Geology on Postage Stamps: #1 Energy Resources, Great Britain 1978

This set of GB commemorative stamps was issued on 25^th January 1978, the first issue of commemoratives for that year. The make the link between energy and geology in a world when alternative energies were a mere twinkle in most peoples eyes.

They were designed by Peter Murdoch (b. 1940) and depict symbols representing energy; oil, coal, gas and (nuclear) electricity coupled with stylised depictions of geological structures and strata associated with hydrocarbon reserves.

The stamps were printed by Harrison & Sons for the Post Office.

First I was afraid, I was petrified … A short history of scary silicified log cabins

This is urban geology at the extreme; petrified wood is not exactly a common building stone, though it is becoming more frequently seen as a decorative stone, used for interior feature walls and even chopping boards and coasters. However it does have a place as a building stone, albeit mostly for novelty purposes … and only in America. Or at least this is the case as far as I am aware.

Silicified or petrified wood is relatively common and always an attractive fossil, with excellent preservation of the wood structure down to cell level. Identification of wood down to species level is often possible from microscopic observation of preserved cell structure. Trees have been around since the Carboniferous but are often much ignored in earth history. They are the background to what are perceived to be more interesting events. Here is an Upper Cretaceous scene whereby a right porker of a Tyrannosaurus is bringing down a green skinny lizard as an amuse bouche, whilst being eyed by circling Pterosaurs. These trees in the background are probably Araucaria sp.

The process of silicification of wood is frequently closely associated with local volcanism. Silica is leached from decaying volcanic ash and carried in groundwaters and stream waters where it can permeate logs and branches. This works particularly well if the logs are buried in ash. Many fossil forest deposits represent preserved log jams in river systems and tree debris becomes silicified in anoxic environments (see Sigleo , 1979). Colours are imparted by trace elements; iron, copper, manganese, even uranium. Black is produced by carbon or finely disseminated pyrite.

Silicified wood deposits are known from Chemnitz in Germany from the Permian Leukersdorf Formation (Luthard et al., 2016) where the coloured stones were used as decorative inlay and as semi-precious gemstones. Also in Europe, deposits occur in Lesvos in Greece (Miocene; see Vasileiadou & Zouros, 2012). Worldwide, silicified wood is known from China (i.e. the Cretaceous ‘Jehol Biota’; see Ding et al., 2016) and exceptionally well-preserved, Pliocene trees are found in Java, Indonesia and Triassic trees occur in Madagascar, and these regions are the origin of most silicified woods on the decorative stone market today (Mandang & Kagemori, 2004; Yoon & Kim 2008). Recent discoveries petrified trees have been made in Brazil (Lower Permian tree ferns; see Maria da Conceiçao et al., 2016) and Turkey (Miocene willows, junipers and oaks; see Akkemik et al., 2016).

Silicified wood panelling supplied by the Emperors of Stone Bling, Maer Charme.          Photo by Ruth Siddall.

There are a large number of deposits of various ages in the USA and it is here that fossil trees have largely been used for building. Deposits are known from the Dakotas, Colorado, Oklahoma, Texas etc. The best known deposit is in the Upper Triassic Chinle Formation represents continental facies and outcrops throughout the southwestern USA. These famous fossil conifer deposits occur in the Lower and Upper Petrified Forest Members of Carnian-Norian age. Conifer logs are also found in the intervening Sonsela Member, representative of an alluvial floodplain (see Ash, 1992; Trendell, 2013). However, there are many other examples through the West and Midwest. Many of these deposits have been raided for garden ornaments and other small-scale structures.

A well-known and rather American Gothic building constructed from petrified wood is the (former) gas station in Lamar, Colorado, described by David Williams in his book ‘Stories in Stone’ and also illustrated in his blog. The gas station was built by one William Brown in the 1930s. The Colorado trees are of Cretaceous age but the precise age of strata of origin is unknown. The ‘logs’ were found in fluvial outwash in farmland around 25 miles south of Lamar.

Lamar Gas Station; photo from Geologywriter.com

And then there is Petrified Wood Park in Lemmon, South Dakota, also constructed in the 1930s. This is a truly terrifying place; cones, pinnacles, pyramids and creepy-looking grotto-like buildings are built from petrified wood as well as dinosaur and mammoth bones. The park occupies a whole block of the town and was built by amateur geologist and ‘visonary’ Ole S. Quammen. To be fair his intention was altruistic, it was to provide work for 50 or so otherwise unemployed men during the depression era West. Quammen’s heirs donated the park to the grateful town of Lemmon in 1954 where it remains ‘the world’s largest petrified wood park of its kind’. Obviously. An on-site museum once house a collection of stuffed animals playing musical instruments. Sadly, these are no longer on display. The petrified wood was sourced locally, potentially from several strata. The Early Cretaceous Lakota Sandstone Formation outcrops in the eastern Black Hills and has fossilised logs of cypress, palm and cycads. The Hell Creek Formation of South Dakota also has petrified Late Cretaceous cypress (and dinosaurs). The Palaeocene petrified wood is found in the Badlands of South Dakota where conifers and broadleaf tree trunks and branches are preserved (Teachout, 1995).

Lemmon Petrified Wood Park sign; photo from SouthDakota.com

According to Snider (2012), Texas is the state for construction in petrified wood, and she cites Austin, Huntville, Decatur (which also has a petrified wood gas station) and Stephenville as all having buildings incorporating this unexpected stone. However, the town of Glen Rose in Somerville County has over 40 buildings and other structures constructed of silicified wood. It was once known as ‘The Petrified City’. The post office, several houses, fountains, a (now disused and ruined) speakeasy and the bandstand are all built from petrified wood. The bandstand also incorporates slabs with spectacular dinosaur footprints too.

The ruins of Glen Rose’s speakeasy; photo by Tui Snider (2013)

Bandstand, Glen Rose, Texas; photo by Tui Snider (2012)

Detail of dinosaur footprint in Glen Rose’s bandstand; photo by Tui Snider (2012)

The origin of this Texan petrified wood building spree, which occurred in the 1920s and 30s was a unexpected consequence of the mechanisation of agricultural machinery. Farmers were able to dig deeper into their soils than before and they hit a petrified wood motherlode in the local fields. The logs were hauled out and used to build the town.

Finally, the most attractive structure built from petrified wood is not all scary. It is Agate House, located in the Petrified Forest National Park, Arizona. It was originally constructed between 1050 and 1300 AD by Ancestral Puebloans from Triassic Chinle Formation petrified trees. Other examples of structures built from petrified wood have also been excavated in the surrounding region, since the 1930s. The excavation of Agate House and its subsequent reconstruction was overseen by archaeologist Cornelius Burton Cosgrove Jr. (1906 – 1999). Petrified wood was also used for arrowheads and similar artefacts by the Ancestral Puebloans.

Above, Agate House; photo by Amusing Planet

Above, the wall of Agate House; photo by NotsofastinBoulder’s Blog.

More petrified wood buildings in the USA …

Gas Station, Decatur, Texas

Carter County Museum, Montana – does anyone have a photo of this? It is partially built of Hell Creek Formation wood.

Petrified wood and petrified wood buildings in Oklahoma

Outside the USA …

The town of Mata – “Cidade da Pedra que foi madeira” (“The city of rocks that once were wood.”) – in Rio Brande do Sul, Brazil. Petrified wood from the Upper Triassic Caturrita Formation is used to build several buildings and can be seen in the Palaeobotanical Garden. The fossils were collected by priest and palaeontologist Daniel Cargnin.

Thanks to Christian Kammerer @Synapsida for this information and photos.

 

If anyone knows of any more structures built of petrified wood outside the US, do let me know …

 

How to cite this blog:

Siddall, R., 2017, First I was afraid, I was petrified … A short history of scary silicified log cabins., Orpiment Blog https://orpiment.wordpress.com/2017/04/13/first-i-was-afraid-i-was-petrified-a-short-history-of-scary-silicified-log-cabins/

 

References

Akkemik, Ü., Arslan, M., Poole, I., Tosun, S., Köse, N., Kiliç, N. K. & Aydin, A., 2016, Silicified woods from two previously undescribed early Miocene forest sites near Seben, northwest Turkey., Review of Palaeobotany and Palynology 235., 31–50.

Ash, S.R. and Creber, G.T., 1992. Palaeoclimatic interpretation of the wood structures of the trees in the Chinle Formation (Upper Triassic), Petrified Forest National Park, Arizona, USA. Palaeogeogr., Palaeoclimatol., Palaeoecol., 96:299 317.

Ding, Q., Tain, N., Wang, Y., Jiang, Z., Chen, S., Wang, D., Zhang, W., Zheng, S., Xie, A., Zhang, G. & Liu, Z., 2016, Fossil coniferous wood from the Early Cretaceous Jehol Biota in western Liaoning, NE China: New material and palaeoclimate implications., Cretaceous Research, 61, 57-70.

Mandang, Y. I. & Kagemori, N., 2004, A Fossil Wood of Dipterocarpaceae from Pliocene Deposit in the West Region of Java Island, Indonesia., Biodiversitas, 5(1), 28-35.

Maria da Conceiçao, D., Saturnino de Andrade, L., Cisneros, J. C., Iannuzzi, R., Pereira, A. A. & Machado, F. C., 2016, New petrified forest in Maranhao, Permian (Cisuralian) of the Parnaíba Basin, Brazil., Journal of South American Earth Sciences 70, 308-323.

Petrified Wood Park, Lemmon, South Dakota.

Petrified Woods from the Indonesian Islands of Java and Sumatra.

Saltarelli, M, G., 2009, ‘Irreplaceable Works of Art’: Petrified wood treasures and dinosaur tracks create a paradise of geology.

Sigleo, A.C., 1979. Geochemistry of silicified wood and associated sediments, Petrified Forest National Park, Arizona. Chem. Geol., 26: 151–163.

Snider, T., 2012, Texas Road Trips: From Dinosaurs to Drive-Ins.

Snider, T., 2013, A to Z Texas: P is for Petrified Wood Buildings.

Teachout, G. E., 1995, Petrified wood of South Dakota.

Trendell, A. L., Nordt, L. C., Atchley, S. C., Lebland, S. L. & Dworkin, S. I., 2013, Determining floodplain plant distributions and populations using paleopedology and fossil root traces: Upper Triassic Sonsela Member of the Chinle Formation at Petrified Forest National Park, Arizona., Palaios, 28, 471-490.

University of Arizona, Laboratory of Tree-Ring Research; Fossil Trees or Petrified Wood.

Vasileiadou, K. & Zouros, N., 2012, Early Miocene micromammals from the Lesvos Petrified Forest (Greece): preliminary results., Palaeobio. Palaeoenv., 92, 249–264.

Williams, D. B., 2009, Chapter 7: Pop rocks, pilfered fossils and Phillips Petroleum – Colorado Petrified Wood., Stories in stone: travels through urban geology., Walker Publishing Inc., New York., 133-151. & Blog.

Yoon, C. J. & Kim, K. W., 2008, Anatomical descriptions of silicified woods from Madagascar and Indonesia by scanning electron microscopy., Micron 39, 825–831.

 

Urban Geology in Birmingham

I was invited to come back to Birmingham by Julie Schroder of the Black Country Geological Society to update and expand previous building stone walks of the city created by Julie, Eric Robinson and Paul Shilston. I was very pleased to do this, having graduated in Geology from the University of Birmingham in 1989. The city has changed a lot since then, with a brand new development around the Bull Ring and New Street Station. Julie, myself and fellow Birmingham Geology graduate Laura Hamilton hit the streets in Easter 2016. We have produced three guides to the city centre which can be downloaded as pdf documents here:

1. The Town Hall to the Cathedral

2. Centenary Square to Brindleyplace

3. Around the shops

The pictures below provide a snapshot of the geodiversity of Birmingham’s built environment …

Prehistoric Animals: A series of illustrations by David Roland

I bought this set of postcards when I was a kid in the 1970s. I can’t remember exactly where I bought them, but it was probably Manchester Museum. They were produced by scientific illustrator David Roland for Birmingham Museum and Art Galleries and represent what was then state-of-the-art interpretations of the appearance of dinosaurs and other prehistoric animals, including Dimtetrodon and Pteranodons. All are very green and scaly. What impressed me at the time, though, is that they all fitted together to make a single, continuous panorama. I loved them!

Featured are: Dimetrodon, Stegosaurus, Diplodocus, Brontosaurus, Iguanodon, Pteranodon, a portly Tyrannosaurus Rex and Triceratops.

The mis-appliance of science in cultural heritage?

Science applied to archaeology and cultural heritage is a thing. It has been happening for decades. Scientific analysis of materials can provide much needed information about materials, trade, manufacture, provenance, foodstuffs, populations, individuals. With today’s kit we can make analyses on tiny samples, or even acquire semi-quantitative analyses without the need for the destruction required to remove a sample. We can identify the components in building materials and pigments, the type of honey a vessel once held, the isotopic signature of metals and therefore their provenance, the isotopic signatures of bone material can tell us were a person lived, where they migrated, where they came from. Science can revolutionise established archaeological chronologies. Amazing information. So why is so much science applied to archaeology and cultural heritage so bad?

I have been to several conferences this year where there has been a cross- and multi-disciplinary approach to materials in cultural heritage. This if course should be a GOOD THING. However, myself and colleagues have been commenting that with the proliferation of analytical techniques and access to them, the science is actually getting worse, and this is not good. We feel that the study of science in cultural heritage is not moving forward. I also see this in papers I review. Scientists are collaborating with archeologists and art historians, but it seems that they are not communicating well and there is little effort on either side to learn about each other’s discipline and what the questions the ‘science’ aims to answer. Inappropriate analytical techniques are used and poor data are produced and these data are, again inappropriately, under- or over-interpreted. The discussions at conferences and recent press activity on the use of a synchrotron to identify the presence of calcium on a Greek vase have spurred me to write this. I admit I have only read the press releases on this latter research, and I presume that the actual publication will provide much more depth to these analysis.

I’m a geologist. My PhD used geothermochronological techniques to look at landscape evolution on a continental scale. I used a radiometric dating technique called fission track analysis which used on the mineral apatite. It was known that the chemistry of apatite, with substitutions between chlorine, fluorine and the hydroxyl group, affected the lengths of the fission tracks and therefore the thermochronometric data obtained. I looked at the subtle changes of chemistry in apatites using Fourier Transform Infra-Red spectroscopy (FTIR). Although I went on to lecture in ‘orthogeology’, much of my research has been associated with the application of geological techniques to cultural heritage. I am so glad – and lucky – to have learned how to use petrology/petrography, geochronology and spectroscopy from first principles, rather than to have stumbled across these techniques as  ‘black box’ methods to reveal more about the material I am working on. I understand what these techniques actually tell us about the materials they are applied to. They are measuring bonds and their configurations on a molecular scale, the number of undecayed v. decayed isotopes (or their proxies), elemental weights or excitation of electron energy levels. These data can then be interpreted to give us information such as a radiometric age, they may be diagnostic of the identification of a mineral (or analogous compound) or the nature and origin of an organic compound, they can elucidate the presence or absence of a certain element or the precise chemical compound present in a sample.

The important word here is interpreted. Yep, that’s right, some guy sits down and writes a computer programme to interpret the results that a machine turns out. This is what the software that comes with your new black box analytical machine is. It is NOT the machine telling you the answers. So if you click on the sample image in your back-scattered electron image of your sample (yes, that’s right it is not a photo) and your spectra tells you that a peak is assigned to silicon, this is a programmers interpretation of what fits that peak. Now of course the vast majority of these analyses will be correct and completely reliable, but they should still be used with caution. I have always found pertinent this quote from the 1990 film The Hunt for Red October, in which a submarine sonar operator Jones (played by Courtney B. Vance) discovers that his interpretation software is not telling him the truth. He has detected the faint but rhythmic sounds of another submarine which is using a new propulsion mechanism and is trying to explain this to his commanding office Bart Mancuso (Scott Glen) …

Jones: When I asked the computer to identify it, what I got was ‘magma displacement’. You see, sir, SAPS software was originally written to look for seismic events. And when it gets confused, it kind of ‘runs home to mama’.

Mancuso: I’m not following you, Jonesy.

Jones: Sorry, sir. Listen to it at times speed. [he plays a tape in which a rhythmic noise is heard] Now that’s gotta be man made, Captain.

Mancuso: Have I got this straight, Jonesy? A forty million dollar computer tells you you’re chasing an earthquake, but you don’t believe it? And you come up with this on your own?

Jones: Yes, sir.

Mancuso: Including all the navigational math?

Jones: Sir, I-I’ve got-

Mancuso: Relax, Jonesy, you sold me!

This is so true. That peak fitting software on your FTIR, WDS, EDS, EMP, XRF, whatever was probably not written with reference to archaeological materials. It was written to identify pure compounds used in pharmacy or precise mineral compositions.

So the lesson to learn here is if you get something unexpected, like a massive peak assigned to tellurium, you should probably question your results, not just accept it as something really unusual and therefore cool. Any scientists watching your presentation or reading your paper (or indeed reviewing it) will immediately see through this. Here is an example from the field of petrology. What you need to know before reading this is that basic mineral identification in rocks is carried out by (experienced) analysts using optical polarising light microscopy (PLM). So for example, a basalt may include a mineral from the pyroxene group where chemistry ranges between iron, magnesium and calcium and variable silica amounts. This group can be conveniently subdivided into two main groups orthopyroxenes and clinopyroxenes which have clear association with different geochemical environments. The ‘ortho’ and ‘clino’ bits refer to differences in crystal structure and these two groups are most easily distinguished using PLM. It would take about three seconds to distinguish the two pyroxenes and we generally just refer to them as orthopyroxene (opx) and clinopyroxene (cpx). If you really want to, you can further subdivide the pyroxenes into named types, say for cpx, the main ones are are augite, hedenbergite and diopside. However most of the time we geologists don’t use this classification unless there is a reason for doing so, i.e. we want to answer specific questions about zoning within a single crystal and how that relates to say, fluctuating melt chemistries (and we need a reason for deciding that fluctuating melt chemistries are important). We just call it clinopyroxene. Therefore when I see a presentation that shows someone has identified diopside in a pot sherd I know they haven’t looked properly at the material and a computer programme written for igneous petrologists has told them it is diopside and they didn’t question it. It is also important to note that the name ‘diopside’ does not denote a specific phase chemistry that will direct you to a particular source/provenance. It doesn’t and it won’t. And it isn’t ‘more scientific’.

Clinopyroxene. I really DON’T CARE whether it is diopside, augite or hedenbergite.

On the subject of mineral names, the misuse that infuriates me most is the use of the mineral name cuprorivaite to describe the synthetic pigment Egyptian Blue. If you Google cuprorivaite, you will get far more references to Egyptian painting than to minerals and this demonstrates the scale of this problem. Cuprorivaite is a rare, naturally occurring mineral. When it occurs it is as a micromineral (very small crystals, < 1 mm) and often disseminated. It occurs nowhere in masses worth of economic extraction, even as a cottage industry. Therefore it has never been used as a pigment and no one has ever detected cuprorivaite on archaeological paintings. However, you would not believe this from the literature. What researchers have found is an analogous synthetic compound called Egyptian blue. It’s a calcium copper silicate if you want to sound scientific. Why is this terminology important? In the world of pigment analysis, there are many phases which can and have been used as pigments derived from natural minerals and analogous synthetic compounds and it is important in answering archaeological and art historical questions to be able to distinguish these forms. Examples are the red pigment mercury sulphide which when natural is cinnabar and when synthetic is vermillion, or (natural) azurite and (synthetic) verditer for the blue copper carbonate hydrates. It can be important to know the differences between these minerals to detect fakes and assess knowledge of technologies or mineral provenances. As a consequence it is essential to have a terminology that clearly differentiates between natural and synthetic pigments.

A photomicrograph of the mineral cuprorivaite.

I think a major problem here is that a lot of the time, scientists and cultural heritage people don’t really get each other. They don’t know enough about each others subjects. Sure, they watch the TV programmes and remembered being into the Ancient Egyptians at school, or they maybe had a toy chemistry set. But now we are all grown-up academics, we are all set in our ways and stuck in the silos that the university academic departments give us (and this structure is a problem). A chemist is probably not likely to meet an art historian at their institution unless they sit together on a college committee – and many academics will avoid these committees like the plague. So we don’t talk and learn about other disciplines, and don’t get me wrong, scientists are just as bad about this as anyone. Many can be arrogant, thinking that they can answer the ‘simple’ questions archaeologists ask and they think its cool to have something they have only seen before in a museum or on the TV in their lab. Something to tell their colleagues and hey, it might just tick some of those ‘impact’ boxes. So an art historian comes along with a Greek vase and the scientist thinks ‘OK I can analyse this for you using my synchrotron. This will cost hours of beam time and thousands of dollars, but we may get a paper from it that will enable me to patronise humanities people with my superior knowledge and show that I can also do novelty science. I have seen Greek vases in museums, I am surprised that no one has made these analyses before because it is so easy to do’. The art historian thinks ‘Brilliant, this will tell us something that all those other techniques won’t tell us. I don’t really understand those techniques, but surely this synchrotron thing is so big and so expensive that it must provide better, richer, more accurate and more precise results than anything else. This will make me look amazing when I present this work! So few people in my position have access to this sort of kit. Just using this technique makes this study novel and innovative. And the scientist guy says it will be easy. He seems to know what he’s doing.’. Maybe, just maybe, one of the parties will search the literature, but because scientific literature database on the whole doesn’t search books and memoirs and vice versa, they don’t find out that this is something that has already been done, using simple analytical techniques which have given better and more informative results.

And this happens within science disciplines too … this (modified) cartoon is on our geochron lab wall. ‘We’ are the geochronologists …

Despite what you may interpret from the above, a geologist or geochemist would never use complex, expensive kit to do routine mineral analyses, so why would you do this to analyse the pigments on a wall painting or on a Greek vase? Many simple techniques such as microscopy and wet chemistry give reliable, accurate and precise results and can’t be improved on. The press releases surrounding the analysis of pigments on an Attic lekythos carried out on a synchrotron only told us that the white pigment present ‘contained calcium’. It stopped short of making the logical (i.e. totally f**cking obvious) observation that this was a form of calcite, a polymorph of calcium carbonate. If I could have taken a tiny sample from the vase (so small that it could not be noticed by the naked eye) I could have told the museum/art history guys that not only was it calcite, but would have been able to differentiate chalk, eggshell, seashell (OK the aragonite polymorph in this case), coral or any other form of calcite derived from a limestone. If sampling was not possible, I could have identified the presence of calcite/aragonite (very unlikely that it would be aragonite though) using a UV light torch and a pXRF in less than a minute. I would have probably charged £50 max for experimental costs. To present a paper on pigment analysis and conclude that you have a Ca-rich pigment or an Fe-rich pigment is simply not good enough and is not moving the subject on.

Mind you, on the subject of portable X-Ray fluorescence machines, these will only give semi-quantitative results and users should be aware of this, but they are good for identifying major elements when sampling is not possible. However pXRF analyses should not be used in isolation. These machines were designed for quick, rough, field checks to look at say, arsenic pollution in ground water. The manufacturers are horrified at how their data has been used to provide ‘confirmed scientific analyses’ in cultural heritage.

My examples here are mainly to do with geological and analogous materials but I am sure that many scientists can quote examples from metallurgy and biosciences where glaring misuse of equipment and terminology are used. How can this be improved? We all have to learn more from each other. Most importantly cultural heritage people need to know what questions they want to ask of their objects and materials. Is it as simple as ‘what is it?’ (a very valid question) or do they want to know more; where does it come from? How was it made? If there are more complex questions to be answered then find a scientist that can work with you to help find the best analytical method to get the answers you need.

My top tip is that scientists and cultural heritage partners work together in a truly collaborative manner, so that the scientists are not just used as technicians, they are an integral part of the research project. I have always worked in close partnership with colleagues and have made time to learn about the materials I am analysing and the contexts of the artefacts or building from which they are derived. I know about comperanda, I read the literature, I know what the expected range of materials are. I double check my results when I find something outside that. If its true I design further experiments or do field work to answer these new questions. I don’t just hand over the data and walk away. And I always, always use the most appropriate technique to perform scientific analysis. As a petrologist I am experienced with polarised light microscopy so that is my ‘go to’ technique. It is great for ceramics, mortars, pigments and of course, stone. I do accept that this takes learning and experience, but go on a course! Learn it! I do a lot of work on pigments so I use FTIR and Raman spectroscopy too. If I have questions about organic binders then I would turn to gas chromatography mass spectrometry, but as this is costly and wouldn’t go there unless it was really important. I use old-skool spot tests and wet chemistry for quick analyses of phases to detect things like lead or phosphate (there are loads of books on how to do this, and you can do it in you kitchen). I use pXRF and SEM/EDS, again for major elements only, but am cautious on how interpret results, knowing these are not quantitative techniques. I occasionally use X-Ray diffraction (XRD) to look at crystallinity in some minerals (mainly hematite and other iron oxides and iron oxide hydroxides in ochres). These techniques answer all the questions I can think of. I used a synchrotron once to try and design some experiments to further the understanding of the blackening of cinnabar/vermillion paints. It didn’t work.

I used a synchrotron to analyse a pigment and all I got was this crappy picture.

Portland Bone

There is a lot of Portland Stone in London, so much of it in fact that I almost blank it out. I am trying to change the way I think about it, partly thanks to Gill Hackman’s inspiring book “Stone to build London” and also, whilst working on the London Pavement Geology project, to give this most iconic of London’s buildings stones its rightful coverage.

London is a good place to see all the varieties of Portland Stone quarried today and in the past, and a variety of facies and fossils can be seen in many buildings (see Siddall & Hackman, 2015; Siddall 2015, Hackman 2014). Notable examples are Green Park Underground Station and BBC Broadcasting House. However the bulk of Portland Stone Buildings in London are of fairly standard Whitbed, with little variation in facies and fossils. Typically these are white to pale-grey weathering, oolitic limestones, sometimes showing cross-bedding and with variable fossil content, mainly shells, shell fragments and occassionally pieces of Solenopora algae. The stone used at St Margaret’s Westminster is, on the whole, fairly characteristic of this description.

Easily overlooked, dwarfed as it is by its next door neighbour, Westminster Abbey, St Margaret’s Westminster is a neat little church clad in Portland Stone Whitbed. Dedicated to Saint Margaret of Antioch, it is the parish church of the House of Commons. There has been a church on this site since the 12^th Century. The current building including its Portland Stone cladding dates from the 1730s refurbishment by the architect John James (1673-1746). St Margaret of Antioch was swallowed by a dragon, but was coughed up alive after she had tickled the dragon’s rib cage from the inside with her cross.

In the passage between St Margaret’s and the Abbey, towards the SE corner of the church, a block sits just above eye-level, containing a pen-shaped, brown clast, truncated by the lower edge of the block, but strikingly different from the standard allochems of the Portland Limestone. The preserved piece is around 15 cm long and 3 cm wide and has been eroded into a flattened pebble (below).

 

My knowledge of vertebrate palaeontology is scant and more influenced by Ray Harryhausen than anything I learned when an undergraduate. But I guessed this looked more ‘bone’ than ‘stone’. However, I have never before seen bone in Portland Stone. In the Purbeck Beds it is relatively common, but usually preserved as jet black phosphate. Thanks to the fantastic research tool that is social media, I contacted geologist Mark Godden of Portland quarry firm Albion Stone. Mark agreed that this worn pebble of brown stuff was ‘probably’ bone, as these occasionally turn up when quarrying and Mark sent some pictures for comperanda. Certainly confirmed bone from Portland Stone, recognisable as vertebra etc are a similar colour and texture. Other options where that it is an infilled burrow; however, if so, what is it that has infilled it? There is extensive bioturbation in Portland Stone, but it is all infilled with white oolitic limestone or shell fragments. So I am fairly convinced that it is a fragment of a disarticulated, much eroded and fairly large vertebrate skeleton.

According to Delair & Wimbledon (1993), the bones from several vertebrates have been found in the Portland Limestone member and equivalent strata of Tithonian (Upper Jurassic) age; crocodile and turtle bones, as well as those of marine reptiles Ichthyosaurs and Pleiosaurs are not unexpected, but dinosaur bones also occur. These may have been derived from paddling saurischians who subsequently keeled over, or more likely, were washed in from adjacent dry land, where dinosaurs such as Megalosaurus and Iguanodon were knocking around.

Reconstructions of these types animals can be seen at Crystal Palace. These Victorian effigies somewhat dated (to say the least) and are not exactly Jurassic Park. The real things are now interpreted to be the sleeker, more streamlined beasts with which the average film goer is more familiar. Nevertheless, you get the idea!

 

Above, left: Crocodiles and Right: Iguanodons at Crystal Palace. Below, a Megalosaurus surveys the scene.

Other notable bones to be found in St Margaret’s include those of William Caxton, England’s first printer of books, Sir Walter Raleigh and poet John Milton.

St Margaret’s Westminster is listed here on London Pavement Geology.

Many thanks to Mark Godden of Albion Stone.

References & Further Reading

Crystal Palace Dinosaurs: http://cpdinosaurs.org/visitthedinosaurs

Delair, J. B. & Wimbledon, W. A., 1993, Reptilia from the Portland Stone (Upper Jurassic) of England: A preliminary survey of the material and literature., Modern Geology, 18, 331-348.

Dino Directory: Megalosaurus: http://www.nhm.ac.uk/nature-online/life/dinosaurs-other-extinct-creatures/dino-directory/megalosaurus.html

Dino Directory: Iguanodon: http://www.nhm.ac.uk/nature-online/life/dinosaurs-other-extinct-creatures/dino-directory/iguanodon.html

Godden, M., 2012, Portland’s quarries and its stones. http://www.dorsetgeologistsassociation.com/Portland-Stone/Portland_Stone_Document_-_7_June_12.pdf

Hackman, G., 2014, Stone to build London: Portland’s legacy., Folly Books Ltd., Monkton Farleigh., 311 pp.

London Pavement Geology: http://londonpavementgeology.co.uk

Siddall, R., & Hackman, G., 2015, The White Cliffs of St James’s: Portland Stone in London’s Architecture., Urban Geology in London No. 31, http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/PortlandStJames.pdf

Siddall, R, 2015, An Urban Geologist’s Guide to the Fossils of the Portland Stone., Urban Geology in London No. 30, http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/PortlandFossils.pdf

St Margaret’s Westminster: https://en.wikipedia.org/wiki/St_Margaret’s,_Westminster

The Worst Stone ever used in the Metropolis: 150 years of Stone Decay in the Houses of Parliament

London’s Houses of Parliament are currently in dire need of restoration and repair. This iconic building is coming apart at the seams due to heavy usage, and quite frankly, inappropriate choice of building materials by men who should have known better 150 years ago. The stone is, of course, not the only problem faced by this building; large amounts of asbestos panelling and insulation were installed in the post-war period and the whole place desperately needs rewiring. The big decision the government needs to make at the moment, is whether to allow the repairs to go on with minimum disruption to the day-to-day business or to move everyone out for 6 years and turn the building over to the contractors. The first option is estimated to cost £5.7bn and take up to 32 years. The six year plan would probably cost in the region of £3.5 bn. This option sounds like a no-brainer, but where would Parliament sit in the meantime?

Following the destruction by fire in 1834 of part of the Palace of Westminster, Prime Minister Robert Peel launched a competition for architects to submit plans for a new seat of British Government in the ‘Gothic or Elizabethan’ style. Charles Barry, in collaboration with Augustus Pugin submitted the winning design. So far so good. The plans were for a superb Gothic palace to sit on the banks of the Thames. Barry’s costing may also have been influential, as he was determined to do this job on the cheap and as quickly as possible. Nevertheless, he contracted the most eminent geologists of the day, Henry de la Beche and William Smith along with master mason Charles Harriot Smith for assistance in selecting the stone (Lott & Richardson, 1997). This team were subsequently known as the ‘Special Commissioners’.

The ensuing cock-up is a well-known story, and the history of the debacle has been described by Lott & Richardson (1997) and Anon (2003). To summarise, a shortlist of suitable freestones were identified. They were: Portland Stone, Darley Dale Sandstone, Bolsover Stone and Anston Stone. The obvious choice in retrospect, and even at the time, would have been Portland Stone, because this had had widespread use in London for the last two centuries and was clearly bearing up well to the smog and pollution, as testified by the good state of repair of buildings such as St Paul’s Cathedral and numerous other churches. But no. The building stone dream team decided to go for Bolsover Stone, on the grounds that it had ‘advantage of colour’ and strong evidence of its durability as demonstrated by its use in Southwell Church (now Southwell Minster) in relatively unpolluted Nottinghamshire.

It should be noted here that Southwell Minster is in fact built from Mansfield White Stone (Thomas, 2006; Lott & Richardson, 1997).

However on investigation of quarry at Bolsover Moor, it was discovered that the quarry was far too small to furnish the stone, and the stone was of poor quality. Disaster averted?

Both Bolsover and Anston Stone are derived from the Permian Cadeby Formation of dolomitic limestone (formerly known as the Lower Magnesian Limestone) which outcrop from Nottinghamshire to Sunderland in an approximately N-S trending band of strata. Many excellent building stones are derived from the dolomitic limestones, but there is considerable variation in facies, texture and durability along strike. Once set on the Magnesian Limestone (as the Cadeby Formation was then known), the Special Commissioners opted for Mansfield Woodhouse Stone and, predominantly, Anston Stone as second choice and stone started being quarried from these localities for the construction of the Houses of Parliament. The stone was transported to London via barge to the Humber Estuary, then down the east coast to London (Lott & Richardson, 1997).

The new parliament building was constructed between 1840 and 1859. It quickly became clear that disaster had indeed not been averted and that the Anston stone began to decay in London’s polluted atmosphere as soon as it was laid. But to quote John Allen Howe (Howe, 2010) ‘The bad state of the dolomitic limestone in the Houses of Parliament does not prove that stones of this class are worse than other limestones for town use, but that slovenly work will produce the results to be expected of it.’

Howe and his colleague James Elsden further elaborated on the levels of slovenliness employed in the Anston Quarries (Elsden & Howe, 1923, quoting a Report of the Government Select Committee on the subject, recorded in Cowper, 1861). Despite most of the stone at Anston being of good quality, there are a number of beds that were known to be poor. But the beds were ‘worked indiscriminately’ and good strata were not followed laterally. Also ‘no supervision of the quarries was provided for and no seasoning of the stone took place. The stone was sent to London within a fortnight of quarrying, even throughout the Winter.’ Soft limestones and dolomites such as these are normally left out to cure in the air for several months before being used. Speed and volume seemed to be of the essence, quantity outstripping quality; ‘So little stone was rejected at the quarries that almost the only waste was that derived from the cutting of the blocks’. Therefore stone of exceedingly poor quality was used along with very sound stone.

Once in London and in the hands of the builders, it was discovered that the stone had not been marked, therefore it was not quarry-laid, i.e. in its strongest orientation. Much of the ashlars ‘were sur-bedded – an example of unpardonable slackness’ (Elsden & Howe, 1923).

The author Charles Dickens has already waded in on the subject of ‘unpardonable slackness’ in 1860, writing in a periodical that the Anston Stone used was ‘the worst ever used in the Metropolis’.

Elsden & Howe (1923) write that Anston Stone was used in the whole building except ‘the upper part of the towers and the front towards Abingdon Street’. Tantalisingly, they do not say which stone was used here, though these authors do say that Steetly Stone (also from the Cadeby Formation) from near Worksop was used to a ‘small extent’.

By the 1920s, several blocks of stone had fallen off the building, so much so that members of the Terrace Bar were recommended to sit as close to the river (and as far from the building) as possible to avoid being hit by falling masonry (Anon, 2003). Around 200 tones of stone were removed and replaced in the 1920s. It was commented in the 1960s that the Houses of Parliament resembled ‘Joseph’s multicoloured coat’. A major programme of works cleaning, repointing, replacing and carving stone happened during the 1980s and 90s, using Clipsham Stone and French Anstrude Stone (Anon., 2003). The latter stone is a Bathonian oolitic limestone from Bierry-des-Belles-Fontaines in Burgundy. It came into use when supply of Clipsham Stone could not keep up with demand. Incidentally, this is the stone that was used in the controversial restoration of the British Museum in the late 1990s.

It is difficult to get close enough to much of the Houses of Parliament these days, for obvious security reasons, however one can examine close-up the grand Peers’ Entrance on Abingdon Street. Certainly a golden yellow stone is used here. The foundations and stone used for the arch are paler than the upper yellow stone, and is a coarse grained calcarenite, cross-bedded and packed with ooids, peloids and shell fragments. It is Clipsham Stone, from the middle Jurassic Inferior Oolite Formation. Running repairs have continued at the Palace of Westminster over the last century and a half. Clipsham Stone has been used since 1928, extracted from Medwells Quarry in Rutland (Anon, 2003).

Above: Clipsham Stone at the Peers’ Entrance

William Smith died in 1839 and therefore did not have to suffer the relentless criticism for the poor choice of stone. The full blame cannot be laid at the feet of either Smith or De la Beche, the fault was clearly with the (lack of) quarrymasters and subsequently cutting corners with stone production. The consequences are, nevertheless, that British taxpayers will have to fork out a £3-5 bn bill in the 21^st Century. Henry de la Beche went on to redeem himself and Anston Stone by choosing this material to build his Museum of Practical Geology on Jermyn Street. However, De la Beche took care in this instance to supervise quarrying and inspect the stone used. This building has subsequently been demolished, so we cannot comment today on whether or not he chose wisely.

The moral of this story is #shouldhaveusedPortlandStone

References & Further Reading

Anon, 2003, (revised 2010), House of Commons Information Office., Restoration of the Palace of Westminster: 1981-94. Factsheet G12 General Series: http://www.parliament.uk/documents/commons-information-office/g12.pdf

BBC News: Parliament restoration plan could cost up to £5.7bn; http://www.bbc.co.uk/news/uk-politics-33184160

Cowper, W., 1861. Report of the Committee on the Decay of the Stone of the New Palace at Westminster.

Dickens, C. 1860. All the Year Round. November Issue. Chapman and Hall.

Elsden, J. V. & Howe, J. A., 1923, The Stones of London., Colliery Guardian, London., p. 132-133.

Houses of Parliament Restoration and Renewal; http://www.restorationandrenewal.parliament.uk

Lott, G. K. & Richardson, C., 1997, Yorkshire stone for building the Houses of Parliament (1839-C.1852)., Proceedings to the Yorkshire Geological Society., 51 (4), 265-272.

Restoration & renewal of the Palace of Westminster; http://www.parliament.uk/about/living-heritage/building/restoration-project/

Thomas, I., 2006, Southwell Minster., Mercian Geologist, 16 (3), 220-222.

Travels of an Urban Geologist: Building Bavaria II

The pretty, Medieval town of Nördlingen lies, fittingly, on the ‘Romantic Road’ in the west-central Swabian Bavaria. Its red-roofed buildings are enclosed within a complete and circular circuit of town walls. This is best viewed, along with the surrounding, rolling green countryside from the 90 m high tower, ‘The Daniel’ which is built over the west entrance of the town’s main church.

St Georg’s Church sits in the centre of Nördlingen, was built in the second half of the 15^th Century. The tower was finally completed in 1639 and, because of the splendid views, it is named after a text from the Book of Daniel; “Then the king made Daniel and […] made him ruler over all the land” (2.48). The grey, breccia used to build the church came from the nearby quarry at Altenbürg, and is generally known as Bavarian Trass. ‘Trass’ generally refers to volcanic derived rocks which form a pozzolanic additive when mixed with lime cement, producing an hydraulic set and in indeed the trass from Nördlingen was also used for this purpose.

 Above left, St Georg’s Nördlingen and Right, Altenburg Quarry

But this is no ordinary, volcanic-derived Trass, such as those quarried in the nearby Rhine Graben. The Bavarian Trass of Nördlingen was formed by a meteorite impact.

Nördlingen lies in the Ries impact crater, created 14.5 million years ago (in the middle Miocene) from the impact of a meteorite. The crater is 24 km in diameter, and the bolide hit a target of Mesozoic limestones, up to 800 m thick which had been deposited on top of Hercynian basement rocks, granitoids, gneisses and amphibolites. At impact, the bolide was vapourised, the extremely high pressures exerted caused melting in the target rocks and of course an explosion, which threw molten rock, molten bolide and rock fragments upwards and outwards, this mix fell back down to Earth into the crater and surrounding areas. Impact melt breccias form a specific and distinctive rock type known as suevite, named after Suevia, the Latin for Swabia. The Ries Suevite has a grey, glass-rich, tuff-like matrix supporting angular clasts of basement derived granite and gneiss, keuper clay, sandstone, Malm limestone and slugs of glass. The latter where incorporated into the breccia whilst still semi-molten and these distinctive textures are known as ’Flädle’.

A map and cross section of the Ries Impact Crater after Osinski (2004), showing the distribution of suevites in black.

 Above left; suevite in a quarry exposure, with coasts of granite and black Flädle. Right Flädle are obvious in the ashlars used in the church walls.

Visitors to Bavaria may well have eaten the delicious Flädlessuppe, which contains noodle-like strips of pancake, the Flädle. The variety found in suevite are somewhat less digestable, and as glass they contain molten components of both the target rocks and the vaporised bolide.

 Above, suevite used as building stone in the Daniel, with clasts of gneiss, limestone and Flädle.

Bavarian Trass, or Ries suevite, is probably one of the world’s most unique building stones. Though many would like to claim so, few churches have been built with an extra-terrestrial contribution.

References and further reading …

Osinski, G. R., 2004, Impact melt rocks from the Ries structure, Germany: an origin as impact melt flows? Earth and Planetary Science Letters 226, 529– 543.

Ries Geopark: http://www.geopark-ries.de/index.php/en/welcome

Travels of an Urban Geologist: Building Bavaria

Bavaria is a huge province of modern Germany. Recently I visited the southern parts, the Allgäu and Upper Bavaria regions, out on the Molasse Basin of the northern Alpine Foreland, staying in the town of Memmingen. The countryside looks like a picture-book, full of toy farms with green manicured grasslands spotted with white, plastered houses with red tile roofs (right). What is noticeable is there is less of the slate and stone seen in the French and Italian Alpine forelands. However in the few towns and villages I visited, churches and modern shop fronts featured stone masonry or cladding. The building stones used were striking in being predominantly fairly recent looking breccias and tufas. I just assumed they were from the Molasse. I pointed one out to my (non-geological) friends and Amira, a local, immediately said ‘my Dad will know exactly what this is’. Amira’s Dad did know, he told us it was ‘Biberstein’ and it was related to the Ice Ages. Dads always know these things. So I started to look into this a little bit more.

Now, I have spent many years studying and teaching Alpine Geology, but have pretty much managed to ignore the Ice Age geology of the region. Somewhere deep I recall the mantra of Günz, Mindel, Riss and Würm, the original glacial periods devised by Alpine geomorphologists and geologists at the turn of the 19^th Century. During the glaciations which define the Pleistocene Stage of the Quaternary, the Alps were covered by a huge ice cap, with enormous glaciers descending towards the southern and northern forelands. In Upper Bavaria, large lobate piedmont glaciers coalesced to form the Inn-Chiemsee Glacier, which was at its greatest extent during the last glacial maximum around 21 thousand years ago (ka). This ice body excavated the famous moraine field around the town of Rosenheim and when it retreated, left behind the enormous glacial Lake Rosenheim, the remnants of which are the present day Simsee and Chiemsee. However this was the last of at least four major glaciations, the Würm. The Victorian glacial chronology has been considerably refined over the last century, but the terminology remains essentially the same. The four main glaciations, Günz, Mindel, Riss and Würm each lasted around 100 thousand years, separated by warm periods of similar lengths.

Glacial Lake Chiemsee, with the Bavarian Alps behind

Quaternary deposits from the Bavarian-Austrian Alpine foreland have been used as building materials since the Roman period. The most famous and most widely used are the Brannenburg Nagelfluh, the Hötting Breccia and the calcareous tufa deposits worked between the Inn Valley and Vorarlberg. A major advantage of these stones, compared with the bedrock of the Alpine series, is that they are soft and easy to quarry, hardening on surface exposure. Despite their young geological age, these Quaternary deposits have been surprisingly resilient to weathering and erosion. They often display large porosity, which, far from being detrimental, has contributed to the resistance to decay; the stones dry out more quickly rather than preserving water in small pore spaces. Having formed at the Earth’s surface and not having undergone major periods of burial or diagenesis, they are at ‘equilibrium with their environment’ (Mirwald et al., 2012). They have been use since at least the Roman period, and some still continue to be quarried today.

A locally sourced and much used stone is the wonderfully named Brannenburger Nagelfluh from Brannenburg am Inn, in the Inn Valley of Southern Bavaria, south of the town of Rosenheim. This is Amira’s father’s ‘Biberstein’ named from the Biber hill near the quarries, and Biberstein is the colloquial name for this stone. The word ‘Nagelfluh’ is used in the German geological literature to refer to young (Tertiary or Quaternary) formations of conglomerate. Nagel means nail, and the name refers to the fact that in outcrop, the rock surface appears to have large nails hammered into it, so that only the heads are seen. Brannenburger Nagelfluh is a (just) coast supported, polymict conglomerate with a ratio of clasts:matrix of 50:50. The clasts are moderately sorted, up to around 10 cm across and composed of a variety of rock types derived from the Alps; limestone, sandstone, gneiss, amphibolite schist, dolerite and quartz. Brannenburger Nagelfluh formed on the shores of the Rissian Lake Rosenheim, and they represent a series of deltaic deposits, which are exposed in the quarries around Brannenburg and Flintsbach (Herz et al., 2014). These sediments were deposited at around 150 ka. The topsets are exposed in the Anton Huber Quarry and foresets are exposed in the Grad Nagelfluhwerk quarry. Grey and yellow varieties are observed in photographs of the quarries published in Herz et al. (2014).

  Above: Shop fronts in Memmingen

 Brannenburger Nagelfluh used in Ludwig II’s crazy fountains at Schloss Herrenchiemsee

A superficially similar stone is the Hötting Breccia. This is an alluvial fan and talus slope deposit, developed on the Northern Calcareous Alps basement. The breccias outcrop around Innsbruck, and are dated between 100-70 ka (Spötl & Mangini, 2006). They are therefore associated with the Riss-Würm interglacial period. Petrologically, they are carbonate-cemented breccias, with poorly-sorted clasts of the underlying Triassic limestones. Local concentrations of red Permian sandstones (Alpiner Buntsandstein) stain the lowest deposits of the Hötting Breccia yellow and red. These are up to 40 m in thickness. The overlying White Hötting Breccia does not contain Buntsandstein, and has only limestone clasts, however this was less well consolidated and was not used for building (Unterwurzacher et al., 2010). Several quarries operated near Innsbruck until the early 20^th Century, the largest of which was Mayr (below). Transportation both north and south along the river Inn was favourable in the distribution of this stone and it is the main building stone in Innsbruck, where it was used for the Cathedral and other examples of civic architecture (Mirwald, 2012, Unterwurzacher et al., 2010).

Many modern buildings are clad with a grey matrix-supported breccia. We need to travel to the southern Alpine foreland to source this stone. This is ‘Ceppo’ from the shores of Lago di Iseo in the Italian Lake District. The Ceppo di Poltragno Conglomerate is a grey, carbonate-cemented breccia. Quarries are located in the Adda and Brembo valleys (Vola et al., 2009). Ceppo is a Quaternary fluvial-glacial conglomerate. It was deposited during the lower to middle Pleistocene (1.8 – 0.125 Ma) as diamictites and colluvial scree deposits. The clasts are matrix supported and are derived from the Triassic dolomites in the Carnic Alps (Forcella et al., 2012).

  Above: Cladding on the ground floor of an office building in Memmingen

Ceppo has been quarried since the Roman period and is actively quarried today, used as cladding and as paving. Vola et al. (2009) have described its use in Bergamo and Bugini & Folli (2008) have described its use in Milan. Varieties known as Ceppo Gentile, Ceppo Gré and Ceppo Poltragno are marketed. Vola et al., (2009) list the following quarries; Camerata Cornello, San Pellegrino and Brembate Sotto (inactive) and Poltragno, Pianico, Grè (at Solto Collina on the lake shore) and Castro (active). The stone is exported throughout southern and central Europe and is widely used in southern Germany.

Back in Austria and Bavaria, tufa limestone occurs commonly across the Northern Calcareous Alps, found in association with cool spring activity, where waters are supersaturated with calcium carbonate. Important quarry sites are at Thiersee, in the Inn Valley south of Brannenburg and at Andelsbuch in the Vorarlberg of westernmost Austria. It could be relatively easily exported to the Allgäu region of Bavaria (Kempten and Memmingen) from this latter locality, along the River Iller. The stone is both strong and light in weight. Like many tufas and also large porosity stones such as the Portland Roach and the Florida Coquina, these stones are extremely effective in the constructions of fortifications as their properties allow them to absorb impacts (of cannon balls etc.) well. Thiersee Tufa was used to construct the Kufstein Fort in the Tirol (Mirwald et al., 2012). However there is good evidence that these stones have been won since the Roman period; they are used in the villa at Rankweil in the Vorarlberg (see Unterwurzacher et al., 2010). It is probably Andelsbuch Tufa that is used in the church at Memmingen (below).

 

At Thiersee, the tufa deposits are up to 10 metres thick and extended laterally, prior to quarrying for several hundred metres. They are primarily ‘moss tufas’ (Unterwurzacher et al., 2010). These deposits are no longer active and the deposits are almost worked out; peak production was in the 18^th Century. At Andelsbuch, moss tufa and phytoclastic tufa have formed on top of the Helvetic Nappe and Flysch formations. The deposit is up to 5 metres thick and extends for 100 m. Tufa deposition here is still active (Unterwurzacher et al., 2010).

References and further reading

Bugini, R. & Folli, L., 2008, Piedras de la arquitectura milanesa (Stones used in Milanese architecture)., Materiales de Construcción, 58, (289-290), 33-50.

Ceppo di Gré: http://www.naturalstoneinfo.com/download/bgcamcom.ceppo_gre.pdf

Forcella, F., Bigoni, C., Bini, A, Ferliga, C., Ronchi, A., Rossi, S. et al., 2012, Note Illustrative della Carta Geologica d’Italia 1:50 000; 078 Breno., Servizio Geologica d’Italia., 313 pp. http://www.cartografia.regione.lombardia.it/metadata/carg/doc/Breno_note_illustrative_dicembre_2012.pdf

Herz, M., Knipping, M. & Kroemer, E., 2014, The Rosenheim Basin: Würmian and Pre-Würmian deposits and the Höhenmoos interglacial (MIS 7). in: Kerschner, H., Krainer, K. & Spötl, C., From the foreland to the Central Alps: Field trips to selected sites of Quaternary research in the Tyrolean and Bavarian Alps., Excursion guide of the field trips of the DEUQUA Congress in Innsbruck, Austria, 24–29 September 2014., 6-17.

Huber Quarry: http://www.nagelfluh.de/INFO/GESCHICHTE/tabid/58/Default.aspx

Mirwald, P., Diekamp, A., Unterwurzacher, D. & Obojes, U., 2012, Weathering of sedimentary stone materials formed under Earth surface conditions., 11 pp.

Sanders, D. & Spötl, C., 2014, The Hötting Breccia – a Pleistocene key site near Innsbruck, Tyrol., in: Kerschner, H., Krainer, K. & Spötl, C., From the foreland to the Central Alps: Field trips to selected sites of Quaternary research in the Tyrolean and Bavarian Alps., Excursion guide of the field trips of the DEUQUA Congress in Innsbruck, Austria, 24–29 September 2014., 81-93.

Spötl, C., Mangini, A., 2006, U/Th age constraints on the absence of ice in the central Inn Valley (eastern Alp, Austria) during Marine Isotope Stages 5c to 5a. Quaternary Research, 66, 167-175.

Spötl, C., Starnberger, R. & Barrett, S., 2014, The Quaternary of Baumkirchen (central Inn Valley, Tyrol) and its surroundings., in: Kerschner, H., Krainer, K. & Spötl, C., From the foreland to the Central Alps: Field trips to selected sites of Quaternary research in the Tyrolean and Bavarian Alps., Excursion guide of the field trips of the DEUQUA Congress in Innsbruck, Austria, 24–29 September 2014., 68-80.

Unterwurzacher, D., Obojes, U., Hofer, R. & Mirwald, P., 2010, Petrophysical properties of selected Quaternary building stones in western Austria. In: Prikryl, R. & Török, A.; Natural Stone Resources for Monuments., Geological Society of London, Special Publication, 333, 143-152.

Vola, G., Fiora, L. & Alciati, L., 2009, Stones used in Bergamo architecture., Studia Universitatis Babe-Bolyai, Geologia, 2009, Special Issue, MAEGS – 16, 137-139.

©Ruth Siddall 2015

Catch a falling star: the strange story of the Tocopilla Meteorite

Think ‘meteorites’ and think ‘art’ and the average human mind will probably conjure up the garish and probably fantastical cover of a sci-fi novel depicting colliding worlds in shades of pink and blue or alternatively an ‘artist’s impression’ of the late heavy bombardment or the precise moment of the Chicxulub impact. Think again.

Tate Britain is hosting a major retrospective of German artist Sigmar Polke’s (1941-2010) work, which is due to end this Sunday (8^th February 2015; Herbreich et al., 2014). If you’re in London you should go and see it. Polke’s work spans decades and genres which at first present no surprises; 1960s Pop Art, for the 1970s, magic mushrooms and sex. And then you walk into rooms with a startling explosion of creativity influenced by Polke’s use of materials from the 1980s onwards. Here we find canvases glittering with mica, a large painting depicting a lump of gold ‘Goldklumpen’ (1982) painted with poisonous pigments orpiment and realgar (arsenic sulphides) and green copper arsenite pigments (Schweinfurt Green). Polke also experimented with even more dangerous substances such as uranium based pigments (i.e. Uranium [Pink], 1992); His intention here was to render his art more ‘harmful’.

Polke continuously experimented with materials. He became interested in ancient and traditional painting materials, even producing his own Tyrian purple extracted from seashells (with minor success) to create the painting Purpur (1986). He was also fascinated with modern pigments with interesting optical effects, predominantly composed of synthetic micas coated with thin layers of metal oxides. These included iridescent and metallic car paints and ‘Magic Purple’; the latter having the effect of appearing purple from one angle and then golden from another – this effect could be optimised by burnishing the painted surface, producing rather beautiful paintings including the triptych Negative Value I, II and III (1982).

Polke was fully cogniscent of the fact that many of his paintings would decay and would be beyond the help of the most skilled of painting conservators. He knew that his materials were difficult and would change over time. For example, the orange arsenic sulphide pigment realgar on Goldklumpen has altered to a yellow shade, indistinguishable from the orpiment. However that is not necessarily always going to be the case. One of his ‘pigments’ has remained unchanged for over 4 billion years, however this fact would not help any future forensic analyst of his paintings identify it.

In 1988 Polke produced a series of five works entitled ‘The spirits that lend strength are invisible I-V’. Inspired by the ancient land and native peoples of America (Garrels, 2010), these works used a variety of materials and media, including powdered nickel, silver leaf and silver oxides, artificial resins and even Neolithic stone tools (in V, left). Perhaps most striking in terms of materiality is ‘The spirits that lend strength are invisible II’ which is a scatter of powdered meteorite dust dispersed onto artificial resin. This painting, currently on tour with the exhibition Alibis: Sigmar Polke 1963-2010 belongs to The Doris and Donald Fisher Collection.

Polke used 1 kg of powdered Tocopilla meteorite to create this painting (right) which is somehow reminiscent of Palaeolithic cave art. Luckily Polke’s use of this material is well documented, otherwise for someone like me, who, perhaps a century from now, will be characterising the pigments used in late 20^th Century art, this would come as an unexpected and unrecognisable surprise. It is an extra-terrestrial encounter with unfamiliar materials not found on Earth, except when delivered my meteorites.

The Tocopilla meteorite is one of many associated with the same bolide that was found in 1875 in the Antofagasta region of northern Chile. It exploded in the air before impact and 266 kg have subsequently been collected from a large area of which Tocopilla is just one locality. The find is now known collectively as North Chile (Grady, 2000) and the various and many fragments found have been shown to be chemically identical. Chemically, this is an iron meteorite, probably representing the core of a small planet that was smashed to bits during the early history of the Solar System. More specifically it belongs to a class of meteorites known as hexahedrites and predominantly composed of the iron-nickel alloy kamacite. These are iron-rich meteorites with only approximately 5-6% nickel present. It also contains 3400 parts per billion of the element iridium, which puts in in a class of hexahedrite meteorites known as IIAB (see Morgan et al., 1995).

Just what might a 22^nd Century pigment analyst expect to find in this painting? Axon & Nasir (1982) analysed a sample of the 2.5 kg Tocopilla mass owned by the British Museum (now the Natural History Museum). They found that the main mineral kamacite formed large crystals composed of 94.51% iron, 5.05% nickel with a trace of cobalt (0.4%). Enclosed within these crystals were lath- or needle-shaped inclusions of another mineral called schreibersite, an iron-nickel phosphide, (Fe, Ni)₃P. Schriebersite is brittle and the needle-shaped crystals form planar features known as ‘rhabdites’ along which the meteorite broke up into its many fragments. Nolze et al. (2006) found tiny inclusions of the chrome-nickel mineral carlsbergite in the schreibersite crystals. Also present are small, massive crystals composed of lamellae of extra-terrestrial sulphide minerals troilite (FeS) and daubréelite (FeCr₂S₄). None of these minerals would be encountered in terrestrial rocks. Iron does not occur native (even in alloys) naturally on our wet, oxygen-rich planet and although the iron sulphide pyrite (FeS₂) is common on Earth, troilite is unknown from terrestrial sources.

Left, needles of schreibersite, included in kamacite, radiate out from an iron sulphide-rich grain. This image is adapted from Axon & Nasir (1982). The field of view is about 1 mm across.

Meteoritic material is in the majority ancient. With the exception of rare meteorites know to have been derived from either the Moon or Mars, the vast majority of meteorites solidified in the primordial Solar System. Morgan et al. (1995) used the rhenium-osmium geochronometer to calculate the age of the Tocopilla meteorite and others of similar composition. They found that it is the age of the earliest material known in our Solar System, over four and a half billion years old (4.5 Ga)., over a million years older than the oldest known minerals on Earth. Most meteorites sit in museums or museum archives or are bought and sold by collectors, but at least a part of the Tocopilla meteorite will endure in a most unexpected way on the walls of art galleries. Many casual observers would not know that ‘The spirits that lend strength are invisible II’ is constructed from a material older than our planet. Polke would have known this and therefore his painting’s resonance with deep time, almost beyond imagination. I think this would have satisfied him. Polke created an unique painting with a unique material legacy and one that takes materiality in art to new extremes.

Links & References

You can see some of Polke’s work on the MoMA website here http://www.contemporaryartdaily.com/2014/07/sigmar-polke-at-moma/

Tate http://www.tate.org.uk/whats-on/tate-modern/exhibition/alibis-sigmar-polke-1963-2010

Image: The spirits that lend strength are invisible II http://superficiecontextual.blogspot.co.uk/2009_07_01_archive.html

Image: The spirits that lend strength are invisible V http://shop.tate.org.uk/alibis-sigmar-polke-19632010/polke-the-spirits-that-lend-strength-are-invisible-v-custom-prints/invt/sigpol006

Axon, H. J. & Nasir, M. J., 1982, A microprobe study of Ni-Co distribution about a schreibersite body in the Tocopilla mass of the North Chile hexahedrite [BM 1931,13]., Mineralogical Magazine, 45, 283-284.

Garrells, G., 2010, http://www.sfmoma.org/explore/multimedia/audio/103

Grady, M. M., 2000, Catalogue of Meteorites: 5^th Edition., Cambridge University Press. p. 371.

Halbreich, K., Godfrey, M., Tattersall, L. & Schaefer, M. (Eds.), 2014, Alibis: Sigmar Polke 1963-2010., Tate Publishing, London., 317 pp.

Morgan, J. W., Horan, M. F., Walker, R. J. & Grossman, J. N., 1995, Rhenium-osmium concentration and isotope systematics in group IIAB iron meteorites., Geochimica et Cosmochimica Acta., 59 (11), 2331-2344.

©Ruth Siddall 2015