August 10th-13th sees the Open University Geological Society come to Worcester University for am ambitious programme of lectures, field trips and gatherings.  For draft programme HERE. Martley is the venue for trips on Saturday afternoon and Monday all day. In order to present our best side to the visitors we are tidying up our key geological sites — Martley Rock, Penny Hill quarry, Southstone Rock, Aymestry exposure on Pudford Hill.  There has been a good response to our appeal for help in carrying out the necessary work, last evening, Sunday being the best example so far. Eight of us moved in on Penny Hill and in two hard working hours made a huge impact on the overgrowth.  Alan (Hughes) had spotted a feature hidden behind bushes, brambles, creepers and this was fully exposed.  Great team work, shows how much can be done in a short time.  Best bit–a beer at the Rodney, enjoying the open air seating.  Thanks everyone!

On our meets a number of fossil full rocks were found and Alan photographed but we need to have them identified please:

On our next meet we had a most enjoyable amble past Rodge Hill farm, on to the ridge and along to the small Aymestry outcrop.  Having cleared this and not thinking too much of it, just alongside, Alan discovered a small cliff covered in trailing ivy and after clearing this became a respectable exposure to be used on our local walks.

Finally on another date, two of us cleared undergrowth at Lower House quarry where we have an interpretation board and there is such a wonderful view of the sweeping valley to Woodbury Hill and beyond.

TVGS WEEKEND FIELD TRIP TO BUDE AREA

This was the first weekend away that Teme Valley Geological Society (TVGS) organised.  Once the base location and dates were set, participants made their own accommodation and travel arrangements, gathering on the Friday evening in a local restaurant for introductions and scene setting.  At the appointed time and place next morning, we all met up, having made arrangements for refreshments.  With the weather and tourist footprint on this area, ice creams were always available and for some became a staple part of the diet. A minimum number of cars was used each day.

All of us would like to express our most sincere thanks to Professor Donny Hutton for his work in making this a very successful visit—the reconnaissance, comprehensive notes, general arrangements and his excellent, empathic way of explaining complex geological structures and processes. We would love to do it again!

Astonishingly we arrived, on time, in non-stop sunshine at a car park in Bude at the appointed hour on Saturday 19^th May.  All 16 of us (was it?).  Breakfasts consumed, lunches packed, sun cream or hats or both, on. Given the quality of Donny’s notes (Intro, S 1, S 2) we looked forward to a quality weekend, and so it proved.  In layers.

North Cornwall beaches reveal many striking features, most best seen at low tide–platforms of worn off folds leaning one way then the other, structures such as the whale back at Bude, natural harbours (Boscastle), fabulous surfing beaches (Crackington Haven). The sites we visited were Hartland Quay, Bude, Millook, Widemouth and on Sunday, Boscastle and Crackington Haven.

Sat navving across country on a rural route to our first ‘site’, Hartland Quay, the formations around and across the bay were breathtaking.  There were folds of every type in all directions, chevron, anticline, syncline; obvious faulting, all as a result of the infinitely slow, continuously exerted forces of plate tectonics, tops planed down by eons of erosion.

This peninsular region of England is a series of ancient, submarine sedimentary basins, contorted beyond imagination by the clashing of continents during what is known as the Variscan orogeny, when the land lay in equatorial regions.

The mountain building episode had its greatest effect on southern UK some 300Ma.  There won’t be many details in this diary entry: two reasons—the ability and knowledge of the writer and the guide, appended, is comprehensive.

As we visited each site, we learned from Donny’s perfectly pitched explanations that there are six basins dividing the length of the SW peninsular.  These formed from the South as the moving plates squeezed out of existence the old Rheic ocean. We spent time on only the Culm basin, whereon in the East, folds are more vertically aligned, in the West, more recumbent.

First appearances were deceiving.  We began to realise that the distortions we could see, large though they are, were in fact only disturbances on the limbs of very much larger folds.  At the other end of the scale we found millimetre sized waves (the more vernacular amongst us termed these, wrinkles).

The layering of sediments making up the whole of this coastline dating from the Carboniferous, was caused by an extended period of turbidity flows (useful reference HERE).  Erosion debris from the land, brought into the oceans mainly by rivers, built up into vast, unstable dumps, teetering on the edge of the continental shelf. Rare but repeated earthquakes on a time scale of incomprehensible duration, occasionally caused the mass to start moving down the slope of the abyss.  Once in motion, huge, gigantic debris laden waters rushed downwards, fanning out for perhaps hundreds of kilometers across undersea plains, with gradual and sequential settlement into distinct layers. This layering is known as the Bouma sequence, from the scientist who described it.  The guide explains the process is more detail.  As a result of the graduated deposits over extreme horizontal distances, the whole set is never to be observed in one place—we saw two and maybe three units. You might recall the talk given to us by Dr Esther Sumner on turbidity flows in the canyon off Monterey in California.

A serious consideration for any geologist is ‘which way is up’?  It is not clear from first view, whether the layers, as one would expect, ‘young’ from bottom to top in the order of laying down.  Fold and over folds leave a puzzle to decipher, but there are clues.  For example, on occasion, lighter deposits had somewhat heavier materials scattered over them, causing the lighter to have positive buoyancy and therefore a tendency to move up in the still fluid sequence. The result of this can be small structures known as Flame Structures and once we tuned in, we found many in the rocks around us, sometimes pointing up, sometimes down.

Fold ‘vergence’ (Wiki HERE) was another important term and concept.  As folds become recumbent (‘lie over’), one limb becomes longer than the other (see the guide).  Smaller folds on each limb change vergence as they are imposed on the longer then the shorter of the main fold limbs.  These smaller features can be used to identify the layout of the much bigger folds of which they are part, as they mimic their shape and hence the direction from whence the thrust. In the early 1900s, there was a well known geologist, Pumpelly, who worked in this arena, giving his name to Pumpelly’s Rule.

A few pictures, there are others….. and to keep your interest at a high pitch, the pictures are not in order but most have captions if you move your mouse over them.

Croft Ambrey was the second in our series of Five Hills Walks–derived from the superb Herefordshire’s Rocks and Scenery ISBN 978 1 9010839 16 4 Logaston Press.

Mike Install has written a report with some photos.  Here they are:

We were met by the three EHT Champion volunteers, Robert Williams, John Charles and Sue Spencer, at the National Trust entrance of Croft Castle.   After a brief outline for the day we set off, stopping to admire the magnificent old Spanish chestnut trees.  Reputedly these trees were said to be grown from chestnuts taken from a captured Spanish Armarda ship.  It is more likely that they were planted in the sixteen hundreds.

The ascent was fairly easy, through National Trust and Forestry Commission land, albeit a little muddy.  On arrival at the Iron Age hill fort Robert described the view over the Vale of Wigmore, which was a little restricted due to the mist.   He also recounted how Murchison had visited the site with the Rev. Thomas Taylor Lewis of Aymestrey and the resulting publication of The Silurian System.  We were told about the re-enactment of a visit by the Woolhope Naturalists Field Club with Murchison and the DVD which was made.  This is available for loan from the TVGS library.  We all agreed that as it was not picnic conditions we would descend back to Croft Castle for lunch.

After lunch we walked up to Fishpool Valley which had once been landscaped in the Picturesque style.  We also visited two quarries.  The first was Limekiln Quarry with the remains of a limekiln under conservation.  The second was Highwood Bank Quarry with its face of nodular limestone and fossils, including brachiopods and crinoids.

On returning to the Croft Castle tea rooms we enjoyed an impressive display of pamphlets, maps and books all set up by Robert.  The highlight of which was a copy of Murchison’s The Silurian System.  This generated a great deal of interest and discussion.  A real highlight of the day.

We all had an informative day out and would like to express our thanks to Robert and the team for giving up their time to lead the walk.

I will finish with a bit of a gripe.  It takes quite a lot of work to organise these trips, from making contact with the leader, arranging dates, publicising at evening meetings, by email and on the web site.  Taking note on the written list, those who wish to come along, setting up an email list and sending at least two reminders of the date and time etc etc.  It therefore gets my goat when those who are committed do not turn up AND have not had the courtesy to at least let me know.

There you are, I’ve said it.

Around a dozen amateur, with a smattering of professional geologists arrived in Martley for three days of tours, commencing Friday 22nd Sept. After showing them our pop-up display and taking some refreshments in the Dave Cropp room at the hall, I led them around a few of our sites. These included Martley Rock, the Nubbins, Scar Cottage (thanks Pam and Ian for opening it up so they could sit in unique surroundings to eat their packed lunches) then after lunch, Penny Hill sites–the main quarry, the Canyon, Lower House then back to the cars for 5pm.

On Saturday, Adrian Wyatt led the group on a full day trip along the Malverns, stopping off at several quarry and view sites.

On Sunday Ian Pennell went into the Teme Valley, to the Brockhill Dyke and to Southstone Rock, the major tufa deposit.

The excellent book, Herefordshire’s Rocks and Scenery ISBN 978 1 9010839 16 4 Logaston Press. refers to the Five Hills of Herefordshire from which can be seen the layout of the county and its chief geological features.  Obviously one is immediately tempted to visit these five hills, so it was that 20 of us under the expert guidance of the book’s editor and contributor Dr John Payne, ventured out on Friday 15th September. Choosing the Herefordshire Beacon as the start to this quest to the 5 hills, that is planned to complete over the next 18 months or so, we spent a couple of hours observing landscape features, with clear explanations from John, on how it all came to be.  Well worth the trip, everyone welcome but limited to around 20, watch out for the next one, still to be announced.  Photos and captions all by Moira Jenkins, and very gratefully received by me–thanks Moira!.

Overview of the Site and its Geology (thanks to i.a. EHT, Prof. Ian Fairchild and Prof. Donny Hutton)

The Carboniferous marks the time in which there was a sharp draw down of atmospheric CO2, which produced cycles of glaciations and also led to the deposition of the massive coal measures that were subsequently exploited during the industrial revolution. It is from the study of these coal measures that the idea of the Carboniferous system was developed. The rocks of this period outcrop in the north and north-western part of Worcestershire, centred on the Wyre Forest Coalfield. They consist of deposits laid down in a flat, swampy, deltaic environment, ideal for coal formation.

The prevalent Coal Measures have not been used for aggregate, however igneous intrusions of the same age have been quarried. Igneous intrusions are discordant bodies, a few cm to over 100 m thick, and can be of any length. They are produced when magma is injected along fractures in the country rock. The tectonic settings responsible for the emplacement of Carboniferous intrusions have not been established, although it may be related to crustal extension following the Variscan Orogeny.

Lithology

A Dolerite dike intrudes in to the Raglan Mudstone Formation at Brockhill in the Teme Valley. It is visible on the east bank of the river in a quarry and again on the opposite side but in smaller exposures. The dike runs in a westerly direction for approximately 1200 metres and has affected the course of the Teme, which runs along it for some way before breaching the hard rock barrier to produce a noticeable meander. At its maximum the dike is around 7.5 m thick at its centre. Its mineralogy is of an alkali gabbro (teschenite) with pyroxene, serpentized olivine, intermediate plagioclase and interstitial calcite. Along its margins it consists of a 2.5 to 5 cm band of quartz dolerite with pyroxene, lathy feldspar and quartz. There is no contact between these two igneous types suggesting that the quartz-dolerite has resulted from acidification of alkali gabbro magma prior to emplacement. The country rock is altered to a distance of around 9 m on each side of the intrusion converting the marls to a purple rock with light spots of calcite, analcite, chlorite and garnet. Tridymite needles have also developed around the quartz grains in the sandstone indicating a temperature range of 870 to 1470˚C. The cornstone has become a quartz-calcite-garnet-hornfels. The age of the dike is likely to be Carboniferous.

Exposed Units: Brockhill Dike, Raglan Mudstone

Conservation Status: Local Geological Site

The Brockhill Dike is a type of intrusive igneous rock with a dolerite/teschenite chemistry, which has been extensively quarried for aggregate at this site. It was intruded into the surrounding sandstones and marls at a time when these country rocks were cold. This created a marked temperature gradient between the sedimentary ‘country rocks’ and the hot, molten igneous intrusion and resulted in the formation of a baked margin.

From TVGS Web Site following a Field Trip led by Prof. Donny Hutton (https://geo-village.eu/?cat=14):

Brockhill Dike, Shelsley Beauchamp, where a teschenite dike is exposed in an old pit. Little remains of the dike except high up in the eastern end of the pit. However, good fresh specimens of the rock were obtained after a stiff scramble halfway up the face. Sodium rich, it belongs to the syeno-gabbro suite of rocks. It’s mineral composition is very similar to gabbro but the inclusion of an alkaline mineral, (either nepheline or analcite – in this case analcite) distinguishes it from gabbro. Plagioclase feldspar, clinopyroxene, analcite (easily distinguishable), minor amphibolites and biotite make up this medium- grained basic rock. The dike extends east-west for about 1200 metres and is exposed in small pits on the western side of the Teme. The river itself runs along the line of the dike until it finds a way through, just below Brockhill Court.  Emplaced in the Downton Series of red marls and sandstones it is about ten metres wide and dips almost vertically.  No in situ examination of the margins was possible but the Droitwich Memoire has it that narrow doleritic edges to the dike can be seen. Loose specimens were found of what may have been a fine grained rock from the chilled margin of the dike. Excellent examples of spheroidal (onion skin) weathering can be found in the debris of the pit and on the exposed face.

The country rock, marls, silts and sandstones were ‘baked’ by the hot (1600 degrees C?) magma. The sandstones are now hornfels, a very hard, metamorphic rock. During the baking some layers of the purple marls were sufficiently plastic to allow the escape of volatile gases and the development of vesicles and tubes which were later lined with calcite, chlorite and analcite.  Extreme baking produced vitrified black specimens with conchoidal fracturing. Good examples of all of these rocks can be found in the garden walls of the nearby Brockhill Court.

An explanation of the cause of the Brockhill dike was given by our very knowledgeable guide, Prof Donny Hutton. The dike is one of a suite emplaced in late Carboniferous times (300 Ma) during the Variscan Orogeny. Similar dikes with similar E-W orientation can be found in Northern England and the Midland Valley of Scotland. Variscan subduction with consequent loading and downbending of the lithosphere induced ‘flexural bulging’ with uplift and tensional fracturing of the crust. Low degrees of adiabatic melting produced buoyant syeno-gabbros which rose and pushed into the fractures.

NOTE ON SOME OF THE WORDS USED IN THE ABOVE

And also a useful note about the commonly used words-acid, basic et al:

Because most igneous rocks are composed of silicate minerals, the earliest chemical classification used was one based on weight percentage of SiO₂ (silicon dioxide) in the rock. This led to the subdivision of igneous rocks into four categories—acid(ic), intermediate, basic, and ultrabasic. Over the years, different authors have varied slightly in the limits of SiO₂ percentage for the four groups, but many petrologists designate igneous rocks with ≫66% SiO₂ as acidic, 52–66% SiO₂ as intermediate, 45–52% SiO₂ as basic, and ≪45+ SiO₂ as ultrabasic (≫65%, 65–55%, 55–45% and ≪45% are also used). Some authors use the term sub-silicic and others use the term mafic synonymously with basic although mafic is mainly used in relation to dark-colored Mg–Fe minerals or rocks rich in these minerals.

GEO-AMBLES IN THE STANFORD AREA–FALL DINGLE AND ST MARY’S STANFORD

T’was a darkening afternoon as we tramped along the bridle path, greeting Robin and Leslie Dean at their Hansel and Gretel house before plunging into the wildwood that surrounds Fall Dingle. Apparently cultivated with fruit trees years ago, a site for springs feeding isolated houses with fresh water, a tangled web of fallen trees, brambles and creepers has obliterated, to all but the most skilled of observers, any trace at all of those days.  I am very thankful to Jon and Tom Pearsall for clearing our route the day before.

The way to the fall traversed not only tricky ground underfoot but also of geological interest, a very muddy tufa seep from a spring a few yards up hill.  The incidence of limey Bishops From calcrete much higher up is well known and observed, we guessed that the seep draws its lime through subterranean channels from that arena.  Anything in the path of the seep was coated with a light beige tufa.  We wondered at the purpose of the small human made pool catching the seep water with drain to the nearby stream.  At the fall itself, an impressive and unexpected sight, evidencing fierce flood flows though only remnant at the time of our visit, we inspected rocks in the bed, noting slabs of the local sandstone (Raglan) and more rarely, nodules of Bishops Frome.  An exciting adventure awaits to inspect the stream bed above the falls and locate where it transits from the Devonian St Maughans, over the Frome to the Raglan.

After the dingle we had been kindly granted permission to climb the church tower of St Mary’s from where stunning views of the lumps and bumps of the Teme Valley can be observed.  Thank you to St Mary’s for this memorable visit, we were able to contribute £24 to church funds.

Following are my field notes for the evening:

Stanford Area Geology

The valley of the River Teme, mainly in this area, soft Raglan Mudstone of Silurian Age, rising up steeply to a layer of calcrete known as Bishops Frome, formed in a dry climate, where the lime in underlying formations leached out and to the surface to develop over millions of years a chemical layer of limestone.  Above that, sandstones and marl of the Devonian period, known as St Maughans, with itself quite a percentage of lime bearing compounds. Age is 416–397Ma and the formation is made up of red/purple/grey mudstones, sandstones, intraformational conglomerates and calcretes, deposited from a braided stream system which ran over a vast, flat, arid landscape. The unit is typified by cyclic sequences moving from an erosive base with basal conglomerate, up into finer siltstones and eventually calcrete (Bishops Frome here) with carbonate nodules. Sandstone lenses infilling river channels and other fluvial features of seasonal streams crossing a semi-arid land surface can frequently be seen. These rocks have often been quarried, both for aggregates and for building stones. The harder bands, such as the intraformational and calcretes have been used for road stone.

It is interesting to see the waterfall in Fall Coppice which I assume is caused by the water tumbling over the harder layer of what I term, Raglan Sandstone into the softer Raglan Mudstone below. According to the geology map (see above), this rib of sandstone extends across the road to near the church of St Mary’s and maybe this is the rock from which the church was built.

Fascinating too to see higher up, a lime kiln right on the limey Bishops Frome and used to burn lime for mortar and for the fields. There is an overgrown quarry just above the kiln and above the Bishops Frome so we deduce this is the St Maughans, here a usable sandstone, to be found in many buildings in the area. The valley below the kiln is notable for its precipitous sides, no doubt because the water having run across hard sandstones and the lime layer found it could cut its way into the softer mudstones below, only forming falls where it came across the harder formations.

Refer to my blog of 21st Feb 2015 as it has quite a bit on this whole area

On yet another very pleasant summery evening we took Hollins Lane to just below Lower Hollings Farm (Hollings = holly), paralleling the East Malvern Fault and Silurian Hills to the West, ourselves on the red Triassic lands of Sidmouth Mudstone. Turning back on field paths towards Martley, we enjoyed a distant view of the church and village framed in the stileway (you can say ‘doorway’ so why not ‘stileway’?) in the hedge.  A short break at the seat commemorating VE day then down to the church to explore its building stones which have arrived from a variety of sources.  The church has been documented HERE as part of the Building Stones Project run by Herefordshire and Worcestershire Earth Heritage Trust.  This tells that most of the stone is Bromsgrove Sandstone and of local origin, from the quarries we call the Nubbins. Repairs, obvious by their precisely cut and uneroded lines are also Triassic-Bromsgrove in origin, taken mainly from quarries in and around Hollington in Staffordshire.  There are also other stones–some light buff coloured, oolitic limestone, no doubt from the Cotswolds and decorations over doorways of Carboniferous Sandstone HERE (I like this link, though not local).  Questions were raised about when repairs were carried out, why the ground level on the North side is higher than the floor inside the church and so on and I have asked those I think might know, for answers.

From the church we crossed the B4204, checked out the Chantry School geology garden (needs a bit of TLC) then back to our start at the Memorial hall.

Thanks for coming!