Take a sharp-eyed walk along the beach at Cocklawburn, just south of Berwick-upon-Tweed, or on the beach just down from St Mary’s parish church on Lindisfarne and with a bit of luck you may find a small disc rather like a tiny petrified polo mint. If you are particularly lucky it will be a well-preserved specimen. A closer look will reveal that the hole in the middle of the “polo-mint” is not circular, but a minutely detailed pentagon. These small discs which bear a resemblance to rosary beads are known locally as St Cuthbert’s beads. The saint must have been particularly heavy handed with his rosary prayers given the numbers of these intriguing objects to be found. They can be discovered not only near to Lindisfarne but also occur commonly in the numerous limestones along the Northumberland coast and north into Fife. They may also be spotted inland throughout the limestones to be found in the middle Carboniferous succession in the Tyne valley and south into the Pennines. A close inspection of the Great Limestone by volunteers on this year’s only geo-walk to Haltwhistle Burn revealed several small discs of this sort.

These beautiful objects are just one part of a particular fossilised animal called a crinoid. Crinoids have a venerable history, over 300,000 times longer than Bede might have supposed, with the first recorded fossils from the Ordovician Period some 480 million years ago. Their close relations in the animal world are more familiar to us and reveal something of the nature of the crinoidal lifestyle.   

Jack Matthison’s Bank is one of my favourite places to be on Lindisfarne, with the enormous sweep of the bay running right up by the massive dune system that reaches north to Cocklawburn with Berwick-upon-Tweed not far beyond. This is a place I have often come across the fragile creamy-white shells of the aptly named sea-potatoes (albeit they can be found on many sandy beaches around the country). Sea potatoes (Echinocardium cordatum) are one of several types of sea urchin found in Northumberland. Looking closely on the top of the animal will reveal a pin-pricked tracery with five-fold symmetry. Similarly, when rock pooling, if you are lucky enough to find a starfish (or even a brittle star) these too usually have a set of five arms. The five-fold symmetry of the starfish, brittle stars, sea urchins and St Cuthbert’s beads is a family brand (carried by most but not all) which gives a clue to their membership of the Echinoderm family. 

The sea urchins, starfish and brittle-stars are predators, wandering the ocean floor to hunt for their invertebrate prey. They get around using hundreds of tube feet covering their apex, akin to the Luggage in the Discworld novels of Terry Pratchett. Rather than muscles or the sinister magic of the luggage, their little feet are operated by a hydro-vascular system with water being pumped in and out of the tube-feet to make them work. These echinoderms also have a “bite” to them. In the case of sea-urchins they have a set of five modified plates which are operated with muscles as teeth to give a powerful bite. This whole structure has become known as Aristotle’s Lantern in consequence of a mistranslation of Aristotle’s description of these animals in his “History of Animals”. Starfish on the other hand have the knack of externalising their stomachs so that they can digest their prey in situ. This gives them the advantage of being able to eat prey larger than their mouths.

Then there are the crinoids. They took a turn down an evolutionary path where they ended up as the couch-potatoes of the echinoderm family. Rather than hunt, some crinoids have developed a skeleton with root like structures at their base (to hold them in the sediment) and a long flexible column made of lots of little discs like a mini Greek column. The animal itself lives like Simon Stylites within a swelling at the top of the column. This swelling, the cyst, has a mouth pointing upwards and is surrounded by arm like structures and it just waits for the food to be delivered by the currents above the seabed. In this upside-down echinoid, the tube feet are to be found in the arm like structures where they are used to help collect the passing food.

To be fair to these creatures, most modern crinoids are only sessile for a part of their lifecycle. They are free swimming as larvae, then as juveniles they go through the couch phase before becoming free-swimming once more as adults.  Their elegant form also belies their lifestyle, their arms can form intricate, feather like structures and the whole animal has a plant-like form. This elegance is reflected in their names, with one group of free-swimming crinoids known as feather stars and the sessile forms commonly referred to as sea-lilies. The name Crinoid derives from this sessile form, coming from the Greek meaning “lily-like”.   

The Carboniferous Period is known for the large number of crinoids preserved as fossils. Rocks laid down in a marine environment are almost invariably limestones and this is where crinoids are often found. The rocks of the middle part of the Carboniferous which underly Hadrian’s Wall between Brampton and Heddon-on-the-Wall have frequent limestone layers in them. Limestones are resistant to weathering and commonly form low lying crags. They are also the raw material for making lime and consequently have been quarried extensively. This means limestones are more commonly exposed and easier to find than many rock types in the Hadrian’s Wall landscape. When you come across a limestone, it’s worth looking for fossils, especially crinoids, an example of which is seen in this month’s mystery rock. Happy fossil-hunting!

@Northumbrianman

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This month’s post from our Community Geologist, Dr Ian Kille discusses ice-cream, sandstone and Mystery Rock 3 which featured in last month’s WallCAP newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.

One of the crucial questions which has come up over the last few weeks as I’ve reached down through the stratigraphy of my freezer towards the Palaeozoic, is why old ice-cream is crunchy?

Knowing why my ice-cream is not in tip top condition is high up my agenda. I was therefore delighted to find that there might be a relationship between this and what happens to some sandstones as they get older (the Mystery Rock 3 for last month’s newsletter included).

On the Northumberland Coast and along the Hadrian’s Wall corridor, there are many examples of sedimentary rocks which have either bands or balls in them which cross-cut sedimentary layering. That these features cross-cut the sedimentary layering tells us they grew after the sediments forming the rock were laid down. The balls are more formally known as concretions (or nodules) and the banding is more formally called, err, banding. Both the banding and the concretion formation are associated with ground water permeating the sediments. The mechanisms for this are enigmatic – scientific code for, “its complex and we’re not really exactly sure how this happened”.

There are some interesting options though, which explain at least in part what is going on.

There are two gentlemen, chemists working at the turn of the twentieth century, whose names are associated with these phenomena: Raphael Liesegang and Wilhelm Ostwald, Liesegang for his rings and Ostwald for his ripening.

Liesegang seems to have stumbled on his rings by accident. He was involved in some of the early work on photography, using the light sensitive chemical silver nitrate. He also used potassium chromate which will react with a solution of silver nitrate to produce the insoluble precipitate silver chromate.  One day, Liesegang had a petri dish of silver nitrate dissolved in a gel and inadvertently dropped a crystal of potassium chromate onto it. To his surprise, as the potassium dissolved and diffused through the gel, rather than producing a growing circle of precipitating silver chromate, it produced a series of concentric rings.

Liesegang was excited by this and made lots of observations which he published. He also got in touch with a colleague, Wilhelm Ostwald who worked out an explanation for this phenomenon. Ostwald proposed that as the potassium chromate diffuses outwards through the gel, nearby silver nitrate heads inwards towards it, anxious to react with it. This creates a band outside the reacting ring depleted in silver nitrate. As the potassium chromate continues to diffuse through the gel, it must cross this depleted band before it can find more silver nitrate to react with and create more precipitating silver chromate. The same process then happens all over again with a ring of precipitate forming, along with a band depleted in silver nitrate outside of it. The Liesegang rings are unsurprisingly often referred to as a diffusion depletion phenomenon.

One of the features of the rings is that they become progressively further apart as you move away from the source. This is a defining feature of the rings and the rate at which they get further apart can be mathematically calculated. Looking at Mystery Rock 3 (figure 2) you can see that there are a series of concentric rings which may be explained in this way.

What about the concretions though? Ostwald we have already seen was interested in how chemical reactions work. One of the areas he studied was the way in which larger crystals tend to grow at the expense of smaller crystals. The atoms at the surface of a crystal are less stable than those in the middle of a crystal, as those at the edge are bonded to fewer atoms. This means that larger crystals, which have a lower proportion of atoms at their surface, are more stable. As atoms move in and out of solution, they will preferentially head towards the larger crystals which will grow and replace the smaller crystals over time. This process, now known as Ostwald ripening, can be seen in many different situations, from the way that copper sulphate crystals grow from solution, to the way that igneous phenocrysts form in magmas and to the way that foam bubbles tend to become larger over time. Not least, this process happens in ice-cream, where the larger ice-crystals will grow at the expense of smaller ones over time. Ice-cream technology is devoted to finding ways of ensuring that this happens as slowly as possible.

In sedimentary rocks a common expression of Ostwald Ripening is in the formation of concretions. These are often formed of calcium carbonate (see figure 6) but may also be formed of iron oxides and other minerals. Concretions vary in size from a few millimetres to many metres across and vary in shape from almost perfect spheres to some curious intersecting sphere and doughnut shapes (figure 7).

It would be satisfying if it were possible to end here offering the rings and ripening as a complete explanation of what we see in the rocks. However, the banding seen in some rocks and the relationship between bands and concretions is more complex than can be explained just by these phenomena.  Even in Mystery Rock 3 it is not clear how the multiple series of bands are formed.

Then there are rocks, known as wonder-stones; a picture of one of these from Birling Carrs near Alnmouth can be seen below (figure 8). Recent research has shown that the darker bands of iron, cementing the sandstone, may have an organic origin. Microbes use the energy from oxidation and concentrate the resulting iron oxide into the bands where they live. The wonder-stones may therefore have formed by a combination of organic activity (microbes) and the self-organising processes described by Liesegang and Ostwald. Part of the reason for the research interest in these easily observed patterns in these sandstones is that they can indicate the presence of life and so used to help understand if there really is life on Mars.

Perhaps this blog post should have started with the question, “what is the relationship between crunchy ice-cream and life on Mars?”. Regardless, the patterns produced by diagenetic iron and other minerals are beautiful in their own right and are potentially useful indicators of the provenance of these stones, both in an archaeological and a geological sense.

@Northumbrianman

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