carboniferousfossils – Vincent Butler Heritage

I found my first fossil when I was ten years old, a gastropod embedded in a piece of Carboniferous limestone. This marked the beginning of my collecting, so it would be remiss of me, even disrespectful, if I were not to highlight this wonderfully interesting sedimentary rock. Around 340 million years ago a sub-tropical sea existed which supported a myriad of marine life including corals, molluscs and sea-lilies. Layers of calcium-rich sediment were accumulating on the seabed and as the corals and other creatures died their remains became incorporated in it. Over millions of years these sediments were transformed into layers of solid rock that we now call limestone and the shells and other hard parts of the marine organisms were preserved as fossils. It makes up around 60% of Ireland’s bedrock.

Fossil solitary coral, Carboniferous Period

Limestone can sometimes be fossiliferous, meaning rich in fossils. These provide us with valuable information about the complex prehistoric marine ecosystem. Corals are a fascinating group of small invertebrates. Each of these soft-bodied animals, known as a coral polyp, secretes calcium carbonate or lime from which it constructs a hard protective exoskeleton. Coming together in large numbers they create individual colonies and when these coalesce form coral reefs. A specimen of a solitary coral still stands upright as it had in life 340 million years ago on the bed of the Carboniferous Period sea.

Although superficially resembling bivalves, brachiopods are a separate group of shellfish. They first appeared in the Cambrian Period some 540 million years ago and at various times in the geological past were extremely common. The lower and larger of its two shells had a round opening at the posterior tip through which a fleshy pedicle or stalk protruded. The animal used this to attach itself to the seabed. Around 200 species are still found today. Perfectly preserved fossil brachiopods sometimes just pop out of the limestone when lumps are broken open, as was the case with the specimen here.

Calcite is the crystalline or pure form of calcium carbonate, the main constituent mineral of limestone. It occurs as veins within the rock, sometimes in large enough concentrations to justify commercial mining.

When rain falls it absorbs carbon dioxide gas from the air and forms a weak carbonic acid which chemically reacts with the limestone, slowly dissolving the calcium. Filtering down through the body of the rock, the water becomes saturated with this mineral. On contact with air when it seeps into an underground passageway or cave, some of the carbon dioxide is released. As a result of this, each drop of water that forms on the roof deposits minute crystals of calcite and over time a downward tapering feature known as a stalactite develops. Where drops splash consistently on the same spot of the floor a stumpy upward growing stalagmite forms.

Another mineral sometimes found in Carboniferous limestone is fluorspar. Also known by the name ‘Blue John’, it was mined at one time in the Burren, Co. Clare. It was used as a flux in the production of iron and was a main ingredient in the manufacture of hydrofluoric acid which among other things, was used for etching glass.

Piddocks are a group of marine bivalves who bore into rock. Tell-tale signs of their presence are small circular holes peppering the surface of the rock or the larger chambers that they excavate by rasping with the sharp edges of the shells. Each piddock is condemned to serving a life-sentence, incarcerated in its self-made stone-walled cell. The wrinkled rock borer or red nose (Hiatella rugosa) bores a flask-shaped chamber and these are a common find in rocks along the shore in areas of limestone.

Best,

Vincent

The compressed remains of ancient plants

When you pick up a piece of coal you hold in your hand the compressed fossilised remnants of a primeval forest. Around 300 million years ago, a time when most of what we now call Ireland lay about 10 degrees south of the equator, giant tree-ferns and clubmosses flourished in hot, humid conditions which saw the development of extensive areas of swamp. Over time, as the plants died, their remains accumulated in the prevailing waterlogged conditions and formed thick layers of semi-rotted dead vegetation or peat. This period was characterised by fluctuations in sea level which rose and fell on numerous occasions. Each rise flooded the swamps and covered the peat layers in deposits of silt and sand. When the water level once again fell, new swamps clothed in vegetation became established and so the cycle repeated itself many times over millions of years. During the passage of time, the alternating layers of silt, sand and peat were compressed and compacted into layers of shale, sandstone and coal respectively, comparable in a sense to a gigantic, multi-layered rock sandwich. This occured in the latter stages of the geological period called the Carboniferous, meaning ‘carbon-bearing’, a reference to the extensive coal deposits which were laid down at that time..

One of the main groups of plants growing in the swamp forests were the lycopods. These were giant clubmosses, some of which attained tree-size proportions. They include a species known as lepidodendron, which could reach heights of up to 38 metres. Its ‘root’ system is known in the fossil record as stigmaria and has a readily recognisable ‘dimpled’ pattern. Another common fossil clubmoss is sigillaria. It takes around 15 cm of peat to form 1 cm of coal. The large sample I am holding was gifted to me in 1977 by a friend whose father owned a fuel depot in Harold’s Cross in Dublin. It is 20 cm in thickness and therefore represents the compressed remains of a 3-metre deposit of ancient peat and weighs 10 kg. The upper surface displays the linear pattern of the trunk of a Sigillaria. It is estimated that as much as 70% of the coal in some deposits may consist of the compacted remains of lycopods. Smaller plants also grew in those tropical forests including many species of fern and Pennsylvania in the USA is famous for them. I regularly check through my coal before consigning it to the flames and over the years have been amply rewarded with some nice fossils.

Clay-ironstone nodules are natural concretions which occur in shale and are commonly found during coal mining. They are composed of a blend of fine clay particles and an iron mineral known as siderite. Also known as ‘iron balls’, they were processed from the mid-sixteenth century onwards as a source of iron when coal extraction began in the Castlecomer area of Kilkenny. Indeed, some of the railings and gates in the locality are made of ‘Castlecomer iron’. The nineteenth-century ironworks in Arigna, Co. Roscommon, was supplied with siderite-rich nodules collected by men and women in the layers of shale exposed along the courses of the local rivers.

Clay-ironstone nodules come in a variety of shapes and sizes and over the years I have amassed a significant collection. When the Bru na Bóinne Visitor Centre in Co. Meath was being built some years ago, the machines removed vast quantities of the local shale and some large nodules surfaced. Some of these were retained and later incorporated in the water feature located just outside the main entrance to the centre. The largest specimen I have ever come across on my travels is on display outside the Natural History Museum in Bordeaux in France.

Best,

Vincent