Geo-Amble 4th September 2017

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 (http://www.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.

Word or Expression Meaning Derivation
Adiabatic Melting Hot magma rises through weak points in the crust but critically retains most of its heat and temperature.  As it rises it is subject to lower and lower pressures.  This allows melting to occur.  Adiabatic refers to constancy of temperature Anglicisation of the Greek for impassable and by Rankine then Maxwell to describe a process whereby heat cannot or does not escape from the process, so the temperature remains constant (in a perfect system of course). See HERE
Amphibole (amphibolite) mineral supergroup, is the name of an important group of generally dark-coloured, inosilicate minerals, forming prism or needlelike crystals Greek amphi–both and ballein–to throw
Analcite White, colourless silicon based mineral, a tectosilicate (cubic crystalline as opposed to sheet crystalline). Found in basalt and other igneous rocks.  Greek analkimos– “weak
Baked Margin That part of the country rock that is immediately adjacent to an igneous intrusion. High temperatures experienced by the country rock during the intrusion of an igneous body can cause clay-rich rocks to become baked in the immediate vicinity of the intrusion. The effect of this baking decreases with distance from the intrusion.
Biotite A common phyllosilicate (i.e. a material where the crystals, based around Silicon, form sheets), part of the mica group Named from French physicist Jean-Baptiste Biot who worked on the optical properties of mica
Calcite Carbonate mineral CaCO3, the most common form of this compound German Calcit from Latin calx for chalk
Chlorite Also a sheet silicate (phyllosilicate). See HERE for silicates and HERE for chlorites. Greek chloros—green, the main tint of chlorite
Conchoidal Fracturing describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Materials that break in this way include quartz, flint, quartzite, jasper, and other fine-grained or amorphous materials with a composition of pure silica, such as obsidian and window glass. Greek Konchos—mussel (shellfish)
Country Rock The host rock into which an igneous rock has been intruded. It is also termed ‘surrounding rocks’ in this entry.
Dike A body of igneous rock that has been intruded into the surrounding rocks and has a ‘sheeted’ geometry. This ‘sheet’ cuts across the sedimentary layering in the surrounding rocks.
Dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt (i.e. basalt that has erupted on to the surface) or plutonic gabbro (i.e. the same material as basalt but that remained underground). Greek doleros-‘deceptive’ (as it is difficult to distinguish from diorite!) via French dolerite
Gabbro a dark, medium- to coarse-grained intrusive igneous rock composed of calcium plagioclase, pyroxene, and minor olivine, but no quartz. It is the intrusive equivalent (i.e. formed underground, not on the surface) of a basalt.
Hornfels Hornfels derives from mudstone or shale (clay rich rocks) being affected by heat from contact with hot magma –contact metamorphism German—literally horn rock as it reminded them of the toughness of animal horns
Intrusive Igneous Rock Rock derived from magma that cooled underground
Mafic an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron and relatively low in Silicon (old term was base or basic) Ma (magnesium) fic (iron—short for Ferric)
Marl A type of mudstone that consists of clay and carbonate, i.e. a lime-rich mudstone.
Plagioclase Feldspar Feldspar-a group of rock-forming tectosilicate minerals that make up about 40% of the Earth’s continental crust. Feldspar from German for field and spar for non-ore containing mineral
Pyroxene The pyroxenes (commonly abbreviated to Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes are silicon-aluminum oxides with Ca, Na, Fe, Mg, Zn, Mn, Li substituting for Si and Al early 19th century: from pyro-‘fire’ + Greek xenos ‘stranger’ (because the mineral group was supposed alien to igneous rocks).
Teschenite Teschenite, coarse- to fine-grained, rather dark-coloured, intrusive igneous rock that occurs in sills (tabular bodies inserted while molten between other rocks), dikes (tabular bodies injected in fissures), and irregular masses and is always altered to some extent. It consists primarily of plagioclase feldspar, analcime, and titaniferous augite, with barkevikite, nepheline, and olivine usually in lesser amounts. The plagioclase crystals often are encased in the augite to give teschenite an ophitic texture. In central Scotland it is abundant in thick sills. Teschenite grades into picrite when the olivine content increases. From Teschen a site in Poland where presumably first noted
Variscan Orogeny A mountain building episode that occurred in the development of super continent Pangaea some 380-280 million years ago Medieval Latin name for the district Variscia, the home of a Germanic tribe, the Varisci; Eduard Suess, professor of geology at the University of Vienna, coined the term in 1880
Vesicles a small, usually spherical cavity in a rock or mineral, formed by expansion of a gas or vapor before the enclosing body solidified. from Latin ‘vesicula’ small bladder

Geo-Amble 29th August 2017

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

Geo-Amble 14th August 2017

HERE is a geology map of the area and notes on the formations

We parked near Ham Bridge at the entrance to the driveway leading to Ham Mill and Ham Farm.  Along the way to the small quarry that was our destination we noted the very steep hillside we assumed cut into by the River Teme and obviously of a more substantial make up than the flat river valley.  According to the geology map this is Raglan Mudstone territory, the lumps of rock we found along the way being a type of fine sandstone, often found within the softer mudstones.  The geology map of the area does not however show these sandstones hereabouts so seems to be somewhat in error.  I stopped the group to show a distinct hollow way, leading up the steep, grassy hillside away from the river and a local crossing point.  I have always taken this to be a drover’s trail created by centuries of passing herds. Further on a very disturbed area, shown on the map as a quarry, the question being for what?  Possibly mudstone for bricks as no rocks visible (although the Building Stones database states it is St Maughans, that is wrong). Scattered about in the grass around here we discovered several giant puffballs and the group volunteered that I should take home the largest and eat it myself.  Writing this from my hospital bed I can tell you that fried in butter with garlic and olive oil it was like eating marshmallow, not unpleasant with quality bacon and a fried egg, but not a repast I am likely with much enthusiasm, to re-eat. As we approached Ham Farm we noted buildings, some very ripe for refurbishment, made from cut stone, with brick infill and timber frames.  Through the farmyard on the right of way, it was a short walk up to the small sandstone quarry and the source of these stones we were sure.

Although the Building Stones database again states that this sandstone is St Maughans formation from the Devonian period it cannot be, as it is far below the Bishops Frome nodular limestone which itself lies under the St Maughans. I sought direction from a noted geologist and she tells me it is definitely in the Raglan sequence, being the sandstone referred to above and obviously in sufficient quantity for it to have been used in the local farm buildings.

The quarry face showed clear signs of bedding at odds with the mainly horizontal layers.  GEO AMBLE 14AUG17 (7)First thoughts were that this is an example of ‘cross’ bedding and I for one have never really grasped what that is all about. For those of you who like me struggle with simple things here are some notes. If sediments are laid down without disturbance (I am thinking underwater for this) then they will form a smooth layer over the underwater ground surface.  Stops and starts in the deposition and/or changes in particle size, will show as joints (see below).  If water flows across this submerged layer, then the particles will tend to be moved along dependent on the speed of the current and the particle size. We have all seen ripples in beach sand and these form under flow conditions. Exactly why they form is not fully understood (I am pleased to report, HERE is more information) but they do. As ripples develop, particles are driven up the shallow slope that faces upstream, reach the top then tumble down the other side, which is steeper. This occurs whether water or air (wind) is the driving force and in air the ripples are usually much larger.

Because the particles vary in size and density, given their origin from (possibly many) different types of rocks upstream, they do form distinct, albeit thin, layers.  If the ripples are preserved by subsequent deluges bringing  more material down and burying them, then eventually rock is formed and the ripples are retained and visible if the rock is cut into.  HERE is an animation that might help.

The other process in operation within, especially, braided rivers, is where channels move around in the bed of the river, cutting through existing deposits (themselves ‘bedded’) and creating new ones on the inside of bends and so on as flow rates vary. You can imagine that this can leave a very confused set of beds built up over time as illustrated below.  It is possible to see these paleo-channels in rock exposures, certainly at the Nubbins and here too I reckon.

braided river 3d levels

Sedimentary deposits act as a geological tape recorder (I remember those); they record the activity of the local environment and in the quarry we looked at this would have been a river.  If there is a change in the process, for example increased flow (and therefore carrying energy) putting down bigger particles, then the layers will show this, sometimes very subtly.  The layers are separated by bedding planes and these tell us that deposition stopped. In effect this is a gap in the record that could last from minutes to thousands of years. Moderate i.e. thick beds, such as we saw at the quarry in the lower sections, tell us that deposition continued in the same way for a very long time with little disturbance. Higher up, and quite abruptly, the beds became much thinner indicating a more disturbed environment.  What could this have been?  Climate change leading to increased flow?  End of the Silurian and start of the Devonian–continents colliding?  Ideas on a post card please. A more lengthy explanation is to be found HERE.

Geo-Amble 7th August

We met at the Talbot, Knightwick, had a look at the large stone apple press, (HERE for Building Stones database information) its quartz conglomerate wheel, past the now converted church (HERE) with its varieties of building stones then tackled the steep road to join the Worcestershire Way going North.  En-route lying on the grassy verge to stop cars parking, we discovered a triplet of foreigners, rocks that definitely do not come from this neck of the woods (see pics).  Later investigation was conclusive in that the two obviously igneous rocks were granites from Shap in NW England and the black rock with quartz vein seems to be Greywacke, probably from Scotland, a very mixed, sedimentary rock formed by underwater turbidity currents (the third picture on the above web page is very similar to the rock we saw by the roadside in Knightwick in my opinion).  Thanks to Prof. Donny Hutton and Moira Jenkins for the detective work. Moira showed us a beautiful polished piece of Shap granite that she has on a shelf at home.

Cross the road, watch the traffic, carry on up the Worcestershire Way, through the dense, mixed woodland that now cloaks the hills.  Closer examination and explorative walks show a hillside pock marked all over with the remains of old quarries and of long-left buildings.  There must be a fascinating social history waiting to be discovered.

According to the geological map, along the road and at the top of the hill, we were in Wyche Formation, Silurian, no lime content (verified with acid afterwards), fine silt and sandstones (Sandstone, Micaceous. Sedimentary Bedrock formed approximately 428 to 436 million years ago in the Silurian Period. Local environment previously dominated by shallow seas. Generally grey, brown and pale green mudstones and siltstones with thin tabular green sandstones. Setting: shallow seas. These rocks were formed in shallow seas with mainly siliciclastic sediments (comprising of fragments or clasts of silicate minerals) deposited as mud, silt, sand and gravel).

Wyche Formation from Top of Ankerdine

Wyche Formation from Top of Ankerdine

The Wyche is older than the more familiar Much Wenlock limestone of the area, which according to the map, we traversed on our way uphill.  One can only presume that the limestones were used for rubble building and for lime burning to put on fields or to make lime mortar, as happened all along this ridge.  On the other hand the Wyche is quite blocky in nature, see pics, and probably lends itself quite well to more regular building.  At any rate at the top of the hill, yards before the East Malvern Fault completely transforms the geological content, lie the remains of quarries, with old entrance ways, ancient yew trees and some small exposures.  In short order, a group could easily make this more visible and accessible but doubt the County Council would appreciate that as it is one of their managed areas and previous proposals of ours have been dismissed, well, dismissively.

From there we strolled along the Worc. Way to the picnic and car park areas where a board explained something about the Common. Good views over towards Bromyard and its plateau of Devonian St Maughans. In not too unseemly haste we tripped downhill into the comfort of the Talbot’s lounge and the health giving properties of its home brewed ales.  Very pleasant.

HERE is a geological map of the area followed below the evening’s pictures.

During the week I have come across two topics worth noting.  The first is a critique of Tolkien’s map of Middle Earth from Lord of the Rings (map).  The second is a fascinating article on minerals that have yet to be discovered (+-5000 known +-1500 to go)  HERE

Geo-Amble 31st July

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!

 

Geo-Amble 24th July 17

A look at Trail 2 around Martley, HERE for a link to the guide.

Trail 2 is the shortest of the three circular geology walks in the parish.  Leaflets are available at the dispenser opposite the Crown.  In two and a half miles the route takes in a huge span of rock history, from the more recent deposits at the start and over the Nubbins to our most ancient (igneous) formations at Martley Rock.

What a glorious evening, so lucky were with the warm sunny weather and the fields in their golden cloths of barley, wheat and indeed oats, no rye tho, looked just beautiful.

Ian (Fairchild) explained the river deposits of fine sands interspersed with bulkier gravels and pebbles brought down in a storm lithified (turned into stone) from the early Triassic period approx. 250 million years ago.  Deposits from a large dry, desert like continent with monsoon rains, when the land was north of the equator (think the Sahara of today).  HERE is a note on cross bedding and HERE is an illustration of a braided river system.

We traversed up into the field above the village with terrific views all around, I never tire of this spot–East across the ancient, wide river valley (HERE is an illustrated guide on its formation) now occupied by the river Severn, to the Cotswolds (Jurassic limestones), NE to the Lickeys (largely Ordovician), the twin peaks of the Clent Hills (from the Permian), North to Penny and Rodge Hills with beyond, Woodbury and Abberley Hills (Silurian), NW to Clee Hills (the tops being a sill of hard, igneous rock forced up into the Carboniferous surrounds around 300 million years ago), West over the deep valley of the River Teme, to the Bromyard plateau, a dome of sandstone and mud/marl deposits from the Devonian period (think deserts formed south of the equator as today) and, in end on view South, the upstanding igneous front of the Malvern Hills.  What a panorama!  If you wish to see how the continents have moved over time, try EarthViewer a free app.  That helps to interprete the mystery of this slowly moving jigsaw and you can always look up weird names such as Ordovician to see where they came from and what they mean.

We followed a track along the edge of the steep river valley, above ancient orchards, skirting the line of the East Malvern fault, crossed the lane so meeting the Worcestershire Way and thence down to Martley Rock. Here Ian and I tried to explain how it was that five geological periods had ended up within 50m, sometimes with older deposits on top of younger ones and with great gaps in the sequence such that major eras were missing, for example no Ordovician, no Devonian.  To finish, having reached the allotted time of an hour and a half, we took the old sunken lane down to Martley and back to the cars at the hall.  Next week we plan to finish the trail by visiting the church and the Chantry geology garden then maybe venture to rocks near the Talbot at Knightwick.  Thanks to those who came along on Monday, hopefully a good turn out again next week–all welcome.

 So, time next week a little later, 6.15pm at Martley Memorial hall to complete the rest of today’s amble then assuming time, probably go down to the Talbot and up to local quarries on Ankerdine but Mike and I need to recce those first.
Links I promised from tonight:
Martley Rock on TVGS site HERE–lots to get your teeth into here
Plan of Martley Rock site temporary trenches HERE
Martley Rocks App notes on TVGS site HERE
Apps for the Geopark way HERE
 

Warwickshire Geological Conservation Group 17th June

10 members of WGCG joined us for a tour of some of our sites on the hottest, bluest, best June day for many a year.  After a short talk and a walk around our pop-up display at the hall, Ian escorted them to Martley Rock, back over the Nubbins to lunch in his and Pam’s garden.  I took over at 1.30 whence we drove to Penny Hill, searched for fossils, walked to the top to view the 360 degree panorama stretching back over 700 million years and enjoyed the profusion of yellow birds foot trefoil, purply clover and dancing white dog roses.  We think we saw a few Dingy Skippers, butterflies for whom the trefoil is a favourite snack and why we assist the West Midlands Butterfly Conservation Group to clear away tree growth so the flowers may flourish.  Descending to the path around the old quarry, including Stairway to Heaven down which we proceeded to the other place, via the Canyon and out through a green doorway into the spectacular hill and vale scape of the hidden valley.  This lies between the older Much Wenlock to the East and the higher and younger Aymestry limestone to the West with softer lower Ludlow Shales between–hence the valley.  After a vote on which route to take, we decided to trek to the Rodge Hill ridge to view the Clees, Hay Bluff, Bromyard Plateau and South to the Malverns and possibly Forest of Dean.  Always an inspiring place to be.  So there was a good slice of geology intermixed with a ramble of suitable duration for such a hot day, finishing for most at the Crown for a nice cold beer.

Downton Gorge 18th May 2017

Simon Cooter, Senior Reserves Manager, Natural England based at the Stiperstones Nature reserve, patiently guided 15 of us on a morning tour of part of the River Teme’s Downton Gorge, upstream of Ludlow. Simon had clearly, long ago mastered the skills of leading from the back, of allowing a linger-long crowd of rockers, birders and plant experts to take their time in this unique place, whilst adding his own sage comments with dashes of poetry as appropriate.  I think that we were quite pleased to hear from Simon that we had not been the slowest group he had led around.  That honour belongs, he said, to the Society of Conchologists, but they would be wouldn’t they?   The weather was fine, what an improvement on yesterday’s non-stop deluge, the vegetation at its spring best, not too verdant to cover over the many rock exposures. We entered near Bringewood House, walking upstream some considerable way beyond Castle bridge. The geology map clearly shows a dramatic change in rock types.

Downton gorge, through which the Teme rapidly flows was at Bringewood, the site of very early iron smelting and forging (from the 1500s) taken over and hugely expanded by the Knight family a couple of centuries later.  The steep descent of the river bed provided plenty of water power to drive the bellows and hammers, whilst the woodlands were given over to the production of charcoal for smelting. It must, in those days have presented a blasted site, with nary a foothold for nature in the area of the works. Now that industry has ceased, only a few stone buildings and several weirs remain to hint at previous activity.  Existing too, are walks and rides constructed by the Richards’ Knight (there were several) subscribers to the Picturesque movement.  We were led into grottoes and caves, on to high promontories by way of slippy steps and ledge like walkways along precipitous hillsides, to be met with sudden openings of dramatic river and woodland vistas.

The route of the Teme has changed over the millennia.  It used to form part of the headwaters of the River Lugg, flowing past Aymestrey, but the last ice age (Devensian), its advances, retreats and final melting changed all that.  It is conjectured that ice melt, blocked on other routes, overflowed the low point en-route East (towards present day Ludlow) and with the difference in height beyond was able to cut deep into the softer shales, developing a gorge back upstream to the flat lands near Leintwardine, site of prehistoric Lake Wigmore. This reversal in the direction of flow, is evidenced in LIDAR views of the area (see below).  Feeder streams normally join the main flow as does a motorway on-ramp, but LIDAR illustrates the opposite and also shows the main river wider upstream.

LIDAR view of the apparently reversed flow of the Teme--it indicates South but actually flows North

LIDAR view of the apparently reversed flow of the Teme–it indicates South but actually flows North

So, the Teme where we joined it cuts through beds of Raglan Mudstone, Whitcliffe Formation (highly fossiliferous) and Downton Castle Sandstone, although this last I am not aware that we saw any in situ but some as building stones, on one of the bridges for example. An article HERE covers the theories appertaining to the formation of the gorge much more thoroughly (thanks to the author, Kathryn Francis)

Visit of SWGA 28th April 2017

South Wales Geological Association visited Martley for a second visit on 28th April.  We met at the Memorial hall, the coincidental charity coffee morning supplying excellent refreshments as the group perused our pop up geology display.  Here is my report of the day that I very much enjoyed escorting around features near Horsham and in the Teme Valley:

An enjoyable and slightly daringly adventurous field trip with South Wales Geological Association, on occasion breaking a few of the rules of the risk assessment, but as experienced adults we considered ourselves capable of making the necessary on the spot decisions in that regard. A little bit of a scramble now and then is good for the soul. Steve Howe, SWGA, genial, knowledgeable leader, only lost one man (found wandering, by others, in Martley at lunchtime), some explaining to do over the border back in West Wales.  No matter. First the Permian, south of the Berrow.  Reached by a stiff climb, the arete of Haffield Breccia, the whole height of the Berrow here, gives a wonderful vantage point over the Teme valley around Knightwick and Whitbourne. A small pit perplexed our experts and in turn their questions flummoxed me:—How do we know it IS Permian? How deep is the bed here? From which mountains do the clasts come? How come there are large rounded clasts abutting and within beds of smaller angled pieces and gravels? What formations are represented here?  Questions notes, answers needed, more exploratory work to be done. The fact of coal measures higher up on the hill also furrowed brows and set the tone for a moderately challenging day both intellectually and at times physically.

Travelling on to Brockhill Court by kind permission of Sir Anthony Winnington to inspect the igneous dyke of putative Carboniferous age, and its heated intrusion, around 300Ma, into the rather moist country rock of Raglan Mudstone.  Averting our eyes, for later enjoyment, from the feature walls flanking the gateway to the parking area, we wandered down the drive to the hidden, unsuspected yet spectacular road stone quarry that is Brockhill Dyke.  Baked country rock with its voids, filled later with zeolites and calcites, crystalline magmatic dyke material and aphanitic (very fine crystals) chilled margin samples, lay in wait for our inspection and speculation.  Intrepid Steve, followed by others, two steps forward, at least one back, up the loose soil slope to the higher faces in order to more closely inspect contact points and spherical weathering.  This latter is sure evidence of the crystalline dyke material.  At the surface the rock takes in moisture that oxidises iron, expanding it, causing shells to break away from the blocks.  Half way up the slope we estimated the width of the magmatic material at around 6-7m.

Back to the cars and the walls—plenty to look at there—collected lunch and up to the lone pine above the dyke where we enjoyed wide ranging views of the valley whilst dining on the delights we had each packed. We wondered how far the dyke extended and whether the BGS geology map is correct (it seems to be not too far out), what all the minor hills and bumps in the valley consist of, whether the dyke shielded land from further erosion and so on.  We fought our way through thick saplings to the top of the quarry and most descended the badger path down the lip of the quarry to the road, wondering if the Lord of the Rings beech contorted tree roots that we used as hand rails en-route, were really an entrance to other worlds.

Into the cars, over New Mill Bridge and further up river so we could find our way to unmatched Southstone Rock, among the largest tufa deposits to be found in England.  One of my ‘Prepare to be Amazed’ moments when out of the trees a giant tufa cliff emerges, complete with rushing water fall, passageways and ferns all set amid primeval woodland   Another of these places is when you round the corner of the shed into Ian’s quarry garden.

At the ‘rock’, the spring gushing from the hillside in its grotto, perhaps a remnant of a once sacred site where on stood a church and later a cottage (see a picture in Cliff Barnard’s Book ‘A Tale of Two Villages’), the top of the waterfall with its giant coppiced yew, (is this another sign of a sacred site) and the precipitous view down valley from tufa cliffs.  Most of the huge deposit seems to have formed around a particular moss that likes the limey, alkaline waters and for its appetite becomes petrified.  In turn though, there are good examples of drip tufa where carbonate saturated water has run down over the mossy substrate and formed laminates of a more solid, less vesicular material. Finally to view an outcropping of an important, but not the only, source of the lime, a continuous covering of Bishops Frome that drapes the Silurian Raglan mudstone and underlies Lower Devonian deposits of St Maughans formation.  This is a calcareous sandstone often in a marly matrix, much used where it occurs, for building. The Bishops Frome is a calcrete, a type of chemical soil formed in this case, in a dry climate where occasional, slightly acidic rain seeps quickly down, takes some of the carbonate out and is later drawn to the surface by capillary action. There are other ways in which calcrete forms.  In the Teme valley where it underlies many square miles of the Bromyard Plateau, it is layer up to a few metres thick, sitting on top of the impervious Raglan Mudstone, giving rise to a line of springs.

Lastly we drove to Shelsley Walsh church, one example of a building made from local tufa, though possibly not from Southstone, rather from now hidden quarries above the church alongside the famous Shelsley Walsh hill climb.  A site worth visiting whatever your interest is, a magical and ancient place of worship in a quiet spot (except on race days of course).

Thanks for your second visit SWGA, your interest in our area and the questions you raised.

Ref HERE from Murchison

HERE for some very interesting references in a book from the late 19th century, copied by Google.  Conjectures about flow rates, the original course of the stream as well as passages and caves in the rock, now collapsed.  Also the thousands of tons of tufa taken away from the site.

HERE from disappointingly, a climbing blog, reference to the church that used to be on top. Personally having done some climbing, I am appalled that ‘rock climbers’; should even think about this as a climbing venue given the fragility rock and the damage that could be caused.

Book Launch Friday 21st April 2017

Herefordshire’s Rocks and Scenery
A Geology of the County ISBN 978 1 9010839 16 4

Friday 21st April, the above book was launched at a short, friendly ceremony at the Shire Hall, Hereford. The purpose of the book is to explain in readable, accessible text the varied geology of the county of Herefordshire and to remind that the county was at the forefront in the mid 1800s when geology was a young science. In my opinion it fully achieves its brief with wonderful perspective drawings, maps, photographs and text uncluttered by Greek or Latin–definitely a book for anyone who wishes to understand the landscape and for whom Herefordshire is a very special county.  The several authors are to be warmly congratulated for completing an epic work commenced ten years ago.  Hilary purchased several for our library and we heartily recommend that at £15, this is a book that should sit on the bookshelf of anyone interested in local natural history.

At top table Tim Logaston, publisher, Laurence Banks of Hergest Croft, Dr John Payne, editor and Dr Paul Olver, President Woolhope Geology

At top table Tim Logaston, publisher, Laurence Banks of Hergest Croft, Dr John Payne, editor and Dr Paul Olver, President Woolhope Geology

Tim Logaston of Logaston Press

Tim Logaston of Logaston Press

IMG_2536 (Copy)IMG_2537 (Copy)