya).? The land sank beneath the waters and Somerset was under a warm semi-tropical sea. From the mountains to the north flowed rivers full of silt, mud and dissolved minerals which deposited layer after layer of Limestone on its seabed. Six distinct beds in our area can be recognised from their chemical or material constituencies. But this deposition came to an end when the two large landmasses of Laurussia and Gondwana collided.

At the boundaries there was a great deal of crumpling but further afield the sideward Tectonic pressure exerted upon the rock beds caused bending and the proto-Mendip Mountains were pushed up. Estimates for their final height are about 1000 metres or three times their current height.

The usual analogy is that in a car crash the front of the bonnet gets crunched up but the rest bends up and down in a series of ripples. This was the same effect on the Limestone beds of this old sea: they were bent up into an anticline to form a line of Mountains.

Hot springs and mineral deposits
There was also a second Tectonic event that occurred both before and during this period that had a great effect upon the Mendips. But this is more related to the driving forces behind Tectonic activity rather than the plates themselves. The solid crustal plates are moved along by the convection currents flowing in the hot Plastic Magma in the Asthenosphere below them.
Our early European Plate was passing over a hotspot in the Earth’s outer molten core and a Pluton of hot Magma rose up into the underlying rocks, but either it ran out of momentum or the plate moved on too fast for it to fully break through to the surface, so its only local showing was a small volcano whose lavas were extruded into a sea, before it gave up. This was in the earlier Silurian period (443 – 419 mya) and its remains can be found on top of the Mendips from Beacon Hill to Stoke St Michael. These deposits are quarried at Moons Hill Quarry and are often referred to as the Shepton Mallet Volcano. The Pluton is still there deep underground and the waters that percolate down through the permeable rocks meet this still-hot mass where they get heated and absorb minerals; they then re-emerge on the surface as the Bath Hot springs.
This process also provided the source of Mineral-rich waters that rose up into the rocks of the central Mendip range and deposited the wide range of Metals and Minerals that were later mined across the area.

The Pluton formations carried on as the European Plate moved over the Hotspot and they rose high enough for the overlying rock to eventually be eroded away to give us the landscapes of Dartmoor and Bodmin Moor, the Scilly Isles and other Cornish Granite outcrops.

Indirect Actions
So those are the 2 mainstream Tectonic events that formed our region, although you should count the movement of the European Plate over the surface of the Globe as the third main event. But that is only part of the story. Other aspects of tectonic activity got to work. First the huge landmass of Pangaea broke up and the large Continental pieces along with smaller chunks began their dance across the globe. With the stresses of moving and interaction with other plates they wobble up and down so that periodically parts are under water whilst other parts are pushed up into mountain chains.

This means that different erosional and weathering effects are exerted upon the landscape. Whilst not directly Tectonic activity they are influenced by the Tectonic interaction.

The sinking landscape can be seen as a direct influence as a deposition area in the submerged regions and on the fringes there is coastal erosion. The Mountains create Weather patterns and the erosion that comes with the water and ice. This can be seen with Equatorial Africa where there is a wide ocean to the West which brings in moist air, but there are no mountain ranges to push this air up and create rainclouds, so the moisture just dissipates leaving a belt across central Africa which is a Desert. However South America is similar with the Pacific Ocean, but it has the Andes, which push the Moist Air up to produce Rainclouds, and so the Amazon area is a Lush Rain forest. The Himalayas do the same job in Asia. But in both instances there are a variety of erosional activities including Snow and Ice in high mountains and water lower down. In the case of deserts the erosion medium comes from Wind-Blown sand and there is also the possibility of rock formation, if the sand dunes are overlain with further deposits and compressed into Sandstones as we found earlier with the Old Red Sandstone.

Mendip erosion
Back to Somerset. Our Tectonic plate moved northward, and we were back to dry land, literally as it was a return to Desert conditions and in some parts Flash Flood plains. Throughout the both the Permian period (298 -252 mya) and Triassic period (252 – 201 mya) the Mendips were subjected to heavy erosional effects and began to be worn down. The water made large gullies which then filled with rocky debris which became cemented together with sand and other minerals, some forming Dolomitic Conglomerate. These can still be seen around Wells and formed large enough deposits to be quarried at Rodney Stoke and can be seen as Gate Posts or building stone around the county.
At this point it could be useful to show that some terms can be confusing. The names of some rock types retain their original names and others have been re-named as processes become refined. Dolomitic Conglomerate is a good case. It has nothing to do with the Dolomites over in Europe. This name has come from the mineral composition of the rock which was first studied in the Dolomites. In this case the principal components of the rocks are Magnesium and Calcium Carbonate. The Conglomerate bit has been retained but it is not a conglomerate: it is actually a Breccia. Both terms define rocks that are composed of smaller pieces of rock, the difference being that a Breccia contains angular non-eroded surfaces of rock fragments whereas Conglomerate has rounded fragments of rock i.e. water-rounded pebbles.

Temperate siltstones and mudstones
Slowly conditions eased: our plate moved away from desert regions to a more temperate climate and the land sank once again. The area to the south of the Mendips became a shallow sea in which fine deposits of Silt or Mud were accumulating; these later became the Siltstones and Mudstones of the Mercia Mudstone Group. These are the rocks that give us the red and/or grey soils along the southern flanks of the hills. Here is another example of re-naming: in earlier publications this group was called the Keuper Marls.
These Siltstones are also interspersed with weak limestone or sandstone bands and belts of clay. Occasionally this sea dried up and the water became concentrated with salts and minerals. As the water evaporated off, these minerals were left behind eventually forming veins of Gypsum or Barytes. Another product from these evaporates is the Potato stone which can be found in several locations near by i.e. Dulcote. It is thought that water in holes in the mud filled with these salts and as they dried out formed crystals lining these holes.

As the Seas became more temperate and less seasonal they filled with various amounts of sea-life and some areas have beds of coastal shore deposits full of shells and sand. All together these remains provide much of the southern Mendip landscape.

Below the waters again
As the European plate moves steadily northward it was subjected to further Tectonic pressure and during the Jurassic (201 – 145 mya) period the Permo-Triassic beds became distorted and eroded and then once again sank below the waters and were overlain by Limestones which now form the White and Blue Lias beds. These have since received very little tectonic activity and are often found still fairly horizontal.

Since the Jurassic period there has been no rock formation here because this area has remained above Sea level; and only erosion has continued. The exception is the Somerset Levels which for most of the time have been shallow seas where only peat has formed and now with human activity they have been drained or dammed from the salt water environment.

"Ice Age" events
Over the last 146 million years the Somerset area has been subjected to several “Ice Age”events. These can also have an element of tectonic activity since low volcanic action reduces Carbon Dioxide so the atmosphere is cooler. In addition, if landmasses are in the polar regions, Ice forms more readily, which combined with Solar activity and Earth orbit variations (Milankovitch cycles) provide the conditions for long bouts of snow and ice.

In our current Geological Period we have had about 6 Glacial and interglacial cycles together with the associated changes to sea-levels. Sometimes the Ice has covered the land; at other times we were on the edge of the Ice Field or close enough to feel the effects. So we have had frost damage and consequently erosion or during the thaw large quantities of water flowing away to sea have scoured out valleys or gorges. The quantities of water have also helped to form the cave systems we have in the Limestone deposits.

So what have tectonics done for us? Quite a bit one way or another, one could even say everything. It created the hills that form our northern ramparts and the side effects formed the fertile slopes below. Even now that the Mendips are just hills they can still influence our local weather. They often channel the banks of cloud along their length so that we have rain to one side and sun to the other or vice versa. The rain for the most part is gentle and the winters not so cold, but even so the pattern of erosion continues; it is just that Geological time scales operate at a different pace to that of Humans so often we miss the subtle clues. The occasional excesses tend to shock us, but even these are small beer compared to some events recorded in the rocks.

Further Information

Mendip AONB Visitor GuidePDF download, shows Mendip formation at foot of page 2.