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 transition from the quiet creaking as my thumbs press slowly against the cork easing it out of the bottle neck, to the cork flying across the room to hit the ceiling is almost instantaneous and explosive. After this ritual build up of excitement I can raise a glass to you all and sip the bottled jollity as the bursting bubbles of CO₂ tickle my nose. This seems like a good place to start a blog for Christmas.

The Whin Sill was once a liquid too. Not chilled but hot, very hot. At over a 1000^oC and with over 215km³ of liquid there would have been enough heat to cook all of the UK’s turkey, duck and goose  dinner’s and nut roasts too, and still have a great deal left to spare. All of this liquid cooled down to make the dense crystaline rock, whose obdurate resistance to the ice of repeated glaciations, made the iconic crags of the central section of Hadrian’s Wall. Pagioclase feldspar, pyroxene, iron-titanium oxide and interstitial alkali-feldspar and quartz interlock to make the rock we see now. There is however just a little bit more to the Whin magma (as well as many other igneous liquids) than the elements that make these strongly interlaced crystals.

Out on the rubbly top surface of the Whin Sill at Harkess Rocks, with a beautful view across the bay to Bamburgh Castle, a rare feature can be found.  It even gets a mention in the Geologcial Conservation Review which documents the most special geological features to be found in the UK. Set in shallow holes within the Whin Sill are small pockets up to 0.5m across of what look like ropy lava (or pahoehoe lava as the Hawaiians and geologists would have it).  These pockets are formed from volatiles coming out of solution in the magma. The ropy lava forms inside the gas pocket (or vesicle) where the magma is chilled sufficiently to form a viscous toffee-like skin which is wrinkled by the still molten magma just beneath, flowing and distorting the skin above. This extraordinary phenomena is not only rare, but gives an insight into the direction of flow of the magma and how it was intruded. Gas only comes out of solution when the confining pressure drops. This equivalent of the cork being pulled only happens near to the surface otherwise the confining pressure from the weight of rocks above keeps the volatiles in solution. This volatile release is also rapid and can be explosive. Some of the most dangerous volcanoes in the world have their extreme explosivity caused by the high volatile content of their magmas.    

In the case of the Whin Sill degassing is gentler. There is no evidence for the magma reaching the surface and erupting, suggesting that the pressure relase mechanism is underground fracturing as the sill pushes into and interleaves the Carboniferous sedimentary rocks it is intruding.

Whilst this is all very fascinating, this was not the main purpose of this blog, I was heading for something rather more visually appealing, with the pizazz needed for the winter solstice. The vesicles do however provide the starting point for this. Nature abhors a vacuum, and when there is heat and water it can do something stylish with holes in igneous rocks. In the Whin Sill we can see the beginnings of this, with some of the vesicles, partially filled with crystline calcite and quartz in cream and white colours. Where the residual heat of cooling magma meets ground water, the water is convected through the rock. Hot water is surprisingly good at dissolving things so that this convection cell offers a great way to dissolve and then precipitate minerals. The vesicles provide handy locations in which these precipitating minerals can make a home. This mechanism may seem prosaic, but the products are works of art.

Silica in various forms is the principle medium for this art work. Silica in its simplest form, silicon dioxide, is not only the most prevelant but is also brilliant in it’s achievements in the form of agates. These semi-precious gemstones are my favourite for the fabulous range of patterns and colours they own, and their durability.

Their formation is enigmatic. The components of agate are pure silica (quartz) and chalcedony which is a mixture of quartz and moganite its polymorph, which has the same chemistry but a different crystal structure. Agates are composed of alternating bands of chalcedony and pure quartz, with colours added by trace elements (iron, manganese, copper and others). The crystals form as fine radial fibres, but exactly why they are able to form such fine bands with such intense changes in colour is not clear. 

It may be the Liesegang depletion-diffusion phenomena described in the blog “Balls and Bands” may be at play. The depletion-diffusion controls the precipitation of repeated quartz then chalcedony into fine bands, and in turn controls the way that trace elements are incorporated. Regardless the results are beautiful.

Sometimes the agates will finish with an additional flourish, as the centre of the agate is filled with crystaline quartz. Occasionally these are hollow so that the interior of the agate is lined with quartz crystals, bringing some extra sparkle to the ensemble.

Quartz and chalcedony are not the only artists in the silica movement. There is another group of silicates called zeolites which are micro-porous, and commonly used as catalysts and absorbants. Zeolites are another mineral that likes to make its home in vesicles. When I was working on the Isle of Mull, some of the vesicles in the Palaeogene lava flow were filled with different types of zeolite. These were often in the form of very fine needles of Natrolite one of the dozens of different types of zeolite. The pictures for this section of this blog have been generously sent to me by Bernardo Cesare, a legend in his own field for beautiful art images made using a polarising microscope. These images show basalts which have vesicles in them, and the vesicles in turn contain zeolites. Most of these basalts are from the Val Lagarina at the northern tip of Lake Garda and are of Middle Eocene age. The last of the four images is from the Predazzo area of the Dolomites and is Triassic in age. When viewed in thin section with a microscope using cross-polarised light the difference in light paths through the mineral structure (anisotropy) generates these fabulous colours. This allows for mineral types to be identified, as well as showing up the amazing textures the minerals form and highlighting the exotic beauty of these rocks. In these pictures you can see the circular zeolite filled vesicles surrounded by the basalt. Most of them show radiating needle like textures as the zeolites have grown into the vesicle, with the 4^th image showing agate like banding. In the 3^rd image and in the core of the 4^th image the crystals form a patchwork suggesting that crystalisation has been initiated across the whole vesicle and not just at its edge. In the basalt of the 4^th image, the dark lined round crystals of olivine and rectilnear laths of plagioclase set in the groundmass of finer crystals add to this fabulous display.

There is so much more to discover here and I have provided some links below to do just that. However, that glass of bubbly is waiting. Seasons greeting to you all, and I look forward to seeing you out along the Wall in the spring time.

Cheers! @Northumbrianman

Resources and attributions

Bernardo Cesare

Some of the images in this blog come from this exploration of the microscopic world of the Dolomites. The Invisible Dolomites:

Professor Cesare has two other websites one on his microscope work and the other where you can buy his art work:

…you can also follow him on Twitter @micROCKScopica 

Agate Resources

Books. There are many out there, but this one published by the Natural History Museum is a beautifully presented introduction. The second book looks like an update and I have a copy in the post. Both books available on Abe Books at a good price. 

Agates. MacPherson, H. G. Published by Natural History Museum Publications (1992)

Agates: Treasures of the Earth. John Cromartie, Peter Tandy, Brian Jackson, Roger Pabian. Published by Natural History Museum Publications (2006)

Online. Again there is much out there.

An interesting article in Geology In about the Types of Agates with good images:

A blog post on the National Museum of Scotland website, “What are Agates” – they also have a fantastic collection.

These are commercial organisations selling agates:

Agate Lady.  Agates Anonymous

Attributions

All images by the author other than:

Figure 4:

1. Agate Lady
2. Photo by Avegaon on Flickr and in Types of Agates article in Geology In
3. Recepimal108 IG in Amazing Geology Facebook group

Figure 5:

1. Crazy lace agate: By zygzee
2. Agate Lady

Figure 7: Natrolite: Nasik District, Maharashtra, India Size : (11x9x7cm) By Didier Descouens – Own work, CC BY-SA 4.0, 

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