We set off early to make sure we could enjoy our trip to Lydney and retreat from the foreshore before the tide rushed in, “faster than a galloping horse.”
We arranged to meet at Lydney Harbour at 1030. As it happened, I was the last one to arrive and I was a quarter of an hour early.
After ensuring that we were all suited and booted for the treacherous Severn Foreshore, we set off.
Anna was leading the trip and had previously carried out a recce but the reeds had grown so much since then that it was difficult to find the way without being able to see the ground ( mud ).
We fought our way through the reeds, over a couple of mud banks and then found ourselves on the rocky foreshore which was mainly made up of very grippy red mudstones of the Raglan Marl Group.
We surmounted these and then had to pick our way over some mud banks. From the erosion surface of the mud banks, deposition layers could be seen so our assumption was that the Severn foreshore was prograding in this area.
Then we came to a beach area underlying a cliff. This is what we had come to see. This is the famous Lydney Cliff SSSI – see citation below.
We walked further on to the point to investigate the strata and cliffs. We also worked out where the Severn Railway Bridge was before it was destroyed in 1960.
You will be glad to hear that we got back to the Harbour before the tide came in.
Height of image approx 3m.
Lydney Cliff showing nodular calcrete.
Soil formation – pedogenesis – in the Silurian and Devonian periods
Soils started to form in the Silurian from the products of chemical weathering from high ground resulting in minerals and clays being deposited in basins. Early organic soils containing plant material have been found in the Early Devonian Rhynie Chert, near Aberdeen, where fossils of early vascular plants have also been found. Before this, in the late Silurian, there were only inorganic, shallow, microbial protosoils. The development of deeper and stronger plant rooting systems on Archaeopteris, Lepidendropsis/Protostigmaria), and Rhacophyton in the Late Devonian resulted in deeper soils containing more organic material by the process of pedoturbation.
Calcrete genesis in the Devonian period.
Clays from the smectite group are found in Devonian palaeosols. These clays take up water and swell in wet conditions and so shrink when they dry out because of evaporation. This results in vertical fissures which allow fluids to penetrate the soil. With each hydration / evaporation cycle, the concentration of calcium and magnesium carbonate minerals increases and these saturated mineral solutions react with the soil altering the aluminium based clays to either calcretes or dolocretes. This alteration is post deposition as can be seen by the limited penetration down from the surface. Older calcretes tend to have been altered to dolocrete.
Calcrete, also known as Caliche in Latin America, cannot form in the present climate of the British Isles as it needs a mean wet season rainfall of 100-500mm and a mean annual temperature of 16 – 20oC. It is forming in modern times where this climate occurs in places such as Arizona USA, Gilgai Australia, Kankar India and areas of Mexico.
Calcrete is the formation of a Duricrust from calcium carbonate, as other minerals are involved, they are called ferrocrete etc.
In the Atacama Desert in northern Chile, vast deposits of a mixture, also referred to as caliche, are composed of gypsum, sodium chlorideand other salts, and sand, associated to salitre (“Chile saltpeter”). Salitre, in turn, is a composite of sodium nitrate (NaNO3) and potassium nitrate (KNO3). Salitre was an important source of export revenue for Chile until World War I.
These deposits were mined because of their high nitrate content. They supplied the material for nearly all the explosives used in WW1. At it’s height, Chile was exporting 25 million tonnes a year.
These deposits are the largest known natural source of nitrates in the world, containing up to 25% sodium nitrate and 3% potassium nitrate, as well as iodate minerals, sodium chloride, sodium sulfate, and sodium borate (borax). The caliche beds are from 0.2 to 5.0 m thick, and they are mined and refined to produce a variety of products, including sodium nitrate (for agriculture or industry uses), potassium nitrate, sodium sulfate, iodine, and iodine derivatives.
This mining was superseded by industrial production of explosives and fertilisers in Europe using ammonia produced by the artificial fixation of nitrogen using the Haber–Bosch process.
In the UK, Calcrete occurs in the Upper Silurian and Lower Devonian rocks and so are used as a marker for the start of the Devonian period.
Depending on the location, it is called Variously: Bishop’s Frome Limestone, Chapel Point Calcrete etc.
The lower part of the cliff consists of mudstones of the Raglan Marl Group but above this, for most of the cliff height it consists of the Bishops Frome Limestone, which is a nodular calcrete. Towards the top of the cliff are cyclothems of nodular calcrete and siltstone / mudstones of the St Maughan’s Group.
Below the current beach level, fish fossils have been found. The cliff is eroding quite fast as shown by the number of nodules on the beach.
More details can be found in the GCR at
BGS. England and Wales sheet 233. Solid and Drift.
Extract from sheet 233 showing Lydney Harbour, a cross section of the anticline and the Silurian / Devonian transition.
We had a long discussion under the cliff about calcrete and it’s polymorphs. Then we wandered further along the shoreline investigating the various strata and noting the calcrete in the cliff above us. It was then time to turn back and retrace our steps to Lydney Harbour. We then decided it was time for lunch so out came our sandwiches.
Then it was time to set off for Huntley Quarry. This took us about half and hour, we regrouped by Huntley Church. Unfortunately the garden centre which used to provided parking, drinks and food is no longer. We set off up the hilly path by the side of the school into a beautiful wood. We followed the path along to the Geological Reserve which is owned by Gloucestershire Geology Trust. We read the interpretation bards along the way so we knew a little of the structural geology by the time we got to the quarry. This was just as well as nature had invaded the quarry and some of the rock faces couldn’t be seen. Never mind, we could see and access the main face so spent some time investigating the many features of sites 2 and 3. Site 1 was completely covered by vegetation.
I won’t go into the detail of the structural geology here as that would just be repeating the text from the excellent guide that is available from Gloucestershire Geology Trust – £2.00 + £2.00 P +P.
Suffice to say, it will keep geologists and other interested people investigating for hours as the rocks vary from the Late Ordovician 485 – 443 mya to the Triassic 251 – 201 mya. These include volcanic ash. Then there were the tectonic movements of the Blaisdon Fault and the accommodation movements, including thrust faults, that were partly caused by the Variscan Orogeny 390 – 310 mya.
After much discussion, we walked along to Ackers Quarry to see the beds of Triassic Bromsgrove sandstone. We then returned to the cars and the trip home after a very interesting day
Huntley Quarry – Geological Reserve Guide – Gloucestershire Geology Trust
Huntley Quarry – Teacher’s guide – Key stage 3 notes – Gloucestershire Geology Trust
Lydney Town and Harbour – Trail Guide – Gloucestershire Geology Trust
Outdoor Leisure 14 – Wye Valley & Forest of Dean – Ordnance Survey
Monmouth, England and Wales sheet 233 – Solid and Drift – BGS
Geological Conservation Review GCR
Lydney OS SO 652017 – http://www.jncc.gov.uk/page-2731 – Fossil fishes of Great Britain. Chapter 3: Late Silurian fishes sites of the Welsh Borders. – GCR
Information on Calcretes
Various sources inc: Dr Dave Green field trip of 9th March 2008
Dr Nick Chidlaw, “Soil Evolution, Arid and semi arid climates, diagrams”Advertisements
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