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             Background information on the geology display


Background information on the geology display


Geology is the study of the Earth, the materials of which it is composed, and the processes by which they change.


Why is Ludlow important to geology?

It is perhaps not surprising that the Marches is renowned for its geology since the region includes representatives from all twelve internationally recognised periods of geological history, from the Precambrian to the Holocene, spanning some 700 million years of earth history. Included within these are four which were defined on the basis of the evidence discovered here: Cambrian, Ordovician, Silurian, Devonian, periods which spanned the evolution of the first hard-shelled fossils and the first life on land, and a fifth, the Permian, which was subsequently defined on the basis of techniques first worked out in the Marches region.

One of the reasons that the area is so interesting and varied is that it is a geological “frontier zone”. For instance:
• The Malvern Fault, running North South, represents an ancient and fundamental division that has asserted considerable influence through time, at the join between two small Precambrian continental terranes (terranes are mini continents).
• The Church Stretton Fault/Neath Valley Disturbance and the parallel Pontesford/Linley Fault, both running North East to South West, form the boundary between areas of Caledonian folding to the west and relatively undisturbed, older continental crustal terrane to the east, quite possibly on, or close to, a major plate boundary during the Silurian.
• The region was uplifted as a huge dome towards the end of the Cretaceous as crustal tensions developed, eventually ripping North America apart from Europe to create the North Atlantic Ocean. One of the associated “hot spots” developed in the eastern Irish Sea basin, raising the elevation of Wales and the Marches. Much of the drainage system recognizable today developed on this uplifted area.
• The Pleistocene Ice Sheets encroached the lower ground, but probably left the hill tops ice-free, creating a complex local topography that includes hummocky terrain as moraine accumulated beneath the ice or was dumped as the ice melted, along with considerable disruption to the drainage system, diverting rivers, flooding valleys and eroding new landscapes.

Our current level of understanding of the 700 million year evolution of the Marches is based upon detailed evidence acquired through diligent collecting and recording by generations of enquirers. Its interpretation is due to the skills that geologists are able to employ to read it. The earliest geological publication by the world’s first geological society is by Arthur Aikin (a pamphlet of 1810 followed by a paper of 1811), since when there have been many more, of the order of one thousand in peer-reviewed publications. However, many questions remain unanswered, awaiting discovery of new facts and interpretation, or re-interpretation, as new ideas emerge.

There have been two meetings over the past decade, both held in Ludlow, which have considered the progress made in our understanding of the geology of the Marches. Summaries of the papers may be found here:


The large cross section

An early Victorian equivalent to PowerPoint, this geological cross-sections was sketched out by Roderick Murchison and drawn by members of the Ludlow Natural History Society for a lecture given in 1852 by the now Sir Roderick to the Society. He is generally acknowledged as the person who unravelled the “Transition” rocks, those beneath the Carboniferous (including the Coal Measures) that hitherto had defied scientific description.

The cross section runs north–south, from Bromfield just north of Ludlow, across the Ludlow Anticline and into Herefordshire. It is based on the pioneering fieldwork undertaken by Murchison in the period 1831-1834.

Another large section – currently hanging in the Hanson Geological Store at the Resource Centre – runs from east to west, from the Cambrian of Wales, through Murchison’s ‘Lower Silurian’ (now Ordovician) of the Stiperstones area of Shropshire and beyond to Ludlow Castle, to the Old Red Sandstone and, ultimately, the Carboniferous of the Clee Hills to the north-east of Ludlow.

There are also several small sections prepared for the same occasion based on newly excavated exposures alongside the then new railway through Ludlow, and along the Teme within the Downton Castle Estate.

 

The geologist’s study

The study is an evocative display of how it might have been for members of the Ludlow Natural History Society in the mid Victorian period, perhaps for the Rev Robert Lightbody, Rev William S Symonds, Rev James D la Touche, Alfred Marston or Colonel John Colville.

Geologist's study

The microscope is an ordinary one. However, a geologist at the time would have instead employed a different kind, specifically designed for the study of rocks. This is known as a petrological microscope and was used to examine thin slices of rock cut by a diamond saw. By slicing to a specific thickness (0.03 mm) this enables minerals and grains to be identified provided the light is polarised. This means that the light is vibrating in just a single direction, achieved by using a crystal of calcite (or nowadays polaroid) both above and below the slide. This is well illustrated here:

http://www.microscopy-uk.org.uk/mag/artapr15/dw-victorian-micro-1.html

 

Note that the rubric for item 1 refers to “lecture notes” but the two Baker pamphlets are not, but are in fact both of the same 1850 publication by William Baker (price sixpence) describing the specimens he brought back from his long tour of duty in India and donated to the Ludlow Museum through the Ludlow Natural History Society. His experiences have been described in a recent booklet about his friend and colleague, John Colvin (Barney Rolfe-Smith (2013) published by Stonebrook Publishing).

Item 2 is a bound volume of The Silurian System. This major work by Roderick Murchison in which he grouped, for the first time, the series of formations which underlay the fossiliferous and hitherto well know Carboniferous System (including the Coal Measures) and underlying, largely unfossiliferous, Old Red Sandstone. Murchison used his 1839 book to publish in Part 1 a detailed account of his researches on the geology and palaeontology of these older rocks in The Silurian System. Part 2 consists mainly of descriptions and illustrations of fossils by Murchison and by others. Part 3 comprises a detailed geological map of the outcrop of these rocks across the Marches. Although Parts 1 and 2 were sometimes bound together forming a somewhat thick and unwieldy volume, Part 3 was often omitted due to the much larger sheet size and consequently has rarely survived.

 

The geological specimens cabinet

The geological specimens are largely fossils, the preserved fragments of living organisms entombed within the rock.

They are arranged in the display cabinet so that the oldest are on the left and get younger from left to right, in roughly two million year steps (the ages of the different formations are currently thought to be a little older and longer than given on the display boards). This was a period of Earth history which saw life emerge from the sea and begin to colonise the land.

There are some errors in the labelling, as follows:

Note that the Coalbrookdale Formation is incorrectly labelled as Wenlock Shales, an old and discarded name.

Specimen 2: On the display card the name should be given as Monograptus priodon (Bronn) with a lower case 'p', and noting that '(Bronn)' has been omitted.

Specimen 6: This trilobite is not labelled but looks like Tapinocalymene nodulosa (Shirley, 1933).

Specimen 7 is misspelt: it should be Favosites, not Flavosites.

Specimen 10: ‘Gastropod’ has been misspelt and should have just one 'd'.

Specimen 15: The genus is not labelled but looks like Pterygotus sp..  A Eurypterid is a lobster-like creature, a voracious predator (see John Norton’s graphic reconstruction of the Silurian Sea). There are also lots of graptolites on this specimen.

Specimen 22 and 23: Meristina obtusa should both have lower case 'o' for obtusa.

Specimen 28: This has been misspelt: it should be euglypha and not eudlypha.

Specimen 33 is probably a water-born plant, not terrestrial. It was discovered in association with graptolites, which are free-swimming creatures often found in deep quiet water environments. The plant may well have been washed in from shallower water, possibly even freshwater. Incidentally, it has nothing to do with Dalmanites, which is a trilobite!

Specimen 34 is misspelt: it should be Saetograptus leintwardinensis after Leintwardine and an important maker fossil for the Ludfordian Zone of the Ludlow Stage of the Silurian.

Specimen 37 is misspelt and should be Sphaerirhynchia wilsoni, the same as 26.

Specimen 39 is misspelt and should be Goniophora and not Gomiophora.

Specimen 41 is misspelt and should be Pteronitella and not Pteromitella.


The video

Picnic in Siluria. Produced by the Woolhope Naturalists’ Field Club based on a re-enactment of their 3rd field trip, on 21st September 1852 to Aymestrey and Croft. This video was prepared on the occasion of the 200th anniversary of the world's oldest geological society, and just over 175 years since the first visit to the area by Roderick Impey Murchison.

The video captures the spirit of the day, from assembly at the Crown in Aymestrey (now the Riverside Inn – in fact, this was actually at Mortimer’s Cross Inn but the Crown was used on other field trips) for a breakfast meeting, through travel by train (the carriage is of the correct vintage but the loco is more modern; and it should have been to Leominster from where horse-drawn carriages would convey the party, but Titley Junction was nearer and had the track ready for our 2007 re-enactment), a picnic on Croft Ambrey overlooking the still active quarry, and a concluding section illustrating the importance of the study of fossils to establishing the environment in which the local rocks were formed.

Note the leather sample bag being carried by Lawrence Banks, great grandson of Richard William Banks, botanist and signatory to the petition to Murchison in 1833 asking him to publish his work on The Silurian System, and the original owner of that bag!

Incidentally, it is untrue that Murchison did not acknowledge his collaborators! For instance on page 198 of his Silurian System concerning the Fish Bed (now more commonly known as the Ludlow Bone Bed) he writes this “might have escaped notice without the vigilant attention of my friends Dr Lloyd and Rev T T Lewis”. Then on page 201 concerning the Aymestry Limestone he writes that “the rock is fully and clearly laid open, and where its fossil contents have been elaborately worked out by my friend the Rev T T Lewis”, and in the footnote Murchison goes on to say that “one day (it may) enable Mr Lewis to confer upon Aymestry the celebrity which White has bequeathed to Selborne”.

Quite an accolade!                                      Click here for a review and how to obtain a copy


Further reading and web links

(1) Teme Bank Trail; latest edition is on-line:

http://www.shropshiregeology.org.uk/SGSpublications/Trail%20Guides/Teme%20Bank%20Trail%20Leaflet.pdf

(2) Building Stones of Ludlow:

http://www.shropshiregeology.org.uk/SGSpublications/Proceedings/2007%20No_12%2005-38%20Rosenbaum%20Ludlow%20building%20stones.pdf

(3) Titterstone Clee Hill:

http://www.shropshiregeology.org.uk/SGSpublications/Table%20of%20Contents%20Proceedings%20No_11.htm#Rosenbaum%20&%20Wilkinson

(4) RGS/IBG Discovering Britain Trail:

https://www.discoveringbritain.org/activities/west-midlands/walks/ludlow.html

(5) Shropshire Geology by Peter Toghill, 2nd edition (available in Castle Bookshop)

(6) Introduction to geology relevant to Shropshire:

http://www.shropshiregeology.org.uk/ShropshireRocks/index.htm

(7) The geological map of The Silurian System has been scanned and can be studied in detail at:

http://www2.odl.ox.ac.uk/gsdl/cgi-bin/library?e=d-000-00---0munahi10--00-0-0-0prompt-10---4------0-1l--1-en-50---20-about---00001-001-1-1isoZz-8859Zz-1-0&a=d&c=munahi10&cl=CL1.6&d=munahi010-abu.8.2


Some misconceptions!

The Ludlow Research Group (LRG) is not the same as the Ludlow Historical Research Group (LHRG)! The LRG was founded in 1951 with its initial focus on Upper Silurian stratigraphy (whose shallow water facies famously outcrop around Ludlow). It has since grown to influence Palaeozoic stratigraphy throughout the world as the foundation of the International Subcommission on Silurian Stratigraphy (ISSS) within the International Commission on Stratigraphy (ICS). The objectives of the subcommission are the development of an internationally agreed scale of time units. The LHRG is more parochial, formed in 1976 with the aim of researching the history of the town of Ludlow and surrounding countryside.




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