If you had asked me three years ago about open water swimming, I would probably have given you a luke-warm response. Having been persuaded by my partner Rachael to give it a go I can conclusively say that luke-warm is not a suitable response to open water swimming. The North Sea is categorically Baltic in the winter months and swimming at these times comes with a generous cake-slice of insanity, albeit mixed in with a vibrant sense of being alive. One of our favoured spots for swimming is in Whitley Bay from the Panama Swimming Club clubhouse.

The beach here is sandy, but with a variable sized collection of stones strewn across the surface and a reef of beautifully striped sandstone pointing from the Spanish City out towards St Mary’s lighthouse. As an unreconstructed geologist I can’t help scrutinizing the pebbles on the beach as I trepidate towards the freezing water. For the most part these are a collection of water-worn sandstones (some with beautiful stripey patterns) and limestones not infrequently spotted with the remains of the communal coral Siphonodendron. On occasion there are small brown, flat slabs of hard sandy limestone to be found, filled with cream-coloured smiles. These are the cross-section of fossil bivalves preserved in multitudes within these stones. It is one of these stones that features as mystery rock number 23 for the WallCAP Newsletter.

A visit to the beach at Whitley Bay in early March 2021 would not have been the time to go for a swim. A storm the previous weekend had almost completely stripped the beach of its sand, leaving a wasteland of boulders and pebbles. It also exposed more layers of bedrock than the regularly visible reefs of stripy sandstone. Amongst these newly exposed layers was the source of the fossil smiles, a layer no more than a few inches thick just above the stripy sandstones and containing thousands of these fossils.

This layer of rock is the Low Main Mussel Band, which lies just above the Low Main coal seam after which it is named. The mussels, a type of bivalve, are of three species, Carbonicola (which used to be called Anthracosia), Anthrocanaia and Naidites. As usual with fossils, their presence helps unravel what was happening when these layers of rock were being laid down.  In addition, this type of shelly layer is found scattered through geography and time in the sequence of late Carboniferous rocks (the Pennine Coal Measures Formation) of Tyneside.  So, what are these fossils and what do they tell us?

In the very dim and distant past, I used to belong to the 1st Central scout group which met in a hut adjacent to the underground sidings behind Morden Station in south London. In this pack I rose to the dizzy heights of being a sixer, a role, which brought variable results. At a scout camp competition where we were to show off our camping and woodsman skills, our “six” successfully dug a magnificent latrine. This was discovered inadvertently during the night by one of the judges who fell into it. What we learned was that the middle of a footpath is not a good place for a lat-pit (and we didn’t do well in the competition). On a brighter and better occasion, we camped at a Longridge on the River-Thames near Marlow, a campsite which mercifully had flushing toilets. One of the main purposes of this camp was to learn how to canoe. Learning how to canoe, it seems, involves a great deal of falling out of canoes and a great deal of close up familiarity with the river. One of the things I discovered through multiple visits into the murk of the Thames water was that my feet on the riverbed squelched into layers of silty mud. To my surprise, within this mud lived some magnificent bivalves several inches long and with beautiful glossy green shells. These were swan mussels (Anodonta cygnea). I recollect my surprise at finding bivalves in a river rather than in what I thought of as their natural habitat, which is in the sea. It is the case that the modern landscape of sea-shells is dominated by the phylum of Mollusca either in the form of bivalves (cockles, mussels, clams and the like) or as gastropods (winkles, whelks, limpets and the like). There was, however, a different story to be told of the Carboniferous bivalves, which finally returns us to the question I asked a paragraph back.

The Carboniferous bivalves to be found in the Low Main Mussel Band are, like the River Thames’ swan mussels an indicator of fresh or brackish water. This marks a significant change from the swamp-land conditions in which the coal of the Main Coal seam was forming and may relate to a rise in sea-level. These mussel bands, of which there are many in the Pennine Coal Measures Formation, are surprisingly continuous over many kilometres. This makes them a useful signpost, not only of the geological conditions in which they were laid down, but also of where you are in the rock sequence and in time. A coal miner coming across the Low Main Mussel Band would know that they were immediately above the Low Main Coal seam. Each of the other mussel bands scattered through the Pennine Coal Measures Formation has different types of bivalves (and other fossils) in them. This was not just down to the mussel beds forming in different environments (which favoured one or other type of creature) but also a result of evolutionary change. In the time interval between the formation of different mussel bands, some creatures had become extinct and new creatures had evolved.

So what happens if we make the comparison between a river or estuarine environment and a marine environment in the Carboniferous as we did for the modern day? It reveals that the dominant species of sea-shell in the Carboniferous were not molluscs of any stripe, but another group of creatures with two shells, the brachiopods. The Brachiopoda form a phylum in their own right completely separate from the Mollusca. The name brachiopod comes from the ancient Greek and means arm-foot. The “foot” is the most obvious part of a live brachiopod, with a muscular column called a pedicle, which extends from the bottom (ventral or pedicle) valve of the animal. Brachiopods use this foot to secure themselves to the sea floor. The “arm” is found on the inside of the shell as part of the upper (ventral or brachial) valve. It is not an arm in any mammalian sense, rather a support structure for a part of the animals feeding apparatus called a lophophore, a whiskery, horseshoe shaped structure. These brachial structures, which support the lophophore vary in their complexity. For example, in the brachiopod family of Spirifers (common in the Carboniferous) they form elegant spiral structures which give the Spirifers their name.

Brachiopods are also known as lamp shells. This comes from the similarity in shape between the brachiopod family of Terebratulids and Roman oil lamps.  The name Terebratulid comes from the Latin for hole borer (terebra) – the reason for the derivation is not clear, but maybe because the small circular hole in the shell, through which the pedicle would have emerged and

looks like it has been drilled, is so clearly seen in these animals. The Terebratulids are also one of a handful of brachiopod orders that survived the largest mass extinction event, known as the Great Dying, at the end of the Permian period 252 million years ago. This event also marked the fulcrum around which the balance of ecological power

between the molluscan bivalves and the brachiopods hinged. The brachiopods are remarkable in their persistence through geological time. There is one species, Lingula, which may be found in brackish estuarine sediments (hostile to many organisms) and there is a remarkably similar (although evolutionary distinct) burrowing brachiopods which is preserved in rocks of Cambrian age (circa 550Ma). However, the rich list of brachiopod species and the range of ecological niches they filled during the Palaeozoic era (including the Carboniferous period), has now been usurped and almost entirely filled by molluscan bivalves. Next time I visit Whitley Bay for a bracing dip, it will be the limpets and cockles (molluscs all) I will share the swim with, whilst I contemplate the brackish waters of the Low Main Mussel Band and their ancient cousins.  

Attributions

Swan Mussel: Anodonta cygnea. Jakob Bergengren, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Mussels: Derrick Mercer, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons

Cockles: Cardium indicum Lamarck, 1819 – hians cockle. James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons

Spirifer: Two specimens of Spirifer striatus (named as Spirifera striata in the original). From Plate XXXI of Monograph of British Fossil Brachiopoda Volume 4 Part 3.

Roman lamp: Ancient Roman oil lamp in D. Diogo de Sousa Museum, Braga, Portugal.. Joseolgon, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Terebratulid: Terebratulid brachiopod from the Campanian (Upper Cretaceous) of southwestern France. wilson44691, CC0, via Wikimedia Commons

Lingula: Lingula anatina shell found in the Mediterranean Sea, in a laboratory of practices of the Faculty of Sciences of the University of Corunna. I, Drow male, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Lingulella: Lingulella lingulaeformis Mickwitz, Leptembolon lingulaeformis (Mickwitz, 1896). Estonian Museum of Natural History, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

 

The post A Foot in the Slime appeared first on WallCAP.

My nose was almost touching the shiny grey-white undulating slab of stone that was slick with moisture. My head was similarly close to the slab of stone above me, though I couldn’t see what it looked like. My elbows and forearms were inching me forward combined with whatever purchase I could get from my toes. In front of me my right hand held a carbide lamp with its silent elegant feather of acetylene flame guiding me forwards. This was me in my first year of doctoral research having decided I wanted something interesting and exciting to do. The Imperial College Caving Club provided this, heading out to the Brecon Beacons in an old ford car for a weekend of underground adrenaline. I can think of no other occasion when I was so closely surrounded by lime in different forms, though a caving trip to the Mendips as part of a management training exercise and trips to the disused chalk quarries above the Silent Pool in Surrey (coming home caked in chalk), come a close second.

The carbide lamp was an elegant testament to the many industrial uses that lime is a part of. The calcium carbide which was one part of the fuel which powered the light is a product of calcium oxide and coke heated to high temperatures in an electric arc furnace. The calcium oxide in turn is a product of limestone which has been heated in a lime kiln to over 800C, breaking down the calcium carbonate (which the limestone is composed of) into calcium oxide and CO₂. The other part of the lamp’s fuel is water, which drips onto the powdered calcium carbide at the base of the lamp, reacting with it to produce acetylene. The acetylene burns with a small, pointed flame of bright light, perfect for caving. The lamp is literally in its element in a limestone cave and is still used by some cavers as their preferred source of light.

Carbide lamps have also been used by many generations of miners albeit the naked flame was a hazard in coal mines. This led to its replacement in this setting by the Davy lamp, with its gauze cover reducing the risk of igniting the coal gas.

The chemistry of calcium carbonate has not just been exploited by industrial man but also by a remarkable range of creatures throughout the history of life on earth. One of the earliest pieces of evidence for life are laminae of calcium carbonate in mounds about a metre across which resemble structures, called stromatolites. These are the product of microbial mats produced by cyanobacteria and growing in shallow water where they trap fine lime-rich sediment. The oldest of these are some 3.5 billion years old in the early Archaean eon and discovered in sandstone from western Australia.

Stromatolites reached the peak of their diversity in the late Proterozoic eon though they can still be seen today. These are not only the earliest life forms but also show the way that organisms started use lime as a way of protecting themselves and in doing so beginning to form reef like structures.  

The journey from here to the astonishing diversity of life on a modern coral reef is a long one with periods of progressive ecological diversification and colonisation, but with some major setbacks too. By the Devonian period, some of the largest reefs that ever occurred on earth had developed.

These reefs were dominated by stromatoporid sponges, along with tabulate and rugose corals, a markedly different mix of animals from our modern reefs. Not only were there different ratios of reef-forming families, with sponges more common than corals, but also these Devonian sponge and coral orders are all now extinct.

Towards the close of the Devonian period, there was an extinction event, the fourth largest of the Phanerozoic (the current geological eon). This extinction event, thought to be caused by climatic cooling – a precursor of the glacial conditions which fluctuated throughout much of the Carboniferous period – wiped out the stromatoporoid corals and severely impacted both the tabulate and rugose coral families.  

It wasn’t until the Triassic period, with the cold global climate of the Carboniferous long in the past that reef building on the scale of the Devonian returned. This also post-dated the “great dying” at the end of the Permian Period, the largest mass extinction the earth has seen, when life on earth was nearly wiped out. This extinction event finally finished off the tabulate and rugose corals and, in the Triassic, these old-order corals were replaced by the modern sclerectinian corals.  These corals had a lighter structure than the rugose and tabulate corals and were made of aragonite (a polymorph of calcium carbonate) rather than calcite (the other polymorph of calcium carbonate). Sclerectinian corals are also know for their symbiosis with the photosynthesising algae, the Zooxanthellae. This partnership gives the corals an advantage in fixing lime from atmospheric CO₂, and it may be that this collaboration gave these corals an edge in colonising and diversifying throughout the world.

It remains an article of debate as to whether the older rugose and tabulate families of corals had this ability. This advantage to growth had its downsides though. When it came to mass extinction events, the zooxanthellate corals were at a disadvantage, constrained as they were to shallow water and more vulnerable to changes in temperature and ocean acidification. This was borne out in the mass extinction at the end of the Cretaceous period where the asteroid impact at Chicxulub in Mexico, not only wiped out the dinosaurs but many other families. The corals that survived this event tended to be deeper water, non-zooxanthellate organisms. You can’t hold a good idea down though, and following this catastrophe, the symbiotic relationship was re-established independently in several coral families.

The diversification of species of reef building animals to fill a wider range of ecological niches and larger areas of the sea-bed is not just controlled by evolution and natural disaster. There is wider rhythm to fluctuations in diversity and range related to changes in climate. One of the consequences of the change from a glaciated world to a hotter global climate with no polar ice caps is that higher sea levels flood continental shelves to create more extended shallow marine environments. As a result, hotter worlds such as the Silurian to mid-Devonian or the Triassic to Cretaceous had much larger habitats in which corals and other reef forming organisms could thrive. It may be that during the Carboniferous period, much of which sported polar ice caps, the consequent low sea levels and restricted shallow sea environments contributed to the slower development of reef habitats.

The Carboniferous period, however, was not without its reefs and was categorically not without limestones which formed from the remains of calcifying animals. Without these limestones the Romans would not have had a source for making lime with which to help stick the Wall together. The remains of reef like structures made up of rugose corals can be seen, for example, on the Northumberland coast. It is during this period that the thick shelled productid brachiopods become common a well as the elegant plant-like crinoids. These three animals are the dominant calcifying creatures of the Carboniferous, and their fossils are the ones you are most likely to find if you happen to come across a limestone outcrop when exploring the Wall.

I am certain that there would have been some of these fossils under my nose in that cave in the Brecon Beacons, however my mind was otherwise occupied at that moment. The steady drip of the water into the calcium carbide of my lamp making the acetylene to light my way forward seemed an appropriate reminder of the slow work of water in the cave I was crawling though. Limestone is soluble in water, particularly when acidified by dissolving CO₂. As the limestone dissolves it produces some beautiful landforms, including the limestone pavement which was Mystery Rock number 22 for the WallCAP newsletter last month. It also completes the cycle started by the corals and crinoids and brachiopods fixing the calcium and carbonate ions into their shells, and now being dissolved and returned via the river to the sea.

It is also a reminder, as levels of anthropogenic CO₂ continue to rise, that rising global sea temperatures and an acidifying ocean make our extraordinarily diverse, complex and beautiful reefs vulnerable. What will be the role of reefs be in the future, as we move into what is now commonly referred to as the Anthropocene?

Attributions

Stromatolite: Stage : Paleoarchean from 3 600 to 3 200 Ma (million years ago). Locality: Strelley Pool Chert (SPC) (Pilbara Craton) – Western Australia. By Didier Descouens – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=15944367

Modern stromatolite: Stromatolites growing in Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia. Paul Harrison, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons

Stromatoporid Sponge: Jstuby at English Wikipedia, Public domain, via Wikimedia Commons.  Stromatoporoids in the Silurian-Devonian Keyser Formation, Old Eldorado Quarry, in Blair County, Pennsylvania.

Corals through time: https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/corals/

Modern corals: By Toby Hudson – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11137678. A variety of corals form an outcrop on Flynn Reef, part of the Great Barrier Reef near Cairns, Queensland, Australia.

Zooxanthellae: By Todd C. LaJeunesse – flickr, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=79980176 Symbiodinium, colloquially called “zooxanthellae”. Corals contain dense populations of round micro-algae commonly referred to as zooxanthellae. A typical coral will have one to several million symbiont cells in an area of tissue the size of a thumbnail.

Healthy reef: Holobionics, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons. Lodestone Reef Valentines Day 2016, Green Chromis on Coral.

Bleached reef: Bleached coral reef (Acropora) (Island of Réunion). The original uploader was Elapied at French Wikipedia., CC BY-SA 2.0 FR <https://creativecommons.org/licenses/by-sa/2.0/fr/deed.en>, via Wikimedia Commons

The post A Reef in Time appeared first on WallCAP.

This month’s blog from our Community Geologist, Dr Ian Kille, discusses geological families.

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Christmas plans are in place and, despite the coronavirus demonstrating once more that evolution is very real, I will cautiously be heading down to stay with my parents. They still live in the same house I was brought up in. So for Christmas I will be returning to the chalk, with a layer of London Clay over the top.  My brother lives not too far away from my parents, still on the chalk but with gravels from the River Thames covering the London Clay and the chalk deeper beneath his feet. My family’s next generation down are more scattered. My youngest son lives above Triassic sandstones of the Chester Formation, with the back of his house mantled in glacial till and the front in river-gravels from the River Irwell. My elder son is above conglomerates from the Helsby Formation also in the Triassic Period. His elevated position means that there is little except a thin layer of organic matter between him and the rock. In my immediate family it seems that I am the only one who has chosen to live on old volcanic rock, as I live above Devonian andesites from the Cheviot volcano, mantled with a fork-breaking layer of fluvio-glacial cobbles.

Many years ago, when I had just started venturing into the intersection between geology and archaeology, I gave a talk on geology and archaeology in Berwick. At the end I was asked a singularly penetrating question about how much I thought that the geology of a landscape influenced the development of culture. The questioner was a certain Lindsay Allason-Jones. At this point I was blissfully unaware of her illustrious career in the world of Roman antiquity, and to this day wonder at just how inadequate my attempt at answering this question was. It is, however, a question that has stuck in my mind, and it returned to me when writing the introduction to this piece about families and geology. I wondered whether the chosen locations for my family might reflect something of our differing cultural values, with the builder in the family closest to solid rock and our family’s geologist closest to volcanic rock (the chosen specialism of my research). This could be a great game to play over Christmas, it’s easy to find your underlying geology by using the BGS geology app: http://mapapps.bgs.ac.uk/geologyofbritain/home.html  Though, thinking about it, it is probably only for those who would want to intersperse their Christmas games with watching back episodes of Star Trek and the Big Bang Theory.

There are many other great family games that can be played by geologists, such as Mine-a-Million, home-made Rock Dominoes, Mappa Mundi with added plate tectonics and an all-time classic, the geologists’ version of rock-paper-scissors. Another sort of game was brought to mind when I was writing a presentation about the history and pre-history of the stones used in Hadrian’s Wall. The presentation was put together from the point of view of a grain of quartz, a mineral which is almost indestructible, despite travelling great distances and being knocked about a great deal. It seemed to me that this was similar in character to Tyrian Lannister in the Wall-related series Game of Thrones, which sees him survive intact through to the end. This led me in turn to observing that quartz has its own family or rather a set of families. So begins the Game of Stones; though to be honest it’s more like a geological version of ancestry than a game.

Quartz is made of silicon and oxygen bonded into an interlocking framework of tetrahedra. Silicon, like its close elemental relative Carbon, is remarkable in its ability to combine with other elements to produce a vast array of compounds. Carbon is the master of this in the biological world, but silicon has the edge in the mineral world.  The silica tetrahedra – a silicon atom surrounded by 4 oxygen atoms and looking similar to one of the jacks from the old fashioned game of Jacks – is the building block which is used to make the dynasty of silicate minerals. The different ways the tetrahedra combine create distinct structures which define the many different silicate families. The tetrahedra may be isolated (Nesosilicates) and sometimes combine in pairs (Sorosilicates). They also make rings (Cyclosilicates) single and double chains (Inosilicates) and sheets (Phyllosilicates). They also make three-dimensional frameworks (Tectosilicates). Within each of these families, these familial structures combine with numerous other elements to create huge numbers of different silicate minerals. I feel certain that with careful use of coloured paper, glue and infinite patience that an absolutely fabulous set of these silicate minerals could be reconstructed using paper chains, to make the most original, brightest and best Christmas decorations ever devised.

This month’s Mystery Rock (number 21) for the Hadrian’s Wall Archaeology project is one of the silicate dynasties. Feldspars along with quartz are part of the tectosilicate family. These alkali-feldspar crystals are in a piece of Shap Granite. Shap is a distinctive granite, with a matrix of coarse crystals of various silicates along with these much larger feldspar megacrysts. There are dozens of different types of feldspar defined by the relative amounts of sodium, potassium and calcium bonded within their three-dimensional structure. More importantly, many of these feldspars are beautiful. For example, labradorite, a calcium-rich feldspar, glows with iridescent hues of deep blue, green and silver. Its cousin Orthoclase, a potassium-rich feldspar, glows with the milky iridescence of the moon and unsurprisingly is known as moonstone.

The other families can claim their beauties too. Quartz, another, tectosilicate, is one of my favourites, forming hexagonal prismatic crystals which interlock in fabulous modernist forms, and glint with a brightness that reflects how hard they are. With names like clear, milky, smoky, citrine, rose, amethyst they give hints of their qualities. Quartz also mixes with other minerals to produce jasper, sunstone, moss-agate and another of my favourites, tiger’s-eye, all of which will be familiar as semi-precious stones.

The cyclosilicates are particularly exotic. Tourmaline is one of these ring-structured minerals. Commonly it is lustrous black and known as schorl, but sometimes it comes in bi-coloured crystals, lollipop-like in pink and green. Then there is Beryl, though this Beryl doesn’t have a stripey top, is not a peril and is indifferent to the smell of paint (cf. Katherine Mansfield). However, it not only has a ring structure but ends up literally on a ring in the form of emerald and aquamarine.

The neosilicates with their isolated tetrahedra also make their appearance on rings. Precious olivine, known as peridot is a mossy-green colour. Garnets, most commonly in a mulled-wine purple, are found in less expensive jewellery. Zircon, harder than quartz and more lustrous than diamond, comes in many colours. When I visited Ratnapura, the gem capital of Sri Lanka, many years ago, zircon was the fake gem of choice to pass off as its more expensive cousin emerald and unrelated grandee, ruby.

This is just a taster of the assorted bling which the silicate syndicate has to offer. I’m sure there is a market out there for a genealogy equivalent website for silicates – findmysilicate.com, mysilicon.com or silicatry.com – as there is still so much more to explore. However, for now, I think that all of the Christmas bases have been covered with family and games and many a brightly coloured things. Time to settle into a repeat of the Christmas Repair Shop and contemplate the ancient lava flows beneath me and pour myself another glass of that mulled wine.

A very Happy Christmas to you all and all good wishes for a fulfilling New Year exploring your landscapes wherever you are.  

The post Game of Stones appeared first on WallCAP.

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On a recent trip to Flamborough Head I found myself both at home and somewhat disconcerted. Flamborough Head is made of chalk, very white and with all the fossils you would expect to find from the Cretaceous Period. Chalk is what I was brought up on. The water that came out of our taps came from an artesian well and had been filtered through chalk. Below the London Clay that made our garden impossibly claggy to dig, there lay a massive saucer of chalk cupping the clays, sands and river terraces which underpinned the London Basin. Less than 10 miles away southwards the chalk emerges as the North Downs, forming the massive pediment of a worn away arch – the other half of which is the South Downs. The Downs, North and South, were my escape – Box Hill and Beachy Head, Hope Gap and the Silent Pool all beautiful places to be as well as offering up a rich array of fossils. Inoceramus liabatus, Spondylus spinosus, Echinocorys scutata, Micraster cortestudinarium and Sporadoscinia aclyonoides were all happily collected and labelled as a teenage buff.

Why then, the Flamborough fluster? Part of it was being in the north when I’m used to going south of London to see chalk. But that shouldn’t really be a surprise as it is well known to me that the chalk outcrops that forms the anticline of the Weald (framed by the North and South Downs) is mirrored by a syncline which emerges the other side of London. After all I lived in St Albans for many years and regularly visit a musical retreat perched above the Hughenden Valley in the Chilterns and know of their chalkiness. These chalklands are part of a great arc stretching from Salisbury through the Chilterns curving north to just east of the Wash and reaching the sea between Kelling and Hunstanton in Norfolk. The chalklands then continue north of the Wash from Skegness right the way up the coast to Flamborough where they once again rise to make cliff scenery nearly as flamboyant as that around Beachy Head.

There were two other things at Flamborough which shifted my pre-conceived notions of chalk and were probably the cause of my disconcert.

Not long after we arrived in our lodging in Bempton, my partner Rachael asked in surprise, if the house opposite was built of chalk, commenting that it didn’t seem to be a plan to build a house out of chalk as it is so soft. I muttered about Chalk Rock and Melbourn Rock, bands of harder chalk within the southern, generally soft chalk, which are used as building materials. I was however surprised to see how many of the older buildings were made of chalk – it seems that northern chalk is, in general, harder than southern chalk.

I was also surprised that the only other common traditional building material was brick. I remember from the chalklands in each of Surrey and Sussex, Hertfordshire and Hampshire, Suffolk and Norfolk, that many of the older buildings frequently featured flints.

Flints make for beautiful looking houses. They are, however, a challenge for the builders as flint is so hard, fiercely sharp when broken and comes in irregular lumps. The history of flint as a building material goes back at least to Roman times. Whilst it is not an easy building material to use, it is extremely durable and freely available (at least in the southern chalk). Having observed and considered the sandstones used to build Hadrian’s Wall, it was fascinating to return to St Albans in February of last year, moments before the pandemic kicked in. Walking from Waitrose (an essential in St Albans) south into Verulam Park towards the Abbey, the path is bordered on your left by a Roman Wall. It is this which featured as Mystery Rock number 20 for the Hadrian’s Wall Community Archaeology Project. It is constructed from a mixture of brick and flint.

The Romans recognized that to make stable walls out of flint it is more effective to mix it with layers of rectilinear material. As with Hadrian’s Wall the Romans once again show their ability to choose materials and build with them to produce remarkably durable structures. Continuing down the hill to the River Ver and then up to the Abbey also allowed for an exploration of the complex built history of the Abbey. This provided another lovely, but very different, example of stone reuse. Significant portions of the transept and the northern wall of the nave are made of reused Roman material. Other walls in both the transept and the southern nave feature newer knapped flint used in much larger faces supported by stone quoins. This reuse matches the way that Roman stone is reused in medieval churches in the Tyne valley except that the materials being reused are very different.

Back in Flamborough we headed out to the coast and found that the absence of flint in the buildings is reflected in an absence of flint in the cliffs. We started our exploration on the beach at South Landing, just south of Flamborough and headed towards Danes Dyke and Bridlington to the west. Here it also dawned on me there was another major difference. The wave-washed cobbles between the chalk boulders by the cliffs consisted of a curious mix of Carboniferous, Jurassic and even older rocks, none of which occur locally. Looking up at the cliffs the reason is clear, with a 10m plus band of boulder clay topping the cliffs. These wave-washed cobbles had hitched a lift on a glacier and were dumped within the boulder clay as the ice departed (some 12 thousand years ago), and now the ice-transported contents are eroding into the North Sea. This ice-sheet didn’t reach Sussex, so the cliff tops at Beachy Head just have a thin layer of chalky soil, and the beach cobbles are exclusively flint (with the odd bit of brick where a house has fallen in). Further exploration of the coast at Flamborough Head and Thornwick Bay, revealed that there were some flints to be found, but different in character from those further south. These flints were grey and not clearly distinguishable from the chalk. At Birling Gap in Sussex the layers of flint nodules band the cliff in dark black, contrasting with the brilliant white of the cliffs. Individual nodules when broken are a beautiful shiny translucent black and have a rind of a porous mixture of flint and chalk.

Back home, reflecting on the trip to Flamborough, I was reminded once again of my university tutor, Professor Harold Reading, and one of his sayings; a geologist is only as good as the number of rocks they have seen. I’m certainly seeing chalk differently thanks to our visit to Flamborough.  Maybe it’s time now to plan trips to the Cap Blanc Nez and then to the Jasmund National Park in Germany and Møns Klint in Denmark, chalklands all. There are also chalk deposits in north America, Australia and Egypt. Apparently, the Champagne region of France is underlain by chalk too – chalk and cheese (and a little wine), now there’s a thought!

@Northumbrianman

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It was like walking in snow, except that we were surrounded by leafy trees in bright summer sunshine and 20 degrees of heat. The creamy white crystals that were reflecting so much light were not the pretty hexagons of ice-crystals but the rhombohedra of calcite. The drift of material we were intent on searching stuck out like a huge tongue between the wooded hillsides, pointing towards the wonderfully named village of Snailbeach in Shropshire. Lichens and moss, mares-tails, stonecrop, cinquefoil, willowherb and even small trees were starting to colonise the bottom edges of the mound where more water collected. Despite this and the surrounding borderland idyll the mine waste maintained a zone of lifeless desolation even 20 years after the mine’s closure in 1955. For my geo-pal Kevin and I, though, it was a world of excitement. We were here on a cycling holiday, or rather a collecting trip which involved some cycling and camping. Along with hunting for trilobites in the county’s famous Silurian and Ordovician outcrops, we were exploring the lead and zinc mining industry of Shropshire to see what beautiful specimens we could find in the disused mine-tips. We were also teenagers, away from home and relishing the freedom that this gave.

In amongst these old tips, it was still possible to find some fine specimens. Along with the brilliant crystals of quartz and the milky crystals of calcite, there were deep brown crystals of sphalerite. Sphalerite (zinc sulphide) is one of the main ores of zinc and valuable in its own right, as zinc is used as an alloy with lead to make solder and with copper to make brass. The name sphalerite and its alternative, blende or zinc blende both refer to sphalerite’s similarities with another ore mineral, galena. To Kevin and I galena was the prize, it felt fabulously dense in the hand and formed beautiful octagonal and cubic crystals which glittered in seductive greys. Galena (lead sulphide) is the principal ore of lead and for early miners sphalerite was a distraction from this valuable lead ore. The name sphalerite come from the Greek word Sphaleros meaning treacherous and Blende come from the German word Blenden to deceive. Clearly the word-coining miners were not happy about Zinc Sulphide.

It is curious that the Romans, who knew about zinc and its use in making brass, don’t appear to have mined the copious amount of zinc available in deposits in Britain. They did however know about the lead and mined large quantities of it exporting it all over the Roman Empire. It seems likely that lead (along with iron, tin, copper and gold) is what drew the Romans to Britain and encouraged them to make it part of the Roman empire. Lead mining started soon after the invasion of Britain with evidence of workings at Charterhouse in the Mendips as early as 49AD. This became a highly organised and productive set of open cast workings which by 70AD had overtaken the lead-mines of Iberia as the principal source of lead for the Empire. There is also a good fictional account of this mine in Lindsey Davis’ book “The Silver Pigs” which gives a real sense of the conditions in which mining took place along with some enjoyable speculations on the political and economic intrigue that may have surrounded such a valuable resource.

Lead is not only very dense but is malleable, durable and waterproof. The Romans understood this, and it was used principally to make pipes to carry water and for lining aqueducts, and also to make pewter plates and coinage. Galena itself (along with stibnite, an ore of antimony) was crushed to a fine paste to make khol which was used as an eye cosmetic. Some Romans also understood that lead was not good for their health. Vetruvius wrote in the 1^st century BCE: “Water conducted through earthen pipes is more wholesome than that through lead; indeed that conveyed in lead must be injurious… This may be verified by observing the workers in lead, who are of a pallid colour; for in casting lead, the fumes from it fixing on the different members, and daily burning them, destroy the vigour of the blood; water should therefore on no account be conducted in leaden pipes if we are desirous that it should be wholesome.” (VIII.6.10-11)

Pliny on the other hand, writing in the 1^st Century CE, noted that “Fresh hogs’ lard, applied as a pessary, imparts nutriment to the infant in the womb, and prevents abortion. Mixed with white lead or litharge, it restores scars to their natural colour” (Book 28: Remedies).  He also advocates that lead could be used as a liniment, or as an ingredient in plasters for ulcers and the eyes, among other health applications. As an aside this underscores that Pliny’s Natural History along with useful direct observation has a great deal that is unverified anecdote. For example: “Sneezing, provoked by a feather, relieves heaviness in the head; it is said too, that to touch the nostrils of a mule with the lips, will arrest sneezing and hiccup” (Book 28: Remedies, Chapter 15), and “For patients affected with melancholy, calves’ dung, boiled in wine, is a very useful remedy.” (Book 28: Remedies, Chapter 67). Taking this into consideration, his common appellation as a scientist does the meaning of science no favours. On the other hand, his detailed records of methods used in mining, extracting and refining metals are valuable resources in understanding what the Romans were making and how they did it.

Pliny gives detailed descriptions of the way that lead is extracted from its ore, galena. From this and other sources we know that lead, despite its value, was a by-product. Galena commonly contains a small fraction of silver, up to 0.5%, making silver the ore’s most valuable component. In writing this article I found a derivation for galena as “From Latin galena – “dross from smelting lead”” – it is a nice idea which I have been unable to verify.

For the Romans silver was not only used for high status ornaments and tableware but was the fundamental currency for the empire. We know from Pliny that the process used to extract the silver was cupellation. Lead melts at a relatively low temperature (327^oC ) whereas silver melts at 960^oC. When heated to circa 1000^oC lead will oxidise to form litharge (PbO) which can be absorbed into a porous calcareous material such as bone ash, leaving the now molten silver fraction. Typically, the bone ash was formed into a truncated cone shaped vessel; a cupel. The litharge having been absorbed would leave drops of silver at the base of the cupel. Later, the lead would be recovered from the cupel by re-smelting and the lead would be formed into an ingot, or pig, each weighing approximately 69kg.

The Romans’ quest for silver and lead in Britain was not confined to the Mendips. Roman lead mining has been identified in Wharfedale and not far from Hadrian’s Wall at Alston Moor. A Roman pig of lead was also discovered not far from the mine at Snailbeach making it another likely location for Roman lead/silver mining and extraction. All of this I neither knew nor cared about on my teenage visit, I was simply happy to have found some beautiful mineral specimens. Mystery rock number 19 for the Hadrian’s Wall Archaeology Project is one of the specimens that came back from that trip… and finding it would have been a good reason to head, once more, to the Stiperstones Inn.

Attributions and References

References:

Roman Lead Working in Britain. R F Tylecote (1964). British Journal for the History of Science vol.2 no.5.

Attributions:

Shropshire view: from http://www.shropshiresgreatoutdoors.co.uk/site/snailbeach-mine/

Galena: Galena with some golden colored pyrite (3.5 × 2.5 × 2.0 cm) from Huanzala mine, Huallanca, Bolognesi, Ancash, Peru. Ivar Leidus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

Lead pipe: By &lt;a href=&quot;https://en.wikipedia.org/wiki/User:Solipsist&quot; class=&quot;extiw&quot; title=&quot;en:User:Solipsist&quot;&gt;Andrew Dunn&lt;/a&gt; – &lt;span class=&quot;int-own-work&quot;&gt;Self-photographed&lt;/span&gt;, <a href=”https://creativecommons.org/licenses/by-sa/2.0″ title=”Creative Commons Attribution-Share Alike 2.0″>CC BY-SA 2.0</a>, <a href=”https://commons.wikimedia.org/w/index.php?curid=606468″>Link</a>

Lead Pig: British Museum. https://www.britishmuseum.org/collection/object/H_1856-0626-1

Sciapod from Pliny’s Historia Naturalae: By Michel Wolgemut, Wilhelm Pleydenwurff (Text: Hartmann Schedel) – http://www.beloit.edu/~nurember/book/images/Miscellaneous/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=490581

@Northumbrianman

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The first time I went to Wales we took a ferry from Aust on the east and English bank of the Severn to Beachly on the west and Welsh side. The next time I went to Wales we drove across the Severn Bridge with barely a glance at Aust a way down below us. I wish I had known then that the nearby cliffs at Aust form a famous exposure of the Rhaetic bone bed – I would definitely have been on the case with Mum and Dad. It wasn’t until several years later, in my teens, through exploring beds of the same age at Blue Anchor (not far from Minehead in Somerset and the home of my grandparents) and a trip to Aust with my old pal and co-geology nerd Kevin that I go to know and see what it was about. Till that point fossils had been about invertebrates – devil’s toenails (gryphaea), crinoids, corals, ammonites and trilobites. Vertebrates were a whole new layer of excitement and in these bone beds there was plenty to get excited about. Fish scales and ichthyosaur teeth and occasionally a piece of plesiosaur vertebra. These fossils have a different look and feel to them, preserved in phosphate minerals rather than carbonates and oxides and they had dark shiny surfaces, particularly the fish scales. Beneath their surface the bones had a distinctive spotty texture revealing a vascular structure where the blood vessels feeding bone growth would have been.

Another few years later whilst on a holiday in Minehead as an older teenager, I borrowed my Nan’s Moulton style bike, with its tiny fat wheels. I headed off for several days exploring the coast, blissfully in control of my own plans, staying in the youth hostel at Quantoxhead. On the last day of my planned trip whilst nosing around in the layered limestone-and-shale cliffs at Kilve Beach, I spotted a ring of material about 6 inches across which had this spotty, vascular texture in it. It was a large piece of bone and in a shape that suggested the snout of a skull. I spent the whole of the rest of the day excavating chasing the bone back into the cliff. The more I dug the larger it got! Late in the afternoon I realized I wasn’t going to get back to Minehead at the agreed time, so found a phone box and called to let my folks know that I was ok, but that I had found a dinosaur and needed a bit more time and a lift. It was later identified as likely belonging to an Ichthyosaur. This chunk of skull is now somewhere in the Oxford Museum of Science, still awaiting preparation even after 40 years in residence. Having recently indulged myself by purchasing an air scribe (the fossil preparation tool of choice) I am now considering the possibility of repatriating the skull to do the work on it myself.

This fossil was from the Lias, the oldest formation of the Jurassic period and at about 200 million years old is approximately 100-140 million years younger than the fossils to be found in the Carboniferous of the Northumberland Coast and the central and eastern part of Hadrian’s Wall. In the Carboniferous Period ammonites and dinosaurs, for which the Jurassic is famed, were in the distant future. The ammonites’ ancestors were there in the shape of goniates. These were spiraled like ammonites, but with much simpler suture lines marking out the division between their floatation chambers as has been explored in my earlier blog “Suckered”.  As for the vertebrates, the Carboniferous precursors of the dinosaurs had made it onto land in the shape of the very first amphibians: this is another story for another blog entry. The other vertebrates that were there were the fishes.

When I started writing this blog, I had intended to map out the evolution of fish. However, it became clear that this would involve writing a book; so here is a quick leap through some of the highlights.

Hag fishes are strange creatures with loose skin and an ability to produce prodigious amounts of slime which combine to make them very difficult to eat. The hagfish and the parasitic lampreys belong to the same class of fish, the Agnatha or jawless fish, which were the first to evolve. The jawless fish arrived early in the evolutionary history of multicellular creatures, appearing at the outset of the Cambrian Period at about 530 million years ago. These were simpler creatures than their distant modern relatives, probably filter feeders and with a notochord (like that found in an embryo) a rod like precursor to the development of vertebrae.

Fish with true vertebrae evolved later in the Cambrian period. Still part of the jawless fish family, they included armoured fish, (Ostracoderms) and Conodonts. The latter are small eel-like creatures previously know only for their teeth-like structures and highly valued as zone-fossils. It is from the Ostracoderms that an evolutionary line can be traced to the first jawed fishes (the epic Placoderms) and then to the explosion of fish types that marked the end of the Silurian and the beginning of the Devonian Period.

These new fish groups included the now extinct spiny sharks (Aconthodii), the cartilaginous fish (Chondrichthyes) which includes sharks and rays, and the bony fishes (Osteichthyes). The bony fish further evolved at this time into the ray-finned fishes (Actinopterygii) and the lobe-finned fishes (Sarcopterygii). We are familiar with the lobe finned fishes in the shape of Coelacanths which were thought to have become extinct in the Cretaceous but in 1938 were discovered living in the Indian Ocean. It is from the lobe-finned fishes that the first tetrapods, the early amphibians, evolved. The ray-finned fishes are even more familiar in the shape of haddock, mackerel, piranhas and goldfish amongst many others.

Having leapt through the evolution of fish, let’s go back to the beautiful foreshore at Cocklawburn Beach just south of Berwick-upon-Tweed, where rocks of the mid-Carboniferous Period are exposed. Not long after I first moved to this part of the world, I was exploring the limestone skerrs at Cocklawburn and was surprised to come across the very same shiny phosphatic bony material with the spotty vascular interior that I had noticed so many years ago at Kilve Beach. This is mystery rock number 18 for the Hadrian’s Wall Community Archaeology Project. It had something of the shape of a shoulder blade about it but about 12 inches across. I also noticed that nearby there were several square cuts in the rock where small slabs of rock had been removed with a saw.  Enquiry revealed that a (then) teenager, Max (who happened to be the son of Mick Manning and Britta Granstrom, then our neighbours and well known as a children’’ author, and illustrator and artist respectively) had made a discovery. The piece of bone was the last piece of this discovery, unnoticed or left behind for some reason, the rest having been collected by the Hancock Museum with permission from the Northumberland Coast AONB partnership. The find has been identified as the remains of a Rhizodont fish, a now extinct member of the lobe-finned class of fish. The remaining bone gives a clue to the size of this creature. As part of what would evolve into a shoulder this implies that this fish was many metres long. The Rhizodonts were predatory fish with vicious teeth so this would not have been a good fish to be swimming with.

The surge of excitement when I found the Icthyosaur skull remains vivid in my mind and finding the bone at Cocklawburn reminded me of that time. I imagine Max may have experienced that excitement too and I know many geologists who retain this feeling, and whilst some of them are paid-up professional geologists many of them are amateurs. The world of geological discovery is there for all, and for the young this maybe the beginning of a lifelong fascination.

Attributions

Aust Ferry: By Adrian Pingstone – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4095255

Goniatite: CeCILL, https://commons.wikimedia.org/w/index.php?curid=64362

Fish evolution diagram: By Epipelagic – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24336974

Hagfish from: https://phys.org/news/2011-03-hagfish-skin.html

Lamprey mouth from:https://www.washington.edu/news/2009/07/23/ancient-sea-lamprey-dramatically-transforms-its-genome/

@Northumbrianman

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Swimming pools are not really the best place to appreciate an earthquake. I was enjoying the benefits of Iceland’s geothermal heat in an open-air swimming pool on the only occasion I have been somewhere when an earthquake took place in that location, and I only discovered it had happened when I was told about it afterwards. Iceland, situated on a constructive plate margin, is tectonically active and earthquakes are not uncommon. They do, however, tend to be rather lower down on the Richter scale, in a way that might make it possible to appreciate an earthquake rather than being terrified by it.  Constructive plate margins are in tension and are located where the crust is already thin (particularly in oceanic crust) or where the crust is in the process of being thinned. In consequence it is possible for the tension to be released more incrementally (unlike compression where long periods of oh-so-quiet suddenly become oh-so-loud and very destructive). Not to say that incremental tension release doesn’t cause problems. The Icelandic attempts at harnessing geothermal energy from the very high heat flows encountered on the island have been severely hampered by the water circulation pipes being bent and broken by earth movement. Expensive and costly but not deadly. The earthquakes can however be a signal of something more deadly about to happen. An increase in earthquake activity is a sign that magma is on the move and therefore a helpful predictor of imminent volcanic eruption.

In contrast to Iceland’s constructive margin earthquake experience, that of a destructive plate margin is altogether more devastating. Destructive plate margins are so named because a slab of oceanic crust and lithospheric mantle (solid) are diving back down into the aesthenospheric mantle (fluid and deeper), dragging its trailing plate behind it. It could equally well be called a destructive plate margin for the naked violence of its volcanic eruptions and its earthquakes. Mount Tambora and Krakatoa in Indonesia, Mount Pele in Martinique, Nevada del Ruiz in Columbia and Santorini in Greece top the list for human casualties, and all are found on destructive plate margins. The same is true for the most devastating earthquakes. Tangshan and Sichaun in China, Haiti, Peru, Kashmir and the Indian Ocean earthquake centered just of the coast of Sumatra top the location-list of recent deadly earthquakes. With fatalities in the hundreds-of-thousands, earthquakes are more deadly than volcanoes. The cost is devastating in so many ways. Not only in terms of in human lives and injury, but also the misery caused by destruction of homes and businesses and infrastructure resulting in loss of income, famine and illness. There are several physical factors that dictate the scale of damage. The amount of energy released by the earthquake (its magnitude) is the principal measure of how much direct damage an earthquake may cause through shaking and ground rupture. Landslides, lahars (mud flows), flooding and liquefaction of soils and unconsolidated sediments are also direct consequences of earthquakes. Additionally, earthquakes may cause tsunamis. I vividly recollect the news and images from Indonesia and Japan on Boxing Day of 2004 showing the astonishing devastation caused by this earthquake-induced tsunami and the consequent tragedies that followed.

It may seem from this that Earthquakes are literally the great levellers, indiscriminate in who and what gets destroyed. As individuals and societies, we do however have knowledge and experience both of where earthquakes are likely to happen and what can be done to mitigate the consequences when an earthquake hits. There are two places I have visited which illustrate this well. The first is San Francisco. I’m not a great lover of any city, but San Francisco is one of my favourites, with its vibrant mix of peoples and its beautiful setting, wrapped around by San Francisco Bay and the Pacific Ocean and its many hills providing beautiful vistas. It is however a city in peril, located as it is on the San Andreas fault, a 1200km-long active transform-fault. The southwestern part of San Francisco is moving north, and the northeastern part is moving south: intermittently and violently. After the devastating 1906 earthquake in San Francisco, much work was done to make buildings and infrastructure more resilient to earthquakes, with good effect. These words from the bible come to mind:

“Therefore whoever hears these sayings of Mine, and does them, I will liken him to a wise man who built his house on the rock: and the rain descended, the floods came, and the winds blew and beat on that house; and it did not fall, for it was founded on the rock.

“But everyone who hears these sayings of Mine, and does not do them, will be like a foolish man who built his house on the sand: and the rain descended, the floods came, and the winds blew and beat on that house; and it fell. And great was its fall.” (Matthew 7:24-27)

However, wise government and judicious engineering is not enough. The central, hilly part of San Francisco is indeed founded on rock, a good place to build not only for rain, but also for earthquakes which have less effect on rock. In contrast the estuarine deposits which fringe the bay area and recent alluvial deposits (sand!) are susceptible to liquification during an earthquake, indeed causing much greater damage. The choices to build and live in these areas are not so much about wisdom versus foolishness as economic necessity. The suburbs built on these soft deposits tend to be the poorest areas in the city. If your house is more likely to be destroyed by an earthquake in an area where earthquakes are inevitable, insurance becomes more expensive and the value of your real estate drops.

In contrast to San Francisco (wealthy albeit with a significant underclass within one of the wealthiest nations) Kathmandu is poor. Nepal is the second poorest nation in Asia. This fact can be attributed to political instability and corruption. I stayed in the Kathmandu valley when I visited Nepal in 2011. We stayed in a lodging house delightfully located in the centre of Bhaktapur. This wonderfully preserved ancient city with its temples and courtyards made of wood, stone, brick and metal is designated as a World Heritage Site.  4 years later, in April 2015, a massive earthquake struck, with an epicentre near to Gorkha to the NE of Kathmandu. The effect of this earthquake was amplified in Kathmandu, Patan and Bhaktapur as they are built on a basin of lake sediments. As distressing as the images were of these ancient temples reduced to rubble it only remains symbolic of what the Nepali people went through, with nearly 9000 killed and nearly 22,000 injured. Nabraj, who had been my guide when in Pokhara, had his family home near to Gorkha. Mercifully his family all survived, but sadly his village was all but wiped out. With no insurance and no government help, rebuilding was a major challenge, with his income from tourism also reduced to rubble.

With so many confounding factors it is apparent that earthquakes scoring highest on the Richter Scale don’t necessarily bring the biggest death toll. Population density, poverty and politics have a strong influence too. These are large and seemingly intractable problems, however as a geologist I would suggest that one of the ways to help with this is to understand how earthquakes work so that engineering and socio-political solutions may be put in place.

This takes us, via a few thousand miles and through 340 million years of history to mystery rock number 17 for the Hadrian’s Wall Community Archaeology Project. It is an example of fault gouge, the material which is produced when rocks fracture and move past each other. This fault is part of a fault complex to be found on the Northumberland coast at Howick and is likely to have formed as this Carboniferous sedimentary basin developed. Faults are responsible for earthquakes. On the Howick fault, there has been a total of about 40m of movement, compared to 3m of movement on the fault plane which created the Nepal earthquake. The fault plane at Howick is likely to be much smaller than that in Nepal, which means that each metre of movement on the Howick fault would have generated less energy than each metre on the Nepal fault.  Additionally, this total of 40m would have been made through many much smaller movements spread over many millions of years. Given that this fault developed under crustal tension it would have created relatively low energy earthquakes.

The early Carboniferous period (in which these rocks formed) would have been home to many creatures including some of the earliest amphibians. I like to think that, like me, these creatures may have failed to appreciate these earthquakes as I did in my geothermal swimming pool in Iceland.

Attributions

Hekla eruption, February 2000. Photo by: Iceland monitor/Rax

Village on the coast of Sumatra: U.S. Navy photo by Photographer’s Mate 2nd Class Philip A. McDaniel, Public domain, via Wikimedia Commons

@Northumbrianman

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Pottery kilns are greedy for power, so in preparation for installing a large second-hand kiln, I had a three-phase supply installed. This required cutting through a concrete path to make a trench and, when the cable had been laid, remaking the concrete path. It was the first big concreting project I had ever done, so when I had finished it, I neatly wrote the names of the people who had helped make it, including my older son Craig, along with the date.  It maybe that in a few hundred million years a geologist, of a highly evolved rat species, will unearth this concrete patch and use it as evidence in a paper on the ritual behaviour of primitive hominids in relationship to concrete structures.

If that writing is preserved over the millennia, it will have become a fossil. A particular sort of fossil.

When I hear the word fossil, the immediate images that come to mind are of ammonites, crinoids, shepherd’s crowns, trilobites and corals. Dinosaurs come to mind too, massive bones, lines of vertebrae, teeth and horns. These are the remains of the actual creature, usually their hard parts and often with their skeletons or shells replaced by a different mineral. The soft parts of an animal are rarely preserved, requiring an exceptional combination of speed of burial and environmental chemistry. Without this the remains would be predated, physically broken up, decayed, oxidized or dissolved. When this intersection of favorable circumstance does happen, it results in fossils which are amazing and tell us so much more about the animals – for example the recent discoveries in China of feathers on a number of dinosaur fossils.

For most ancient creatures, the information we have about their cells, muscles, nerves, brains, hearing, seeing and so on can only be inferred from the hard parts to which they attach or within which they are contained.  We can measure skull cavity sizes to infer brain size, bone size and density along with muscle attachment points to work on musculature. We can also see the gas chambers and siphuncle of ammonites which tell us a bit about their flotation mechanism. We can also put these ammonites into a flume to see how water-dynamic their shapes are and infer something of how well adapted they are for moving.

All of these things are fascinating and help build a picture of what these animals were like and what they were capable of.  What they don’t tell us is what they actually did. If Hamlet had looked at Yorick’s skull without knowing him well, alas he would not have been able to say anything of his infinite jest.

For ancient creatures, there is however, another type of fossil which helps us understand more of what these animals actually did. These are trace fossils, and ichnology is an important branch of paleontology which not only tells us what animals did, but also provides another set of diagnostic information which helps us understand the environment in which they are preserved.

The most obvious of trace fossils are footprints and trackways. Some of these even make an appearance in Roman remains. Just like my writing in the drying cement, there are some tiles at an undisclosed site where the paw prints of a dog can be found. Whether this is a particular dog that likes the feel of clay, or a potter’s dog that the owner wanted immortalized or whether potteries were particularly dog-rich environments is not clear. It simply tells us that dogs were around and dipping their paws where they probably weren’t welcome!

Fossil footprints and trackways are not uncommon, with dinosaur footprints making news in recent years with discoveries in the Jurassic strata on the Isle of Skye as well as in the Cretaceous rocks of the Isle of Wight. More locally, tracks discovered by Maurice Tucker, have been found in the lower Carboniferous rocks at Howick on the Northumberland coast. These proved to be from an early amphibian, probably from the Temnospondyl group and are one of the oldest amphibian footprints ever found.

Not all trace fossils are so obvious or so glamorous. Many of them are simply burrows or feeding trails and unlike the footprints, it is often hard to work out what animal made them. This is in part because many burrowing animals only consist of soft parts, so that what they did in chewing their way through soft sediment is the only record of their existence. This reminds me of a lecture we had at college from Professor Jim Kennedy on early molluscan evolution and their development to manage the relative positions of mouth and anus in their simple guts. My recollection is that Jim said something along the lines of, “much of their evolutionary effort was directed at working out how not to crap on their own heads”. This seems like a hard almost futile existence, but evolution is nothing if not a long game!

The rocks of the Carboniferous Period in Northumberland and beyond have a rich variety of trace fossils preserved within its many kilometers of deltaic and marine limestones. This month’s mystery rock, number 16 in a series, is one of them. This particular gem comes from the geologically fabulous Cocklawburn Beach just south of Berwick upon Tweed. Until very recently I had thought that these beautiful three-dimensional patterns in these siltstones glorified in the name of Eione monilforme. However, in trying to discover what sort of creature made these remarkable traces, my learned colleagues pointed me towards a paper in which they have acquired the even more remarkable name of Neoeione monilforme. It is a shame that scrabble doesn’t allow proper names! As far as the animal is concerned, I quote from Dr Katie Strang’s reply (an expert on all things Carboniferous – particularly sharks) “It was originally thought to be made by a mollusc, but has now been attributed to a deposit-feeding endobenthic (ie a lived in sediment at the lowest level in a lake or the sea) worm-like animal, that actively back-filled its burrow, but…”. As with many things geological, there is clearly still room for speculation, debate and further observation…

…and maybe this is a fitting sentiment to end this piece. My concrete scribblings have lasted all of 10 years so far and maybe some of the pots that were made in the kiln will appear, at some distant point in the future, in a Raturnine archaeological trench as a definitive marker for the late Anthropocene.

Attributions

Iguanodon footprint along the foreshore at Compton Bay.from https://ukfossils.co.uk/2016/06/17/compton-bay/

Dinosaur footprint An Corran: from https://www.nature.scot/dinosaur-sites-skye-be-given-official-protection

Amphibian footprint from Howick: in David Scarboro and Maurice Tucker: “Amphibian footprints from the mid-Carboniferous of Northumberland, England: Sedimentological context, preservation and significance” Palaeogeography Palaeoclimatology Palaeoecology 113(2):335-34

@Northumbrianman

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It may be that my memory is not quite right, but I have a strong recollection of visiting Ghiradelli Square Factory in San Francisco and being fascinated by the massive stone rollers grinding the chocolate to produce the beautifully smooth finished product. It was nearly 40 years ago, but I also have a recollection of the way that the chocolate was formed into beautiful folds against the bar at the bottom of the roller. The inexorable movement of the roller pulled the base of the viscous chocolate onwards while the bar held the top of the delicious smoothness in place. The rollers I am sure about, the ripples might just be in my imagination. A chocolate sundae and a sun-soaked view out over San Francisco Bay to Alcatraz and the Golden Gate Bridge after the event does do things to soften the memory.

It is a slightly tenuous, albeit irresistible analogy to make to describe the process which formed this month’s Mystery Rock for the Hadrian’s Wall Community Archaeology project.  Chocolate clearly isn’t magma, but both magma and chocolate are viscous liquids when heated albeit the magma is rather hotter than the chocolate. Basaltic magma is liquid at temperatures over 1000C and flows rapidly, as can be seen from the spectacular images coming from the current eruption at the Geldingadaler volcano in Iceland. What can also be seen from this eruption is the way that this magma becomes stickier as it cools from bright yellow heat to red heat to black. If the magma is continuing to move and the crust of cooling magma is kept hot enough that the surface doesn’t cool so much that it becomes brittle, then, just like the Ghiradelli chocolate, the cooling sticky lava will be pulled into ripples and braids which have a texture much like a coil of rope. As the top of the curve in the coils points along the direction that the underlying magma is flowing, ancient examples of ropy lava give useful evidence about the volcano from which it was erupted.

Mystery Rock 15 doesn’t come from a volcano though. This image was taken just north of Bamburgh Castle at Harkess Rocks at an exposure on the top of the Whin Sill. The shape and form of the ropy braids is unmistakable and the curve in the braids tells us how the magma was flowing. This all occurred under the ground, though, away from the cooling air which formed the ropy braids at Geldingadalir and in many other basaltic volcanoes. How could this have happened? The clue is in the curved outline of this small section of the Whin Sill which has this ropy texture. This whole process is contained within a gas bubble like a little world inside a very hot “snow globe”. Volatiles are a common component of magmas and the formation of gas bubbles a regular occurrence as the pressure on the magma is released as it reaches the surface. What is unusual is that the volatiles have come out of solution within the Whin Sill under the ground. This tells us that either the intrusion of the sill was close to the surface and/or there was a pressure release caused by the magma pulsing forward as it wedged its way between the layers of Carboniferous sedimentary rock. It is not surprising that this rare phenomenon is recorded as a significant point of interest within the series of Geological Conservation Reviews carried out to record the best of British geological exposures.

Just a short blog this month as there is so much fieldwork to organize. Maybe just a little time to visit the Doddington Milk Bar and enjoy one of their excellent chocolate sundaes whilst taking in a view of the Cheviots.

Attributions & Links

Chocolate Grinding: By Sanjay Acharya – Own work by uploader. Picture taken at Ghirardelli Square, San Francisco, California USA, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5178665

Alcatraz: By Centpacrr (talk) (Uploads) – Own work, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=38140500

Ropy Lava: By Tari Noelani Mattox,[1] USGS geologist[2][3] – https://web.archive.org/web/20070102035046/http://volcanoes.usgs.gov/Products/Pglossary/pahoehoe_ropy.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=700082

Link to Geldingadalir ropy lava flow:

https://www.youtube.com/watch?v=aZenXClZn4U

@Northumbrianman

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The taxi drivers at Arughat Bazar wished to proclaim their faith with slogans emblazoned on their battered cars like “Jesus Dead For You” (sic). It could also be taken as a warning of what lay ahead on the narrow winding dirt roads with vertiginous drops of hundreds of feet to rocks strewn in the powder blue Budhi Gandaki River in the gorge below. This heart-stopping drive took us to the start of a 14-day hike around Manaslu in the Nepalese Himalayas. The Himalayas are a good place to learn new perspectives and to let go of preconceptions. The sheer scale of the place provides a cold and implacable sense of one’s place in the world.

This is a story I have already told in an earlier blog (“Scratching the Surface”) but it seems to me worth re-telling to set the scene for explaining this month’s WallCAP mystery rock. One of the (many) preconceptions I brought with me to Nepal was about glaciers. From the pictures I had seen I assumed that they would be snow-strewn and that in the crevasses you would be able to see icy layers turning deeper shades of blue. In preparation for the summit of the Manaslu Circuit at the Larkye La pass I had asked my guide, Roshan, about crampons, but he had said they wouldn’t be necessary. This puzzled me at the time, but he was oh so right.  There was some snow and some ice as we reached the pass just after sunrise, but the way was mostly covered in rock debris ranging in size from boulders to sand. This was in part because the bits of the glaciers we were walking on were at relatively low level in the zone where the argument between snowfall and melting is played out. It is however also to do with ice’s phenomenal ability to break up and move rock around.

Not only were the glacier surfaces we walked on covered in rock of all grain sizes, but around the margin of the glaciers were huge ridges of sediment. These marginal moraines were more prominent as the surface of the glacier was much lower than the moraine ridges. Each year the glacial tongue will extend during the colder months as more snow, compacting to ice, flows down the mountain side. As the seasons turn and the sun fights back against the ice, melting becomes faster than the ice flow. This annual argument is usually balanced, but now it is turning in favour of the warming climate and these glaciers are clearly in retreat, leaving behind massive piles of rock debris.

Mountain glaciers are impressive, not least for their mountainous setting, but they are not on the same scale as the polar ice sheets. Where the mountain glaciers measure in the tens of kilometers, even the relatively small ice sheets of Iceland measure in the hundreds.  Flying to San Francisco many years ago the view down to Greenland was clear and cloudless and we flew for hours across unbroken ice.

When thinking about the effect of ice on our landscape in the Hadrian’s Wall corridor, it is this sort of ice that would have been the cause of many of the landscape features we now see. The margins of Iceland’s icesheets as they retreat provide a good analogue for what we see here.

Ice is a powerful tool, and this is because of two things. The first, that within the normal temperature range on our planet it passes between a solid, a liquid and a gas. The second is that when it solidifies from a liquid, it expands. This is very unusual and has consequences. The first is that it operates like a natural crowbar. Water that infiltrates rock, when it freezes, will happily break open even the hardest of rock types. The second is that it floats on top of its own liquid, water, rather than sinking. Without this feature ice-skating would never have become a thing and not only because of the skating-surface that ice forms as it floats. There is yet another feature, which is the way that ice responds to pressure. Usually, pressure will compact a solid so that it becomes more difficult to melt, however ice under pressure will tend to melt. This is what happens at the knife edge of skate-blades, where the intense pressure this causes against the ice results in the formation of a thin layer of water, lubricating the skater’s path. Without this, the steel blade would stick to the ice, and the skater’s glide would become a skater’s face-plant.

A similar principle applies to icesheets. There is a balance of pressure and temperature where the advance of the icesheet is enabled by a layer of water at its base. It is not only the ice-fractured rock which falls onto the ice which creates rock debris, but the water and ice underbelly of the glacier which grinds and flushes out the rock. We can see examples of this within the Hadrian’s Wall landscape, for example in one of the early WallCAP Mystery Rocks from near to Heavenfield. Here the surface of sandstone has been flattened and smoothed, and linear grooves have been raked into its surface, all by the action of ice.

What happens, then, when the ice melts? At the poles where ice flows out into the sea, as the ice melts, the rocks contained in the ice fall to the sea floor. These random stones are referred to as dropstones and can be seen in the arctic deep. They can also be found in the geological record and are one of the pieces of evidence used to show that there have been a few periods of the earth’s history where the entire planet has been glaciated – popularly named as Snowball Earth.

On land we can see the systematic remains of glacial action and retreat. Ground moraines and the drumlins that are created from them by the continued movement of the ice, scatter the landscape. Other features like eskers, and subglacial channels which have formed as a result of water-movement under the ice leave ridges and furrows in the landscape. As the ice retreats, the meltwater from the ice is added to the prevailing precipitation creating huge volumes of water. Sometimes this may be trapped by retreating ice to create lakes and consequent flat-lying lake deposits. This water also creates large high energy river channels which rework the glacial debris leaving behind meandering deposits, which often border contemporary rivers now much shrunk in size. Around Hadrian’s Wall this means that most of the landscape has a covering of one sort of glacial deposit or another.

Ice’s rock-moving super-power is manifested not only in the massive volumes of rock debris it creates, but also in the distance it moves this material and the incredible size of some of the boulders it drops into the landscape. One such boulder is the Loch Maben Stone which features as this month’s Mystery Rock for the WallCAP project. This huge, rounded stone is made of granite, a rock type which is foreign to the bedrock it overlies (the Triassic St Bees Sandstone). The nearest granite to the Loch Maben Stone is approximately 30km away, beyond Dumfries. Glacial erratics like this come in all sizes, but they have all wandered from their place of origin within a carpet of ice, hence the name erratic. Erratics weighing many thousands of tons have been recorded as well as pieces that have wandered for thousands of kilometers. On the Northumberland Coast It is possible to find a particularly characteristic volcanic rock, with elegant, elongated phenocrysts, which have travelled from Norway.  This means that the glacial ice not only walked 500 miles and walked 500 more but also walked on water for this stone to fall down on the Northumberland shore.

These erratics were one of the pieces of evidence that convinced Louis Agssiz, a Swiss geologist of the 19^th century, that there had been a past ice-age during which glaciers had extended all over Switzerland from the Alps and out into the plains of Europe. Working with William Buckland, the only person in Britain he could convince of his ideas, they looked for evidence of glaciation in Britain and concluded that all of Scotland and Ireland had been covered in ice.  These findings were published in 1840 in a 2-volume work “Études sur les glaciers”. This was a moment in the history of geology where pre-conceptions about past climate took a radical turn, providing a crucial piece of understanding to build our contemporary picture of climate change.

Back to more local concerns, it has been fascinating spending time at the recent Heritage at Risk dig undertaken by WallCAP at the Cambeck Crossing, the first this year with volunteers. Near to the trace of the wall, an area was uncovered containing cobbles which were probably sourced from the nearby riverbed in which there are sizable banks of water-worn cobbles. Examination of these cobbles shows them to be made of sandstones (including some distinctive poorly sorted grey sandstones which are likely to be turbidites), low grade metamorphosed sediments and granites. These are all erratics and likely to have been derived from the Ordovician and Silurian terranes and Devonian granites to the north west. I discussed the sequence of events that brought this about with the volunteers: the coming of the ice and how it levelled the landscape, leaving behind a rich variety of glacial erratics and how this material would have been re-worked by the rivers.  We also discussed how lifting the weight of the ice allowed the land surface to rise, like a super-tanker being unloaded, so that the Cambeck cut down through the glacial material and the soft sandstone underneath to create the gorge we now see. That our landscape, which seems set and permanent, can change so much in a relatively short time is remarkable and the role which climate change plays in this important. With the rate of man-made climate change exceeding that of the last ten thousand years what will be the consequent changes that we will see? It seems we have reached another moment in geological history where there is a need to revise our preconceptions and take action.

Attributions

Example of a glacial dropstone from Namibia, in rocks that date to the second Snowball Earth. The stone was likely carried and dropped by a floating ice shelf, and when it plunked into seafloor sediment below, that sediment folded around it. (Penny shown for scale.) Image by Paul Hoffman in: https://astronomy.com/news/2019/04/the-story-of-snowball-earth

@Northumbrianman

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